The Pentium as a Navajo weaving

Hurrying through the National Gallery of Art five minutes before closing, I passed a Navajo weaving with a complex abstract pattern. Suddenly, I realized the pattern was strangely familiar, so I stopped and looked closely. The design turned out to be an image of Intel's Pentium chip, the start of the long-lived Pentium family.1 The weaver, Marilou Schultz, created the artwork in 1994 using traditional materials and techniques. The rug was commissioned by Intel as a gift to AISES (American Indian Science & Engineering Society) and is currently part of an art exhibition—Woven Histories: Textiles and Modern Abstraction—focusing on the intersection between abstract art and woven textiles.

"Replica of a Chip", created by Marilou Schultz, 1994. Wool. Photo taken at the National Gallery of Art, 2024.

"Replica of a Chip", created by Marilou Schultz, 1994. Wool. Photo taken at the National Gallery of Art, 2024.

I talked with Marilou Schultz, a Navajo/Diné weaver and math teacher, to learn more about the artwork. Schultz learned weaving as a child—part of four generations of weavers—carding the wool, spinning it into yarn, and then weaving it. For the Intel project, she worked from a photograph of the die, marking it into 64 sections along each side so the die pattern could be accurately transferred to the weaving. Schultz used the "raised outline" technique, which gives a three-dimensional effect along borders. One of the interesting characteristics of the Pentium from the weaving perspective is its lack of symmetry, unlike traditional rugs. The Pentium weaving was colored with traditional plant dyes; the cream regions are the natural color of the wool from the long-horned Navajo-Churro sheep.2 The yarn in the weaving is a bit finer than the yarn typically used for knitting. Weaving was a slow process, with a day's work extending the rug by 1" to 1.5".

The Pentium die photo below shows the patterns and structures on the surface of the fingernail-sized silicon die, over three million tiny transistors. The weaving is a remarkably accurate representation of the die, reproducing the processor's complex designs. However, I noticed that the weaving was a mirror image of the physical Pentium die; I had to flip the rug image below to make them match. I asked Ms. Schultz if this was an artistic decision and she explained that she wove the rug to match the photograph. There is no specific front or back to a Navajo weaving because the design is similar on both sides,3 so the gallery picked an arbitrary side to display. Unfortunately, they picked the wrong side, resulting in a backward die image. This probably bothers nobody but me, but I hope the gallery will correct this in future exhibits. For the remainder of this article, I will mirror the rug to match the physical die.

Comparison of the Pentium weaving (flipped vertically) with a Pentium die photo. Original die photo from Intel.

Comparison of the Pentium weaving (flipped vertically) with a Pentium die photo. Original die photo from Intel.

The rug is accurate enough that each region can be marked with its corresponding function in the real chip, as shown below. Starting in the center, the section labeled "integer execution units" is the heart of the processor, performing arithmetic operations and other functions on integer numbers. The Pentium is a 32-bit processor, so the integer execution unit is a vertical rectangle, 32 bits wide. The horizontal lines correspond to different types of circuitry such as adders, multipliers, shifters, and registers. To the right, the "floating point unit" performs more complex arithmetic operations on floating-point numbers, numbers with a fractional part that are used in applications such as spreadsheets and CAD drawings. Like the integer execution unit, the floating point unit has horizontal stripes corresponding to different functions. Floating-point numbers are represented with more bits, so the stripes are wider.

The Pentium weaving, flipped and marked with the chip floorplan.

The Pentium weaving, flipped and marked with the chip floorplan.

At the top, the "instruction fetch" section fetches the machine instructions that make up the software. The "instruction decode" section analyzes each instruction to determine what operations to perform. Simple operations, such as addition, are performed directly by the integer execution unit. Complicated instructions (a hallmark of Intel's processors) are broken down into smaller steps by the "complex instruction support" circuitry, with the steps held in the "microcode ROM". The "branch prediction logic" improves performance when the processor must make a decision for a branch instruction.

The code and data caches provide a substantial performance boost. The problem is that the processor is considerably faster than the computer's RAM memory, so the processor can end up sitting idle until program code or data is provided by memory. The solution is the cache, a small, fast memory that holds bytes that the processor is likely to need. The Pentium processor had a small cache by modern standards, holding 8 kilobytes of code and 8 kilobytes of data. (In comparison, modern processors have multiple caches, with hundreds of kilobytes in the fastest cache and megabytes in a slower cache.) Cache memories are built from an array of memory storage elements in a structured grid, visible in the rug as uniform pink rectangles. The TLB (Translation Lookaside Buffer) assists the cache. Finally, the "bus interface logic" connects the processor to the computer's bus, providing access to memory and peripheral devices. Around the edges of the physical chip, tiny bond pads provide the connections between the silicon chip and the integrated circuit package. In the weaving, these tiny pads have been abstracted into small black rectangles.

The weaving is accurate enough to determine that it represents a specific Pentium variant, called P54C. The motivation for the P54C was that the original Pentium chips (called P5) were not as fast as hoped and ran hot. Intel fixed this by using a more advanced manufacturing process, reducing the feature size from 800 to 600 nanometers and running the chip at 3.3 volts instead of 5 volts. Intel also modified the chip so that when parts of the chip were idle, the clock signal could be stopped to save power. (This is the "clock driver" circuitry at the top of the weaving.) Finally, Intel added multiprocessor logic (adding 200,000 more transistors), allowing two processors to work together more easily. The improved Pentium chip was smaller, faster, and used less power. This variant was called the P54C (for reasons I haven't been able to determine). The "multiprocessor logic" is visible in the Pentium rug, showing that it is the P54C Pentium (right) and not the P5 Pentium (left).

The Pentium P5 on the left and the P54C on the right, showing the difference in die and package sizes. If you look closely, the P5 die on the left lacks the "multiprocessor logic" in the weaving, showing that the weaving is the P54C. I clipped the pins on the P5 to fit it under a microscope.

The Pentium P5 on the left and the P54C on the right, showing the difference in die and package sizes. If you look closely, the P5 die on the left lacks the "multiprocessor logic" in the weaving, showing that the weaving is the P54C. I clipped the pins on the P5 to fit it under a microscope.

Intel's connection with New Mexico started in 1980 when Intel opened a chip fabrication plant (fab) in Rio Rancho, a suburb north of Albuquerque. At the time, this plant, Fab 7, was Intel's largest and produced 70% of Intel's profits. Intel steadily grew the New Mexico facility, adding Fab 9 and then Fab 11, which opened in September 1995, building Pentium and Pentium Pro chips in a 140-step manufacturing process. Intel's investment in Rio Rancho has continued with a $4 billion project underway for Fab 9 and Fab 11x. Intel has been criticized for environmental issues in New Mexico, detailed in the book Intel inside New Mexico: A case study of environmental and economic injustice. Intel, however, claims a sustainable future in New Mexico, restoring watersheds, using 100% renewable electricity, and recycling construction waste.

Fairchild and Shiprock

Marilou Schultz is currently creating another weaving based on an integrated circuit, shown below. Although this chip, the Fairchild 9040, is much more obscure than the Pentium, it has important historical symbolism, as it was built by Navajo workers at a plant on Navajo land.

Marilou Schultz's current weaving project. Photo provided by the artist.

Marilou Schultz's current weaving project. Photo provided by the artist.

In 1965, Fairchild started producing semiconductors in Shiprock, New Mexico, about 200 miles northwest of Intel's future facility. Fairchild produced a brochure in 1969 to commemorate the opening of a new plant. Two of the photos in that brochure compared a traditional Navajo weaving to the pattern of a chip, which happened to be the 9040. Although Fairchild's Shiprock project started optimistically, it was suddenly shut down a decade later after an armed takeover. I'll discuss the complicated history of Fairchild in Shiprock and then describe the 9040 chip in more detail.

A Navajo rug and the die of a Fairchild 9040 integrated circuit. Images from Fairchild's commemorative brochure on the opening of a new plant at Shiprock.

A Navajo rug and the die of a Fairchild 9040 integrated circuit. Images from Fairchild's commemorative brochure on the opening of a new plant at Shiprock.

The story of Fairchild starts with William Shockley, who invented the junction transistor at Bell Labs, won the Nobel prize, and founded Shockley Semiconductor Laboratory in 1957 to build transistors. Unfortunately, although Shockley was brilliant, he was said to be the worst manager in the history of electronics, not to mention a notorious eugenicist and racist later in life. Eight of his top employees—called the "traitorous eight"—left Shockley's company in 1957 to found Fairchild Semiconductor. (The traitorous eight included Gordon Moore and Robert Noyce who ended up founding Intel in 1968). Noyce (co-)invented the integrated circuit in 1959 and Fairchild soon became a top semiconductor manufacturer, famous for its foundational role in Silicon Valley.

The Shiprock project was part of an attempt in the 1960s to improve the economic situation of the Navajo through industrial development. The Navajo had suffered a century of oppression including forced deportation from their land through the Long Walk (1864-1866). The Navajo were suffering from 65% unemployment, a per-capita income of $300, and a lack of basics such as roads, electricity, running water, and health care. The Bureau of Indian Affairs was now trying to encourage economic self-sufficiency by funding industrial projects on Indian land.4 Navajo Tribal Chairman Raymond Nakai viewed industrialization as the only answer. Called "the first modern Navajo political leader", Nakai stated, "There are some would-be leaders of the tribe calling for the banishment of industry from the reservation and a return to the life of a century ago! But, it would not solve the problems. There is not sufficient grazing land on the reservation to support the population so industry must be brought in." Finally, Fairchild was trying to escape the high cost of Silicon Valley labor by opening plants in low-cost locations such as Maine, Australia, and Hong Kong.

These factors led Fairchild to open a manufacturing facility on Navajo land in Shiprock, New Mexico. The project started in 1965 with 50 Navajo workers in the Shiprock Community Center manufacturing transistors, rapidly increasing to 366 Navajo workers.

Fairchild's manufacturing plant in Shiprock, NM, named after the Shiprock rock formation in the background. The formation is called Tsé Bitʼaʼí in Navajo.
    From The Industrialization of a 'Sleeping Giant', Commerce Today, January 25, 1971.

Fairchild's manufacturing plant in Shiprock, NM, named after the Shiprock rock formation in the background. The formation is called Tsé Bitʼaʼí in Navajo. From The Industrialization of a 'Sleeping Giant', Commerce Today, January 25, 1971.

By 1967, Robert Noyce, group vice-president of Fairchild, regarded the Shiprock plant as successful. He explained that Fairchild was motivated both by low labor costs and by social benefits, saying, "Probably nobody would ever admit it, but I feel sure the Indians are the most underprivileged ethnic group in the United States." Two years later, Lester Hogan, Fairchild's president, stated, "I thought the Shiprock plant was one of Bob Noyce's philanthropies until I went there," but he was so impressed that he decided to expand the plant. Hogan also directed Fairchild to help build hundreds of houses for workers; since a traditional Navajo dwelling is called a hogan, the houses were dubbed Hogan's hogans.

Workers in Fairchild's Shiprock plan, 1966. Photo by Jack Grimes. Photo courtesy of Computer History Museum, Henry Mahler collection of Fairchild Semiconductor photographs.

Workers in Fairchild's Shiprock plan, 1966. Photo by Jack Grimes. Photo courtesy of Computer History Museum, Henry Mahler collection of Fairchild Semiconductor photographs.

In 1969, Fairchild opened its new facility at Shiprock and produced the commemorative brochure mentioned earlier. As well as showing the striking visual similarity between the designs of traditional Navajo weavings and modern integrated circuits, it stated that "Weaving, like all Navajo arts, is done with unique imagination and craftsmanship" and described the "blending of innate Navajo skill and [Fairchild] Semiconductor's precision assembly techniques." Fairchild later said that "rug weaving, for instance, provides an inherent ability to recognize complex patterns, a skill which makes memorizing integrated circuit patterns a minimal problem."7

However, in Indigenous Circuits: Navajo Women and the Racialization of Early Electronic Manufacture, digital media theorist Lisa Nakamura critiques this language as a process by which "electronics assembly work became both gendered and identified with specific racialized qualities".5 Nakamura points out how "Navajo women’s affinity for electronics manufacture [was described] as both reflecting and satisfying an intrinsic gendered and racialized drive toward intricacy, detail, and quality."

Fairchild's Shiprock plant, 1966. From the patterns on the floor, this photo may show the time period when Fairchild set up manufacturing in the school gymnasium. Photo by Jack Grimes. Photo courtesy of Computer History Museum, Henry Mahler collection of Fairchild Semiconductor photographs.

Fairchild's Shiprock plant, 1966. From the patterns on the floor, this photo may show the time period when Fairchild set up manufacturing in the school gymnasium. Photo by Jack Grimes. Photo courtesy of Computer History Museum, Henry Mahler collection of Fairchild Semiconductor photographs.

At Shiprock, Fairchild employed 1200 workers,6 and all but 24 were Navajo, making Fairchild the nation's largest non-government employer of American Indians. Of the 33 production supervisors, 30 were Navajo. This project had extensive government involvement from the Bureau of Indian Affairs and the U.S. Public Health Service, while the Economic Development Administration made business loans to Fairchild, the Labor Department had job training programs, and Housing and Urban Development built housing at Shiprock7.

The Shiprock plant was considered a major success story at a meeting of the National Council on Indian Opportunity in 1971.7 US Vice President Agnew called the economic deprivation and 40-80% unemployment on Indian reservations "a problem of staggering magnitude" and encouraged more industrial development. Fairchild President Hogan stated that "Fairchild's program at Shiprock has been one of the most rewarding in the history of our company, from the standpoint of a sound business as well as social responsibility." He said that at first the plant was considered the "Shiprock experiment", but the plant was "now among the most productive and efficient of any Fairchild operation in the world." Peter MacDonald, Chairman of the Navajo Tribal Council and a World War II Navajo code talker, discussed the extreme poverty and unemployment on the Navajo reservation, along with "inadequate housing, inadequate health care and the lack of viable economic activities." He referred to Fairchild as "one of the best arrangements we have ever had" providing not only employment but also supporting housing through a non-profit.

Navajo workers using microscopes in Fairchild's Shiprock plant. From "The Navajo Nation Looks Ahead", National Geographic, December 1972.

Navajo workers using microscopes in Fairchild's Shiprock plant. From "The Navajo Nation Looks Ahead", National Geographic, December 1972.

In December 1972, National Geographic highlighted the Shiprock plant as "weaving for the Space Age", stating that the Fairchild plant was the tribe's most successful economic project with Shiprock booming due to the 4.5-million-dollar annual payroll. The article states: "Though the plant runs happily today, it was at first a battleground of warring cultures." A new manager, Paul Driscoll, realized that strict "white man's rules" were counterproductive. For instance, many employees couldn't phone in if they would be absent, as they didn't have telephones. Another issue was the language barrier since many workers spoke only Navajo, not English. So when technical words didn't exist in Navajo, substitutes were found: "aluminum" became "shiny metal". Driscoll also realized that Fairchild needed to adapt to traditional nine-day religious ceremonies. Soon the monthly turnover rate dropped from 12% to under 1%, better than Fairchild's other plants.

Unfortunately, the Fairchild-Navajo manufacturing partnership soon met a dramatic end. In 1975, the semiconductor industry was suffering from the ongoing US recession. Fairchild was especially hard hit, losing money on its integrated circuits, and it shed over 8000 employees between 1973 and 1975.8 At the Shiprock plant, Fairchild laid off9 140 Navajo employees in February 1975, angering the community. A group of 20 Indians armed with high-power rifles took over the plant, demanding that Fairchild rehire the employees. Fairchild portrayed the occupiers, part of the AIM (American Indian Movement), as an "outside group—representing neither employees, tribal authorities nor the community." Peter MacDonald, chairman of the Navajo Nation, agreed with the AIM on many points but viewed the AIM occupiers as "foolish" with "little sense of Navajo history" and "no sense of the need for an Indian nation to grow" (source). MacDonald negotiated with the occupiers and the occupation ended peacefully a week later, with unconditional amnesty granted to the occupiers.10 However, concerned about future disruptions, Fairchild permanently closed the Shiprock plant and transferred production to Southeast Asia.

An article entitled "Navajos Occupy Plant". Contrary to the title, MacDonald stated that many of the occupiers were from other tribes and were not acting in the best interest of the Navajo. From Workers' Power, the biweekly newspaper of the International Socialists, March 13-26, 1975.

An article entitled "Navajos Occupy Plant". Contrary to the title, MacDonald stated that many of the occupiers were from other tribes and were not acting in the best interest of the Navajo. From Workers' Power, the biweekly newspaper of the International Socialists, March 13-26, 1975.

For the most part, the Fairchild plant was viewed as a success prior to its occupation and closure. Navajo leader MacDonald looked back on the Fairchild plant as "a cooperative effort that was succeeding for everyone" (link). Alice Funston, a Navajo forewoman at Shiprock said, "Fairchild has not only helped women get ahead, it has been good for the entire Indian community in Shiprock."11 On the other hand, Fairchild general manager Charles Sporck had a negative view looking back: "It [Shiprock] never worked out. We were really screwing up the whole societal structure of the Indian tribe. You know, the women were making money and the guys were drinking it up. We had a very major negative impact upon the Navajo tribe."12

Despite the stereotypes in Sporck's comments, he touches on important gender issues, both at Fairchild and in the electronics industry as a whole. Fairchild had long recognized the lack of jobs for men at Shiprock, despite attempts to create roles for men. In 1971, Fairchild President Hogan stated that since "semiconductor assembly operation require a great amount of detail work with tiny components, [it] lends itself to female workers. As a result, there are nearly three times as many Navajo women employed by Fairchild as men."7

The role of women in fabricating and assembling electronics is often not recognized. A 1963 report on electronics manufacturing estimated that women workers made up 41 percent of total employment in electronics manufacturing, largely in gendered roles. The report suggested that microminiaturization of semiconductors gave women an advantage over men in assembly and production-line work; women made up over 70% of semiconductor production-line workers, with 90-99% of inspecting and testing jobs. and 90-100% of assembler jobs. Women were largely locked out of non-production jobs; although women held a few technician and drafting roles, the percentage of woman engineers was too low to measure.

The defense contractor General Dynamics also had Navajo plants, but with more success than Fairchild. General Dynamics opened a Navajo Nation plant in Fort Defiance, Arizona in 1967 to make missiles for the Navy. At the plant's opening, Navajo Tribal Chairman Raymond Nakai pushed for industrialization, stating that it was in "industrialization and the money and the jobs engendered thereby that the future of the Navajo people will lie." The plant started with 30 employees, growing to 224 by the end of 1969, but then dropping to 99 in 1971 due to a slowdown in the electronics industry. General Dynamics opened another Navajo plant near Farmington NM in 1988. Due to the end of the Cold War, Hughes Aircraft (part of General Motors) acquired General Dynamics' missile business in 1992 and sold it to Raytheon in 1997. The Fort Defiance facility was closed in 2002 when its parent company, Delphi Automotive Systems, moved out of the military wiring business. The Farmington plant remains open, now Raytheon Diné, building components for Tomahawk, Javelin, and AMRAAM missiles.

Navajo workers at the General Dynamics plant in Fort Defiance, AZ. From the 1965 General Dynamics film "The Navajo moves into the electronic age". From American Indian Film Gallery.

Navajo workers at the General Dynamics plant in Fort Defiance, AZ. From the 1965 General Dynamics film "The Navajo moves into the electronic age". From American Indian Film Gallery.

Inside the Fairchild 9040 integrated circuit

The integrated circuit die image in Fairchild's commemorative brochure has an exceptionally striking design and color scheme. It's clear why this chip brings weaving to mind. Studying the die photo of the 9040 carefully reveals some interesting characteristics of integrated circuit design, so I will go into some detail.

Die photo of the Fairchild 9040 flip-flop. From the commemorative brochure.

Die photo of the Fairchild 9040 flip-flop. From the commemorative brochure.

The chip was fabricated from a tiny square of silicon, which appears purple in the photograph. Different regions of the silicon die were treated (doped) with impurities to change the properties of the silicon and thus create electronic devices. These doped regions appear as green or blue lines. The white lines are the metal layer on top of the silicon, connecting the components. The 13 metal rectangles around the border are the bond pads. The chip was packaged in an unusual 13-pin flat-pack, as shown below. Each of the 13 bond pads above was connected by a tiny wire to one of the 13 external pins.

The Fairchild 9040 packaged in a 13-pin flatpack integrated circuit. The chip was also available in a 14-pin DIP, a standard way of packaging chips. Photo from the commemorative brochure.

The Fairchild 9040 packaged in a 13-pin flatpack integrated circuit. The chip was also available in a 14-pin DIP, a standard way of packaging chips. Photo from the commemorative brochure.

The Fairchild 9040 was introduced in the mid-1960s as part of Fairchild's Micrologic family, a set of high-performance integrated circuits that were designed to work together.13 The 9040 chip was a "flip-flop", a circuit capable of storing a single bit, a 0 or 1. Flip-flops can be combined to form counters, counting the number of pulses, for instance.

The most dramatic patterns on the chip are the intricate serpentine blue lines. Each line forms a resistor, controlling the flow of electricity by impeding its path. The lines must be long to provide the desired resistance, so they wind back and forth to fit into the available space. Each end of a resistor is connected to the metal layer, wiring it to another part of the circuit. Most of the die is occupied by resistors, which is a disadvantage of this type of circuit. Modern integrated circuits use a different type of circuitry (CMOS), which is much more compact, partly because it doesn't need bulky resistors.

Resistors in the 9040 die.

Resistors in the 9040 die.

Transistors are the main component of an integrated circuit. These tiny devices act as switches, turning signals on and off. The photo below shows one of the transistors in the 9040. It consists of three layers of silicon, with metal wiring connected to each layer. Note the blue region in the middle, surrounded by a slightly darker purple region; these color changes indicate that the silicon has been doped to change its properties. The green region surrounding the transistor provides isolation between this transistor and the other circuitry, so the transistors don't interfere with each other. The chip also has many diodes, which look similar to transistors except a diode has two connections.

A transistor in the 9040 die. The three contacts are called the base, emitter, and collector.

A transistor in the 9040 die. The three contacts are called the base, emitter, and collector.

These transistors with their three layers of silicon are a type known as bipolar. Modern computers use a different type of transistor, metal-oxide-semiconductor (MOS), which is much more compact and efficient. One of Fairchild's major failures was staying with bipolar transistors too long, rather than moving to MOS.14 In a sense, the photo of the 9040 die shows the seeds of Fairchild's failure.

The 9040 chip was constructed on a completely different scale from the Pentium, showing the rapid progress of the IC industry. The 9040 contains just 16 transistors, while the Pentium contains 3.3 million transistors. Thus, individual transistors can be seen in the 9040 image, while only large-scale functional blocks are visible in the Pentium. This increasing transistor count illustrates the exponential growth in integrated circuit capacity between the 9040 in the mid-1960s and the Pentium in 1993. This growth pattern, with the number of transistors doubling about every two years, is known as Moore's law, since it was first observed in 1965 by Gordon Moore (one of Fairchild's "traitorous eight", who later started Intel).

The schematic below shows the circuitry inside the 9040 chip, with its 16 transistors, 16 diodes, and 22 resistors. The symmetry of the 9040 die photo makes it appealing, and that symmetry is reflected in the circuit below, with the left side and the right side mirror images. The idea behind a flip-flop is that it can hold either a 0 or a 1. In the chip, this is implemented by turning on the right side of the chip to hold a 0, or the left side to hold a 1. If one side of the chip is on, it forces the other side off, accomplished by the X-like crossings of signals in the center.15 Thus, the symmetry is not arbitrary, but is critical to the operation of the circuit.

Schematic of the Fairchild 9040 flip-flop chip. From Fairchild 1970 Data Catalog.

Schematic of the Fairchild 9040 flip-flop chip. From Fairchild 1970 Data Catalog.

Despite the obscurity of the 9040, multiple 9040 chips are currently on the Moon. The chip was used in the Apollo Lunar Surface Experiments Package (ALSEP),16 in particular, the Active Seismic Experiment on Apollo 14 and 16. This experiment detonated small explosives on the Moon and measured the resulting seismic waves. The photo below is a detail from a blueprint17 that shows three of the nineteen 9040 flip-flops (labeled "FF") as well as two 9041 logic gates, a chip in the same family as the 9040.

Detail from Logic Schematic Type B Board No.4 ASE.

Detail from Logic Schematic Type B Board No.4 ASE.

Conclusions

The similarities between Navajo weavings and the patterns in integrated circuits have been described since the 1960s. Marilou Schultz's weavings of integrated circuits make these visual metaphors into concrete works of art. Although the Woven Histories exhibit at the National Gallery of Art is no longer on display, the exhibit will be at the National Gallery of Canada (Ottawa) starting November 8, 2024, and the Museum of Modern Art (New York) starting April 20, 2025 (full dates here). If you're in the area, I recommend viewing the exhibit, but don't make my mistake: leave more than five minutes to see it!

Many thanks to Marilou Schultz for discussing her art with me. For more on her art, see A Conversation with Marilou Schultz on YouTube.18 Follow me on Mastodon as @[email protected] or RSS for updates.

Notes and references

  1. The original Pentium was followed by the Pentium Pro, the Pentium II, and others, forming a long-running brand of high-performance processors. Pentium was Intel's flagship line until the Core processors took over in 2006. 

  2. Sheep hold a key role in Navajo culture and economy, which I'll briefly summarize here. Domestic sheep were brought to the Americas during the Spanish colonization, reaching the Navajo in the late 1500s. Since sheep were able to graze on semi-arid land unsuitable for crops, sheep became very important to the Navajo. Although the Navajo had used cotton for weaving in the past, the availability of wool made weaving a fundamental industry; the production and trading of woven Navajo blankets became an important economic factor in New Mexico by the 1750s (details).

    Navajo leader Peter MacDonald described the role of sheep: "Sheep were like money in the bank: the more you had, the better your life, your future, and your family's future." The number of sheep grew exponentially in the early 1900s, resulting in overgrazing of the land. The drought and Dust Bowl of the 1930s led the government to restrict the number of sheep on Navajo land, imposing the Navajo Livestock Reduction. This heavy-handed program purchased and slaughtered over half the livestock, which was catastrophic to the Navajo, both economically and culturally, destroying the Navajo's wealth and self-sufficiency.

    The Navajo-Churro sheep is a breed that the Navajo developed from the Churra sheep brought from Spain during the Spanish colonization of the Americas. These sheep have a long, lustrous fleece that is excellent for weaving. The Navajo-Churro is also called the Navajo Four-Horned Sheep as some rams have four horns, a rare trait. The Navajo-Churro breed was severely depleted when American troops killed livestock during the Navajo Wars (1863) and then brought close to extinction by the Livestock Reduction of the 1930s to 1950s. In the 1970s, the Navajo Sheep Project started efforts to preserve and revitalize the Navajo-Churro. The breed is still rare, but currently numbers in the thousands. Now, climate change and water shortages are putting more pressure on sheep grazing.  

  3. A photo of the rug was published in American Indian Science & Engineering Society 1994 Annual Report. This photo shows the "physically accurate" side of the rug, not the side that is currently on display.

    A photo of the rug from 1994.

    A photo of the rug from 1994.

    Which side of a die image is the top is mostly arbitrary. Intel usually presents die photos with the tiny text on the die right side up, so I will use that convention. For the Pentium die, this text is in the lower right corner and says "80P54C (m) (c) intel '92,'93". Of course, this text is much too small to be part of the woven rug. 

  4. Strengthening the Indian Economy (Indian Affairs, 1966) discusses various industrial development projects, of which Fairchild was the largest. Other projects included a plant at Rolla, ND to produce sapphire and ruby bearings, a Seminole project with Amphenol to produce electronic connectors, and a Hopi project with BVD to produce garments. Other economic development projects included timber and mining; extractive industries provided over half of Navajo income. 

  5. Racialization is defined by Nakamura as "the understanding of a specific population as possessing traits and behaviors that belong to a race, not an individual." 

  6. Many photos of workers at the Shiprock plant are in Fairchild VIEWS, March 1969. Fairchild deserves credit for referring to the workers by name rather than viewing them as anonymous props for photos. Fairchild followed the same practice in its annual reports

  7. NCIO (National Council on Indian Opportunity) News, Oct/Nov 1971 described a high-level meeting with industry to discuss "new development on Indian reservations" with industry. US Vice President Spiro Agnew ran the meeting, with Attorney General John Mitchell a speaker along with Navajo Tribal Council chairman Peter MacDonald. Bizarrely, all three ended up convicted of felonies for different reasons. Within a few years, Mitchell was imprisoned for Watergate crimes and Agnew pled guilty to federal tax evasion. In 1990, MacDonald was convicted of fraud, riot, extortion, racketeering, and conspiracy by a Navajo tribal judge and then a federal judge, spending eight years in prison until pardoned by Bill Clinton (details). The story of Peter MacDonald is complex and many view his prosecution as politically motivated; MacDonald's memoir provides his perspective. 

  8. Although Fairchild was highly successful at first, it suffered from chaotic management and economic decline. Fairchild steadily lost key employees, many of whom started competing companies. Most important was Intel, started in 1968 by Moore and Noyce, two of the "Traitorous Eight". Eventually, hundreds of companies (called the Fairchildren) could be traced back to Fairchild. Economic factors also battered Fairchild; the semiconductor industry had barely recovered from the 1970-1971 recession when it was hit by the severe 1975 recession. As a result, Fairchild had large layoffs, of which the Shiprock layoffs were a small part. Fairchild's business continued to decline; it was purchased by Schlumberger in 1979 and went through various acquisitions, mergers, and spinoffs until it finally ended in 2016, acquired by ON Semiconductor. 

  9. Were the employees "laid off" or "layed off"? Curiously, the New York Times article said "layed off" but sources uniformly state that "layed off" is grammatically wrong. The New York Times has extensively used "layed off" so this isn't a one-time typo. I hypothesized that usage had changed since the 1970s but Google Ngram Viewer shows laid off as the consistent and overwhelming winner. Maybe "layed off" was a stylistic quirk of the New York Times? 

  10. Looking back, MacDonald questioned his decision to let the occupation of Fairchild's plant continue rather than ordering the tribal police to forcibly remove the occupiers from the plant. In his view, his decision to let the occupation led to the closing of the plant and the loss of 1200 jobs. On the other hand, forcibly removing the occupiers risked violence and loss of life: "I would have become the chairman who killed his own people instead of the chairman who allowed Navajo to lose their jobs."

    The risk of bloodshed was not theoretical. In 1989, a riot between MacDonald's supporters and the police resulted in two Navajos being shot and killed by the police. MacDonald pressed for a federal investigation into police brutality, but instead MacDonald and Benally (a council delegate) received long prison sentences for inciting the riot even though they were not present at the time. 

  11. Alice Funston was Forewoman for the Reliability and Quality Assurance Section at Shiprock. In a Fairchild employee newsletter, she said, "Fairchild has not only helped women get ahead, it has been good for the entire Indian community in Shiprock. Before the plant was built here, there weren't many jobs available. You could work for the Bureau of Indian Affairs, the Navajo Tribe or other government agencies, but there just weren't enough jobs to go around. I started in assembly in 1965 and was recently promoted to Production Supervisor in R & Q.A. Since the beginning of the year, a number of women have been promoted into supervisory positions. When I joined Fairchild, most of the members of management were non-Indian. Today, almost all of our supervisors and managers are Indian."

    I quote this at length, since it was the only example I could find of an employee discussing Shiprock in their own words. It must be recognized, of course, that this is a company publication, so the comments may not be completely candid. See "Affirmative Action: A growing consciousness of the needs of the individual" in Fairchild HORIZONS, May-June, 1973. 

  12. See Interview with Charlie Sporck, 2000 February 21, timestamp 0:27. From "Silicon Genesis: oral history interviews of Silicon Valley scientists, 1995-2024," Stanford Digital Repository.

    I view Sporck's comments on the failure of Shiprock as highly questionable. First, Sporck left Fairchild in 1967, so he was not present for most of the Shiprock project. Moreover, he implies that Fairchild's closing of Shiprock was in the best interest of the Navajo, which is a morally convenient justification for Fairchild's decision, but contradicted by most other sources. 

  13. Fairchild's 9040 logic family was called LPDTμL for "low-power diode-transistor Micrologic". Some sources label this family as TTL (Transistor-Transistor Logic), probably confusing it with the 9000-family, which was TTL. 

  14. Fairchild's failure to recognize the importance of MOS transistors and transition from bipolar transistors is described in History of Semiconductor Engineering, page 170. 

  15. I'll provide more details of the 9040 schematic in this footnote. The 9040 is a flexible flip-flop. It can be wired as an R-S (reset-set) flip-flop, set to 1 or reset to 0 as needed. It can also be wired as a J-K flip-flop, a flexible circuit that can store a value, hold a value, or toggle, based on the settings of the J and K inputs.

    The 9040 is a "dual-rank" flip-flop, meaning it holds its value in two latches: a primary latch and a secondary latch. (This type of flip flop was generally called "master-slave", a name that is now controversial). Looking at the schematic, the primary latch at the bottom of the schematic passes its value to the secondary latch at the top under the control of the clock. This structure makes the flip-flop "edge-triggered", changing its value at the moment when the clock signal changes.

    This circuit uses diode-transistor logic. Diodes perform most of the logic operations by combining input signals, while the transistors provide amplification. Diodes play a different role in the "push-pull" output circuit, raising the level of the high-side transistor. Because the output circuit has a transistor, diode, and transistor stacked vertically, it is often called a totem pole output, a name that seems questionable in this context.

    One curious feature of the 9040 is that it contains two pull-up resistors that are not assigned any role. The user of the chip can attach them to unused inputs to keep the input at the desired value.

    Looking at the schematic shows 13 pins, corresponding to the 13 pins of the flat-pack integrated circuit. All but three of these pins are symmetrical; power (Vcc), ground, and the clock (CP) have single connections. The ground pad is in the bottom-center of the die, which maintains symmetry. The clock and power pads are side-by-side in the top-center of the die. If you study the die photograph closely, you will see that they subtlely break the chip's symmetry as the clock signal runs down the center of the die while the power connection runs down both sides. There are a few other subtle violations of symmetry when signals cross from one side of the chip to the other, as well as the obviously asymmetrical text. 

  16. I haven't been able to prove that the Apollo program used chips from the Shiprock plant rather than a different facility. Fairchild President Hogan stated that workers at Shiprock assembled guidance, communications, and gyro systems that were used on Apollo rockets. 

  17. The ALSEP schematic is from Miller, K. Logic Schematic Type B Board No.4 ASE, A4, technical drawing, January 27, 1967, University of North Texas Libraries, The Portal to Texas History; crediting Lunar Planetary Institute Library. 

  18. Marilou Schultz had another chip weaving on display at the National Gallery of Art. It is labeled "Untitled (Unknown Chip), 2008", but Antoine Bercovici identified it for me as the AMD K6 III processor, released in 1999 and comparable to the Pentium III.

    A weaving created by Marilou Schultz, "Untitled (Unknown Chip)".

    A weaving created by Marilou Schultz, "Untitled (Unknown Chip)".

    If you're interested in computer-related weaving, the exhibition also had "Copper Tapestry (Riva 128 Graphics Card, Nvidia, 1997)" by Argentinian artist Analia Saban, created on a computer-automated Jacquard loom. This weaving represents a PC graphics card, specifically, the STB Velocity 128, which uses the Nvidia Riva 128 GPU chip. This chip was released in 1997, at a point when Nvidia was in a dire financial position, thirty days from going out of business. The Riva 128 saved Nvidia and now Nvidia is the world's third most valuable company.

    A tapestry created by Analia Saban, "Copper Tapestry (Riva 128 Graphics Card, Nvidia, 1997)".

    A tapestry created by Analia Saban, "Copper Tapestry (Riva 128 Graphics Card, Nvidia, 1997)".

     

Inside the guidance system and computer of the Minuteman III nuclear missile

The Minuteman missile was introduced in 1962 as a key part of America's nuclear deterrent. The Minuteman III missile is currently the only US land-based intercontinental ballistic missile (ICBM), with 400 missiles ready for launch, spread across five central states.1 The missile contains a precision guidance system, capable of delivering a warhead to a target 13,000 km away (8000 miles) with an accuracy of 200 meters (660 feet).

The diagram below shows the guidance system of the Minuteman III missile (1970). This guidance system contains over 17,000 electronic and mechanical parts, costing $510,000 (about $4.5 million in current dollars). The heart of the guidance system is the gyro stabilized platform, which uses gyroscopes and accelerometers to measure the missile's orientation and acceleration. The computer uses the measurements from the platform to determine the missile's position and guide the missile on its trajectory to the target. Other key components are the missile guidance set controller, which contains electronics to support the gyro stabilized platform, and the amplifier, which interfaces the computer with the rest of the missile. In this blog post, I take a close look at the components of the guidance system that was used until the early 2000s.2

The Minuteman III guidance system (NS-20). Click on this image (or any other) for a larger version. Original image from National Air and Space Museum.

The Minuteman III guidance system (NS-20). Click on this image (or any other) for a larger version. Original image from National Air and Space Museum.

Fundamentally, the guidance computer constantly compares the missile position to the desired trajectory and generates the appropriate steering commands to keep the missile on track.3 The diagram below shows how directing the engine nozzles causes the missile to rotate around its three axes: roll, pitch, and yaw.4 In the silo, the roll angle (the azimuth) is aligned with the direction to the target. The missile takes off vertically and then the missile gradually rotates along the pitch axis to tilt over toward the target. During flight, adjustments along all three axes keep the missile on target. The Minuteman III has four rocket stages so the guidance computer jettisons each rocket stage and ignites the next stage in sequence.

The roll, pitch, and yaw axes for the Minuteman missile. The engine diagrams show how the nozzles are directed to rotate around each axis, Modified from A Simulation of Minuteman Trajectories, with changed axes.

The roll, pitch, and yaw axes for the Minuteman missile. The engine diagrams show how the nozzles are directed to rotate around each axis, Modified from A Simulation of Minuteman Trajectories, with changed axes.

The guidance platform

The idea behind inertial navigation is to keep track of the missile's position by constantly measuring its acceleration. By integrating the acceleration, you get the velocity. And by integrating the velocity, you get the position. Inertial navigation is self-contained, a big advantage for a missile since the enemy can't jam your navigation. The hard part is measuring the acceleration and angles with extreme accuracy, since even tiny errors are multiplied as the missile travels.

In more detail, the Minuteman's inertial guidance is built around a gyroscopically stabilized platform, which is kept in a fixed orientation. The platform is mounted on two beryllium gimbals. Feedback from gyroscopes drives three torque motors to rotate the gimbals to keep the stable platform in exactly the same orientation no matter how the missile twists and turns.

The Minuteman III stable platform. Original image from National Air and Space Museum.

The Minuteman III stable platform. Original image from National Air and Space Museum.

The diagram below shows the components of the stable platform, in approximately the same orientation as the photo above. Three accelerometers are mounted on the stable platform to measure acceleration. The accelerometers are oriented along three perpendicular axes so each one measures acceleration along one axis. (The accelerometer axes are not aligned with the platform axes; this distributes the acceleration (mostly "up") across the accelerometers, increasing accuracy.) The two alignment mirrors allow the stable platform to be aligned with a precise device called an autocollimator, as will be described below. The gyrocompass uses the Earth's rotation to precisely determine North, providing a backup alignment technique. Both the alignment mirrors and the gyrocompass can be rotated to a precise angle, reported by the resolver.

The stable platform for Minuteman II and III. Modified from Minuteman weapon system history and description.

The stable platform for Minuteman II and III. Modified from Minuteman weapon system history and description.

To target a Minuteman I missile, the missile had to be physically rotated in the silo to be aligned with the target, an angle called the launch azimuth. This angle had to be extremely precise, since even a tiny angle error will be greatly magnified over the missile's journey. Aligning the missile was a tedious process that used the North Star to determine North. Since the star was not visible from inside the silo, a complex surveying technique was used, using a surveyor's theodolite to measure the angles between the North Star and three concrete monuments outside the silo. Inside the silo, the closest monument was visible through a sighting tube, allowing the precise angle measurement to be transferred to the silo. After many more measurements inside the silo, a special device called an autocollimator was positioned precisely 90° from the desired launch azimuth. The autocollimator shot a beam of light through a window in the side of the missile, where it bounced off a mirror on the stable platform and returned to the autocollimator. If the returning beam wasn't exactly parallel, the autocollimator sent a signal to the missile, causing the stable platform to rotate as needed. The result of this process was that the stable platform was exactly aligned with the desired angle to the target.5

The guidance platform was completely redesigned for Minuteman II and III, eliminating the time-consuming alignment that Minuteman I required. The new platform had an alignment block with rotating mirrors. Instead of rotating the missile, the autocollimator remained fixed in the East position and the mirror (and thus the stable platform) was rotated to the desired launch azimuth. The new guidance platform also added a gyrocompass under the alignment block, a special compass that could precisely align itself to North by precessing against the Earth's rotation. At first, the gyrocompass was used as a backup check against the autocollimator, but eventually the gyrocompass became the primary alignment. For calibration, the alignment block also includes electrolytic bubble levels to position the stable platform in known orientations with respect to local gravity.6

The alignment block with mirrored surfaces. Image from National Air and Space Museum.

The alignment block with mirrored surfaces. Image from National Air and Space Museum.

The photo above shows the alignment block on top of the gyrocompass. The front and back of the block are the precision mirrors that reflect the light beam from the autocollimator. The circles on top of the block and at the right are two level detectors, with set screws for exact adjustment. The platform has four level detectors, allowing it to be aligned against gravity in multiple positions. Like the gimbals, the gyrocompass assembly is made of beryllium due to its rigidity and light weight; it has a warning sticker because beryllium is highly toxic.

The diagram below shows how the axes align with the gimbals of the stable platform.7 Note the window at the top of the photo. Light from the autocollimator shines in through the window, reflects off the mirror on the alignment block, and returns through the window to the autocollimator. The autocollimator detects any error in alignment and signals the guidance system to correct its position accordingly.

Coordinate system for the stable platform. Note that these axes don't match the missile axes; the stable platform axes remain constant as the missile turns. Original image from National Air and Space Museum.

Coordinate system for the stable platform. Note that these axes don't match the missile axes; the stable platform axes remain constant as the missile turns. Original image from National Air and Space Museum.

The stable platform uses gyroscopes to maintain its fixed orientation as the missile turns. The idea behind a gyroscope is that a spinning disk will tend to maintain its spin axis. The problem is that any friction, even from precision ball bearings, will reduce the accuracy. The solution in the Minuteman is a "gas bearing", where the gyroscope rotor is supported by an extremely thin layer of hydrogen. As shown below, the gyroscope is built around a stationary marble-sized ball (blue), fastened to the gyroscope frame at the top and bottom. The rotor (pink) is clamped around the equator of the ball and spins at high speed, powered by an induction motor (windings green, rotor yellow). If the gyroscope frame is tilted, the rotor will stay in its orientation. The resulting change in angle between the frame and the rotor is detected by sensitive capacitive pickups (purple). The gyroscope is sensitive to tilt in two axes: left-right, and front-back. Since nothing touches the rotor except the thin layer of gas around the ball, the influence of friction is minimal.

A gas-bearing gyroscope. Based on patent 3,025,708.

A gas-bearing gyroscope. Based on patent 3,025,708.

A gas-bearing gyroscope has the problem that when it starts or stops, the gas layer dissipates, allowing the rotor and the bearing to rub. The Minuteman missile's guidance system was kept continuously running, so starts and stops were infrequent. Moreover, when the gyroscope did need to be started, the electronics gave it a 40-volt jolt to get it up to speed quickly. Because the Minuteman's guidance system was always running—and its solid-fuel engines didn't require fueling—the missile could be launched in under a minute.

To summarize the guidance trajectory, a Minuteman flight is typically about 35 minutes,8 but only the first few minutes are powered by the rockets; the warheads coast most of the way on a ballistic trajectory. The first three rocket stages are active for just 180 seconds; this completed the boost phase for Minuteman I and II. However, the innovation of Minuteman III was that it held three warheads, a system called MIRV (Multiple Independently-targeted Reentry Vehicles). To direct these warheads to their targets, Minuteman III has a fourth stage, called PSRE (Propulsion System Rocket Engine), mounted just below the guidance system. The PSRE was active for 440 seconds, directing each warhead on its specific path. (Meanwhile, a retro-rocket sent the third stage in a random direction. Otherwise, it would tag along with the warheads, acting as a giant radar beacon for enemy anti-ballistic-missile systems.) The warheads travel very high, typically over 800 nautical miles (1500 km), more than three times the altitude of the International Space Station. As for the multiple-warhead MIRV, the Minuteman III missiles were converted back to single warheads as part of the New START arms reduction treaty, with the last MIRV removed in June 2014.

A MIRV configuration with three W78 warheads on the Minuteman III MK-12A reentry vehicle system. The conical reentry vehicles are smaller than you might expect, just under 6 feet tall (181 cm). In comparison, the Titan II had a reentry vehicle that was 14 feet long (4.3 m), holding a massive 9-megaton warhead. Photo from GAO-21-210.

A MIRV configuration with three W78 warheads on the Minuteman III MK-12A reentry vehicle system. The conical reentry vehicles are smaller than you might expect, just under 6 feet tall (181 cm). In comparison, the Titan II had a reentry vehicle that was 14 feet long (4.3 m), holding a massive 9-megaton warhead. Photo from GAO-21-210.

The Minuteman D-17B computer

The guidance computer has a key role in the Minuteman missile, determining the missile's position from the stable platform data, executing a guidance algorithm, and steering the missile on the desired trajectory. Before explaining the D-37 computer used in Minuteman II and III, I'll start by discussing the D-17B computer used in the first Minuteman, since its characteristics strongly influenced the later computers. The Minuteman I computer was very primitive by modern standards. Although it was a 24-bit machine, it was a serial computer, operating on one bit at a time. The big advantage of serial processing is that it dramatically reduces the hardware requirements. Since the computer only processes one bit at a time, it uses a one-bit ALU. Moreover, the buses and datapaths are one bit wide rather than 24 bits. The disadvantage, of course, is that a serial computer is slow; the D-17B took 27 clock cycles (24 bits and three overhead) to perform any operation. At best, the computer could perform 12,800 additions per second.

The computer has an unusual cylindrical structure, 29 inches (74 cm) in diameter, designed to fit the diameter of the Minuteman missile. The computer itself is the bottom half of the cylindrical shell. The top half is the electronic equipment chassis, holding the power supplies for the computer and the stable platform, as well as servo control amplifiers, oscillators, and converters.

The Minuteman I guidance computer. The computer itself is the bottom half of the cylinder, with the disk drive in the 4 o'clock position. The upper half is electronics to drive the IMU and rocket. The IMU itself would be mounted in the center. Photo by Steve Jurvetson, CC BY 2.0.

The Minuteman I guidance computer. The computer itself is the bottom half of the cylinder, with the disk drive in the 4 o'clock position. The upper half is electronics to drive the IMU and rocket. The IMU itself would be mounted in the center. Photo by Steve Jurvetson, CC BY 2.0.

The computer doesn't have any RAM. Instead, all instructions, data, and registers are stored on a hard disk, but not like a modern hard disk. The disk has separate, fixed heads for each track so it can access tracks without seeking. (This approach is similar to a computer built around drum memory, except the drum is flattened.) In total, the disk held just 2727 24-bit words (approximately 8 Kbytes). The computer's serial processing and its disk-based storage worked well together. The disk provided data one bit at a time, which the computer would process serially. The results were written back to the disk, one bit at a time as calculation proceeded. The write head was positioned just behind the read head so a value could be overwritten as it was computed.

The photo below shows the numerous read and write heads for the D-17B's hard disk. Note that the heads are fixed (unlike modern hard drives), and the heads are widely distributed across the surface. (There is no need for different tracks to be aligned.) I believe that the green and white heads in pairs are for the "regular" tracks, while the heads with other spacings implement registers and short-term storage called loops.9

Disk head assembly from the D-17B. Photo by LaserSam, CC BY-SA 40.

Disk head assembly from the D-17B. Photo by LaserSam, CC BY-SA 40.

The D-17B computer was transistorized. The photo below shows one of its circuit boards, crammed with transistors (the black cylinders), resistors, diodes, and other components. (This board is a read amplifier, amplifying the signals from the hard disk.) The computer used diode-resistor logic and diode-transistor logic to minimize the number of transistors; as a result, it used 6282 diodes and 5094 resistors compared to 1521 silicon and germanium transistors (source).

A read amplifier circuit board from the D-17B. Photo from bitsavers.

A read amplifier circuit board from the D-17B. Photo from bitsavers.

The computer supported 39 instructions. Many of the instructions are straightforward: add, subtract, multiply (but no divide), complement, magnitude, AND, left shift, and right shift. The computer handled 24-bit words as well as 11-bit split words, so many of these instructions had "split" versions to operate on a shorter value. One unusual instruction was "split compare and limit", which replaced the accumulator value with a limit value from memory, if the accumulator value exceeded the limit.

The focus of the computer was I/O with 48 digital inputs, 26 incremental inputs, 28 digital outputs, 12 analog voltage outputs, and 3 pulse outputs for gyro control. The computer had special instructions to support the various inputs and outputs.10 For example, to integrate pulse signals from the stable platform, the computer had instructions to enter and exit "Fine Countdown" mode, which caused two special registers to operate as digital integrators, in parallel with regular computation (details).

The D-37 computer

For the Minuteman II missile, Autonetics built the D-37 computer, one of the earliest integrated circuit computers. By using integrated circuits, the guidance computer was dramatically shrunk, increasing range, functionality, and accuracy. The photo below compares the size of the older D-17B computer (half-cylinder) with the D-37B (held by the engineer).

The Minuteman D-17B computer (cylinder) and D-37B computer (being held). From Microcomputer comes off the line, Electronics, Nov 1, 1963. Using modern definitions, the computer was a minicomputer, not a microcomputer.

The Minuteman D-17B computer (cylinder) and D-37B computer (being held). From Microcomputer comes off the line, Electronics, Nov 1, 1963. Using modern definitions, the computer was a minicomputer, not a microcomputer.

Although the main task of the computer is guidance, with the increased capacity of the D-37, the computer took over many of the tasks formerly performed by ground support equipment. The D-37 managed "ground control and checkout, monitoring, communication coding and decoding, as well as the airborne tasks of navigation, guidance, steering, and control" (link).

The D-37 had several models. The D-37A was the prototype system, while the D-37B was deployed in the first 60 Minuteman II missiles. The Air Force soon realized that nuclear radiation posed a threat to the computer, so they developed the radiation-hardened D-37C.11 The Minuteman III used the D-37D, an improved and slightly larger version. Even with additional disk space, program memory was so tight that software features were dropped to save just 47 words.

As far as architecture and performance, the D-37 computer is almost the same as the D-17B, but extended. Most importantly, the D-37 kept the serial architecture of the D-17B, so it had the same slow instruction speed. The D-37 kept the instruction set of the D-17B, with additional instructions such as division, logical OR, bit rotates, and more I/O, giving it 58 instructions versus 39 in the older computer. It expanded the hard disk storage, but with a double-sided disk providing 7222 words of storage in the D37-C.12 The D-37 included division implemented in hardware (which the D-17B didn't have), along with a faster hardware implementation of multiplication, improving the speed of those instructions.13 The D-37C added more I/O lines, as well as radio input and 32 analog voltage inputs.

The diagram below shows the D-37C computer, used in the Minuteman II. At the left is the hard disk that provides the computer's memory. Most of the computer is occupied by complex circuit boards covered with flat-pack integrated circuits. At the right is the advanced switching power supply, generating numerous voltages for the computer (±3, 6, 9, 12, 18, and 24 volts). The connectors at the top provide the interface between the computer and the rest of the system. Because the computer has so many digital (discrete) and analog signals, it uses multiple 61-pin connectors (details).

The D-37C computer. Image courtesy Martin Miller, www.martin-miller.us.

The D-37C computer. Image courtesy Martin Miller, www.martin-miller.us.

The D-37C computer was built from 22 different integrated circuits, custom-built by Texas Instruments for the Minuteman project. These chips ranged from digital functions such as NAND gates and a flip-flop to linear amplifiers to specialized functions such as a demodulator/chopper. Texas Instruments sold the Minuteman series integrated circuits on the open market, but the chips were spectacularly expensive ($55 for a flip-flop, over $500 in current dollars) and not as popular as TI's general-purpose integrated circuits.14 The circuit boards were very complex for the time, with 10 interconnected layers. Each board was about 4 × 5½ inches and held about 150 flatpack integrated circuits, with components on both sides.

The growth of the integrated circuit industry owes a lot to the Minuteman computer and the Apollo Guidance Computer, both developed during the early days of the integrated circuit. These projects bought integrated circuits by the hundreds of thousands, helping the IC industry move from low-volume prototypes to mass-produced commodities, both by providing demand and by motivating companies to fix yield problems. Moreover, both computers required high-reliability integrated circuits, forcing the industry to improve its manufacturing processes. Finally, Minuteman and Apollo gave integrated circuits credibility, showing that ICs were a practical design choice.

The Minuteman III used the D-37D computer, which had about twice the disk capacity, 14,137 words. The layout is similar to the D-37C above, with the disk drive on the left and the power supply on the right. Since the computer is mounted "upside down", the boards are not visible inside, blocked by the interconnect board.15 Note the use of flexible PCBs, advanced technology for the time, soldered with low-melting-point indium/tin solder.

The D-37D computer. Image from National Air and Space Museum.

The D-37D computer. Image from National Air and Space Museum.

By 1970, the D-37 computer had made the cylindrical D-17B obsolete. The government gave away surplus D-17B computers to universities and other organizations for use as general-purpose microcomputers. Dozens of organizations, from Harvard to the Center for Disease Control to Tektronix jumped at the chance to obtain a free computer, even if it was slow and difficult to use, forming a large users group to share programming tips.

The P92 amplifier

The amplifier provides the interface between the computer and the rest of the missile. The amplifier sends control signals to the missile's four stages, controlling the engines and steering. (The electronic circuitry from the Minuteman I's nozzle control units was moved to the amplifier, simplifying maintenance.) Moreover, the Minuteman has explosive ordnance in many places, ranging from small squibs that activate valves to explosives that separate the missile stages. The amplifier sends the high-current (30 amp) signals to detonate the ordnance, while monitoring the current to detect faults.16 The amplifier acts as a safety device for the ordnance, blocking signals unless the amplifier has been armed with the proper code. The amplifier sends control signals to the reentry system (i.e. the warheads) as well as the chaff dispenser, which emits clouds of wires to jam enemy radar. The amplifier also sends and receives signals through the umbilical cable from the ground equipment.

The PS 92A amplifier. Image from National Air and Space Museum. Click this (or any other image) for a higher-resolution version.

The PS 92A amplifier. Image from National Air and Space Museum. Click this (or any other image) for a higher-resolution version.

The photo above shows the amplifier with its cover removed. The amplifier is constructed as two stacks of six circuit boards, on top of a double-width power supply board. At the top and bottom of each board, connectors with thick cables connect the boards to the rest of the system. Each board is a multi-layer printed-circuit board built on a thick magnesium frame for cooling. The amplifier has five power switching boards, a valve driver board, three servo amplifier boards, and an ACTR control board (whatever that is). The system board is visible on the left, with large capacitors and precision 0.01% resistors. To its right is the decoder board, presumably decoding computer commands to select a particular I/O device. Note the extensive use of Texas Instruments flat-pack integrated circuits on this board, the tiny white rectangles.

Missile Guidance Set Control

The Missile Guidance Set Control (MGSC) contains the electronics to power and run the inertial measurement unit (IMU), providing the interface to the computer. The MGSC handles the platform servo loop, accelerometer server loops, gyroscope torquing, gyrocompass torquing and slew, and accelerometer temperature control.17 One unexpected function of the MGSC is powering the computer's hard disk, supplying 400 Hz, 3-phase power at 27.25 volts (source).

The Missile Guidance Set Control with the modules labeled. Original image from National Air and Space Museum.

The Missile Guidance Set Control with the modules labeled. Original image from National Air and Space Museum.

The MGSC is constructed from hinged metal modules, each with a particular function, shown above. The modules are constructed around printed circuit boards. Two large connectors at the right of the MGSC provide electrical connectivity with the IMU and computer. At the top and bottom of the MGSC are connections for coolant. The MGSC is roughly equivalent to the top half of the Minuteman I's cylindrical guidance system, opposite the computer half. The MGSC is unchanged between the Minuteman II and Minuteman III. The MGSC is normally covered with a metal cover that provides radiation protection, but the cover is missing in the photo above.

Battery

The battery in the Minuteman Guidance System is very unusual, since it is a "reserve battery", completely inert until activated. It is a silver/zinc battery with the electrolyte stored separately, giving the battery an essentially infinite shelf life. To power up the battery during a launch, a gas generator inside the battery is ignited by a squib. The gas pressure forces the potassium hydroxide electrolyte out of a tank and into the battery, energizing the battery in under a second. The battery can only be used once, of course, and you can't test it. The battery was built by Delco-Remy (a division of General Motors) (details). It provides 28 volts at 14.5 Amp-hours, powering the guidance system and most of the missile; a separate battery powers the first-stage rocket.

The battery inside the Minuteman III. Original image from National Air and Space Museum.

The battery inside the Minuteman III. Original image from National Air and Space Museum.

The photo above shows the battery mounted inside the guidance system. Note the two thin wires attached to the posts on the left front of the battery to enable the battery, and the thick power wires bolted to the posts on the right. Above these posts is an "electrolyte vent port"; I'm not sure what prevents caustic electrolyte from spraying out under high pressure.

The photo below shows the construction of a Minuteman I battery, similar but with two independent battery blocks. The two round gas generators on the front of the electrolyte tube force the electrolyte into the battery sections.

Inside the remotely-activated SE12G battery. (source)

Inside the remotely-activated SE12G battery. (source)

Squib-activated switch

Another unusual component is the squib-activated switch. This switch is activated by a small explosive squib; when fired, the squib forces the switch to change positions. This switch may seem excessively dramatic, but it has a few advantages over, say, an electromagnetic relay. The squib-activated switch will switch solidly, while the contacts on a relay may "chatter" or bounce before settling into their new positions. An electromagnetic relay may require more current to switch, especially if it has large contacts or many contacts. However, like the battery, the squib-activated switch can only be used once.

The squib-activated switch, next to a coolant line.
The manufacturer of this part is Boeing, as indicated by the Cage Code 94756 on the part.
Image from National Air and Space Museum.

The squib-activated switch, next to a coolant line. The manufacturer of this part is Boeing, as indicated by the Cage Code 94756 on the part. Image from National Air and Space Museum.

The purpose of the switch is to disconnect important signals, known as critical leads, during launch. The Minuteman missile has an umbilical connection that provides power, cooling, and signals while the missile is in the silo. Just before the umbilical cable is disconnected, the switch severs the connections for the master reset signal along with an enable and disable signal. Presumably, these control signals are cleanly disconnected to avoid stray signals or electrical noise that could cause problems when the umbilical connection is pulled off.

The photo below shows the umbilical cable connected to a Minuteman II missile in its silo. Also note the window in the side of the missile to allow the light beam from the autocollimator to reflect off the guidance platform for alignment.

A Minuteman II missile in its silo. Photo by Kelly Michals, CC BY-NC 2.0.

A Minuteman II missile in its silo. Photo by Kelly Michals, CC BY-NC 2.0.

Cooling

The guidance system is water-cooled while in the silo, using a solution of sodium chromate to inhibit corrosion. After launch, the guidance system operated for just a few minutes before releasing the warheads, so it operated without water cooling. (The stable platform has a fan and heat exchanger to keep it cool during flight.) The diagram below highlights the cooling lines. Coolant is provided from the ground support equipment through the umbilical connector in the upper right. It flows through the computer, diode assembly, MGSC, and stable platform. Finally, the coolant exits through the umbilical connector.

Original image from National Air and Space Museum.

Original image from National Air and Space Museum.

Diode assembly

In the middle of the guidance system, the diode assembly consists of seven power diodes. These diodes control the power flow when switching from ground power to battery power. The photo below shows the diode assembly, with coolant connections at the top and bottom. The thick gray wire in the center of the diode assembly receives power from the battery just to the left.

The diode assembly. Image from National Air and Space Museum.

The diode assembly. Image from National Air and Space Museum.

Permutation plug

The Permutation plug (or P-plug) was the key cryptographic element of the guidance system, defining the launch codes for a particular missile. The P-plug looked similar to a hockey puck and plugged into a 55-pin socket attached to the amplifier. The retaining bar held the P-plug in place.

The connector that receives the Permutation plug. Image from National Air and Space Museum.

The connector that receives the Permutation plug. Image from National Air and Space Museum.

Because the security of the missile hinged on the P-plug, the P-plug was handled in a highly ritualized way, transported by a two-person team, an airman and an officer, both armed (source). After the guidance system underwent maintenance, the P-plug team would ensure that the plug was properly installed, just before the missile was bolted back together. There was also a lot of ritual around the disk memory, since it held security codes and targeting information.18 Before anyone could work on the computer, a special team would come to the silo and erase the memory. Afterward, another team would load up the computer from a magnetic tape (in the case of Minuteman III) or punched tape (earlier).19

The missile launch codes are said to be split between the hard disk and the permutation plug. In particular, the missile software holds a two-word code for each of the five launch control facilities.22 The launch code in an Execute Launch Command (ELC) must match the combination of the P-plug value and the site-specific value on disk.23 Thus, the launch code is unique to each launch control site and each missile.24 As another security feature, a launch requires messages from two launch control sites, unless only one was available.25

Transient current detector

A nuclear blast has many bad effects on semiconductors and can cause transient errors. A rather brute-force approach was used to minimize this risk in the D-37C and D-37D computers: if a nuclear blast is detected, the computer stops writing to disk until the burst of radiation passes by. When the radiation level drops, the computer carries on from where it left off, extrapolating to make up for the lost time26 to minimize the error. Since all data is stored on the hard disk, the system doesn't need to worry about memory corruption as could happen with semiconductor RAM.

The Minuteman documents euphemistically refer to "operating in a hostile environment" for the ability to handle large pulses of radiation from a nearby nuclear explosion. Another euphemism is "seismic environment", when a nuclear blast near a silo could disturb the missile's targeting alignment. To get an idea of the expected forces, note that the launch officers were strapped into their seats with four-point harnesses to protect against the seismic environment.27

The Transient Current Detector. Image from National Air and Space Museum.

The Transient Current Detector. Image from National Air and Space Museum.

The "transient current detector" above detects dangerous levels of radiation. I couldn't find any details, but I suspect that it contains a semiconductor and detects transient current through the semiconductor induced by radiation. It would make sense to use a semiconductor similar to the ones in the computer so the detector's response matches the response of the computer, perhaps a matching Texas Instruments IC.

The Minuteman III also has two "field detectors" mounted on the outside of the guidance ring. These presumably detect large fluctuations in the electromagnetic field, indicating an electromagnetic pulse (EMP), different from the ionizing radiation picked up by the Transient Current Detector.

Conclusions

The Minuteman guidance system is full of innovative technologies. Among other things, Minuteman I used an early transistorized computer, and Minuteman II used one of the first integrated circuit computers. The Minuteman missile isn't just something from the past, though. There are currently 400 Minuteman missiles in the United States, ready to launch at a moment's notice and create global devastation. Thus, its technical achievements can't be glorified without reflecting on the negativity of its underlying purpose. On the other hand, Minuteman has succeeded (so far) in its purpose of deterrence, so it can also be viewed in a positive, peacekeeping role. In any case, the Minuteman technology is morally ambiguous, compared to, say, the Apollo Guidance Computer.

I plan to write more about the role of Minuteman and Apollo in the IC industry, so follow me on Mastodon as @[email protected] or RSS for updates. Probably the best overview of Minuteman is Minuteman weapon system history and description. The book Minuteman: A technical history has thorough information. For information on the missile targeting and alignment process, see Association of Air Force Missileers Newsletter, December 2006. The Minuteman guidance system is described in detail in The evolution of Minuteman guidance and control. Much of the imagery in this article is from the National Air and Space Museum. Thanks to Martin Miller for providing a detailed D-37C photo. He has taken amazing photos of nuclear equipment, published in his book Weapons of Mass Destruction: Specters of the Nuclear Age, so check it out.

Notes and references

  1. The Minuteman missile was introduced in 1962, followed by the improved Minuteman II in 1965 and the Minuteman III in 1970. From 1966 to 1985, the US had 1000 Minuteman missiles fielded, but the number has been reduced since then due to various arms control agreements. At present, there are 400 active Minuteman III missiles spread among 450 launch sites. The Minuteman guidance system was updated in the early 2000s to a platform called the NS-50, using a computer based on a MIL-STD-1750A microprocessor. I'm not discussing that system in this post for reasons of space.

    Although the Minuteman has undergone modernization projects, it is reaching the end of its life and is scheduled to be replaced by the Sentinel missile. The Sentinel program is encountering delays and is over budget by 80%, raising the risk of cancellation but the Sentinel program is proceeding as of July 2024. 

  2. Disclaimer: This information is all from published sources. There's nothing secret, and it's mostly obsolete from 60 years ago. I don't have access to a Minuteman system (unlike the Titan), so this post is based on publications and photos, rather than hands-on experience. I've tried to be accurate, but I'm sure there are errors. 

  3. Different guidance algorithms can be used, such as Q-guidance, delta guidance, explicit guidance, and numerical integration; the more advanced algorithms require better computers but provide easier targeting, better accuracy, and more ability to correct for course deviations (see Present and Advanced Guidance Techniques). Q-guidance uses a precomputed "Q matrix" to constantly determine the direction in which velocity needs to be gained, while delta guidance attempts to keep the missile along a precomputed trajectory by using polynomials. In explicit guidance, the equations of motion are solved to determine the steering direction. Minuteman used delta guidance at first, but moved to "hybrid explicit" guidance when the computer became more advanced. See Minuteman: A technical history, page 234 for more on targeting algorithms. 

  4. On Minuteman I, the three stages were steered by changing the direction of the rocket nozzles. Minuteman II, however, used a single fixed nozzle on the second stage but injected fluid into the exhaust to steer the missile, a technique called liquid injection thrust vector control. The Minuteman III used this technique on the third stage as well, injecting a strontium perchlorate solution. (Small nozzles powered by a gas generator are used for roll control, since directing the exhaust won't produce roll motion.) The thrust control liquid was Freon 114B2, which turned out to be harmful to the ozone layer, so it was replaced in the 1990s with perfluorohexane

  5. Strictly speaking, the launch azimuth wasn't aimed at the target. Because the Earth rotated during the missile's flight, the launch azimuth was aimed at where the target would be when the warhead landed. Another factor was the Minuteman I had a limited ability to steer off the launch azimuth, about 10°, allowing the missile to switch between two targets at launch time. 

  6. The Minuteman guidance system is designed to achieve as much accuracy as possible. One problem is that the gyroscopes and accelerometers aren't perfect, but have small errors due to friction and other factors. Moreover, the construction of the stable platform isn't exact; components that should be parallel or perpendicular will have tiny angle errors. To deal with these problems, the missile performs periodic calibrations ranging from some every 15 minutes to some every few months.

    To assist with calibration, the guidance platform contains electrolytic bubble levels, similar to an ordinary carpentry level, but extremely sensitive. Each bubble level contains wires positioned partially in the bubble and partially in the conductive electrolyte fluid. As the bubble shifts, the amount of wire in the fluid changes, changing the measured resistance. These levels are so sensitive that The levels allow the stable platform to be rotated to known positions relative to gravity for calibration.

    The top of the gyrocompass has two mirrors for calibration, allowing the missile platform to rotate exactly 180° relative to the autocollimator. Every 15 minutes, the platform would flip over to measure the gyroscope and accelerometer signals in the opposite orientation. This allowed much better calibration, canceling out errors and improving the missile accuracy. Other calibrations were performed less frequently, such as checking each accelerometer in the up and down positions. Every 90 days, a calibration called PSAT (Perturbation Self-Alignment Technique) pitched the platform by 90° and then slowly rotated the gyrocompass around the vertical to simulate the Earth's rotation (details).

    Another alignment measurement checks the angle between the two mirrors. The two mirrors on the alignment block are supposed to be parallel, but they won't be exactly parallel. The guidance platform periodically rotates the mirror assembly to check one mirror and the other against the autocollimator to compute the angle between the mirrors, called zeta. (See Software Validation Study, page A-94.)

    These calibrations permitted the measurement of small biases and imperfections in the gyroscopes and accelerometers; this data was fed into the guidance calculations to squeeze out as much accuracy as possible. These measurements also provided statistical tracking of the devices so they could be replaced if their performance started to deteriorate. 

  7. Inconveniently, I found contradictory sources about the Minuteman coordinate system. Most sources specify Z as the roll axis, but one detailed paper swaps the X and Z axes, maybe to match simulation software. Examining Figure 5 closely shows that the new axis names were drawn in by hand. 

  8. The flight time of Minuteman depended on the distance and trajectory. The Minuteman's range is said to be 13,000 km. For a closer target, there are two possible trajectories: a high path and a low path. Being direct, the low path could take about 25 minutes, while the high path would reach over 1500 nautical miles (almost 3000 km, seven times the altitude of the ISS) and take 45 minutes. See A simulation of Minuteman Trajectories

  9. The disk holds a timing track, which provides the timing for the computer, giving it a 345.6 kHz clock speed. Note that all operations in the computer are synchronized to the disk, rather than a clock inside the computer. One consequence of this is that the processor speed depends on the disk speed, so it isn't as precise as most computers, which generate the clock from a quartz crystal. The processor timing is very important for a guidance computer, since its calculations of positions depend on the time step. If the processor is running fast or slow, the position will be correspondingly wrong. The solution is that the computer calculates a parameter "tau", the ratio between processor time and wall clock time. The computer receives an interrupt exactly once per second; by counting the number of instructions executed between interrupts, the computer can compute tau and ensure that the calculations are accurate. 

  10. The computer has 8-bit analog-to-digital converters. The D-37C supports 32 analog inputs with a range of +/- 10 volts (source). It also has four digital-to-analog outputs with 8-bit accuracy, also +/- 10 volts.

    In the D-17B, nine analog outputs control the rocket steering, providing roll, pitch, and yaw to the three stages, while three analog outputs go to the stable platform, probably positioning the gimbals. 

  11. The housing for the stable platform provides radiation shielding; it is one of the few parts of the guidance system that is officially secret, but is said to be tantalum sheeting (see Minuteman: A technical history page 224). Although the computer is also said to have radiation shielding, it is curiously not on the secret list. 

  12. Sources give different memory capacities. The reason is that in addition to the regular memory, part of the disk is used for special purposes including registers and rapid access loops. The problem with the regular memory is that the processor may need to wait for an entire disk revolution to access a particular word. The solution is rapid access loops: by putting the write head just upstream of the read head, the data can be accessed more rapidly. For instance, if the write head is positioned one word length upstream, the word can be read (and rewritten) every cycle, providing immediate access to a single word. Putting the write head further upstream allows storage of longer values, with a corresponding longer wait. The D-37C has ten rapid-access channels of one to 16 words (source). The regular memory in the D-37C consists of 56 channels (i.e. tracks) of 128 words, totaling 7168 words. Counting the loops and registers yields the higher memory capacity of 7222 words. 

  13. The differences between the D-17B and D-37C instruction sets are described here

  14. The schematic for the Minuteman's flip-flop IC is shown below. This is a complex circuit for the time, with six transistors along with numerous resistors, diodes, and capacitors.

    Flip-flop schematic. From Integrated circuits go operational, Electronics, Feb 15, 1963.

     

  15. The diagram below shows an exploded view of the D-37D computer (rotated 180° from the earlier photo).

    Exploded view of the D-37D computer. Modified and fixed from Minuteman weapon system history and description.

    Exploded view of the D-37D computer. Modified and fixed from Minuteman weapon system history and description.

     

  16. The danger of these explosives is illustrated by a bizarre accident summarized by "The warhead is no longer on top of the missile." At 3:00 pm on December 5, 1964, two airmen were in the missile silo, troubleshooting a fault in the security system. One airman removed a fuse, triggering a loud explosion and the nuclear warhead fell off the missile, falling 75 feet to the floor of the silo. Nobody was injured and the warhead was hoisted out a few days later without incident.

    The problem was that the airmen used an "unauthorized tool" (a screwdriver) to remove the fuse, briefly shorting power to ground. This caused a current on a ground line connected to the missile through an umbilical cable. Inside the missile, the retrorocket for the warhead had an igniter, but a short on its connector caused another connection to ground. This ground went out through a second umbilical, closing the circuit. (Apparently, the resistance between the two grounds was high enough that the path through the two shorts had enough current to ignite the igniter.) The force of the retrorocket flung the warhead off the rocket.

    More details are in this report and this report. (This incident is not the 1980 Damascus Titan incident, where a dropped 8-pound wrench socket led to the explosion of the missile, killing one person and injuring 21 others, while flinging the warhead out of the silo. The very interesting book Command and Control discusses the Damascus incident and other mishaps with nuclear weapons.) 

  17. The functional diagram below shows the interactions between the stable platform and the guidance set. Shaded circuits are mounted on the stable platform, while others are in the control set. This diagram is for the later NS-50 platform, but it should be mostly relevant to the NS-20 used in Minuteman III earlier. At the top are the feedback loops for the PIGA accelerometers (top). The torque motors (TM) in the middle provide feedback through the gimbals for the gyroscopes. Below that, the gyrocompass has a a feedback loop with its internal torquer. The torque motor at the bottom rotates the gyrocompass and mirrors with feedback through the optical resolver.

    Platform Control Functional Diagram. From Technical Reference Handbook, SELECT WS133A, D2-27524-5, Fig. 3-12, page 3-68.

    Platform Control Functional Diagram. From Technical Reference Handbook, SELECT WS133A, D2-27524-5, Fig. 3-12, page 3-68.

     

  18. The Air Force was especially concerned with keeping the targeting information secret; the people launching the missiles had no idea what the targets were. It occurs to me, though, that since the Minuteman I missile had to be physically rotated in its silo to exactly line up with the target, one presumably could draw an azimuth line on the map and know the target was along the line. 

  19. The Minuteman computer has a conditional fill mode, where the computer can't be loaded with a new program unless the first four words match the first four words in memory channel 12. This ensures that the computer can't be loaded with unauthorized software. This four-word code must be different from the P-plug value for two reasons. First, the P-plug value is not allowed to be stored in memory. Second, the filling code is four words, while the P-plug value is two words.

    The P-plug held two hardwired code words that could be read by the processor.20 For security, the two words were not allowed to be in memory (i.e. the hard drive) at the same time. I assume it is called a Permutation Plug for historical reasons; the Saturn V booster used in Apollo used a security plug that provided a permutation of the 21-character code.21 (That is, it mapped 21 inputs to 21 outputs as a permutation.) 

  20. The processor read the P-plug code words by first triggering the discrete output #25 with the DOB 25 instruction (Discrete Output B) and then reading the value (twice for reliability). The process was repeated with output #6. Finally, the discretes were cleared with DOB 0 (reference). 

  21. The Apollo flights used "code plugs" to protect the Range Safety system from unauthorized access, since this system was capable of blowing up the Saturn V rockets (details). Signals were transmitted in a 21-symbol "alphabet" (encoded by 2 tones out of 7). The code plug permuted the 21 symbols in an arbitrary way. This wasn't a lot of security, just a simple substitution cipher, but it was sufficient for its role. A command consisted of 11 characters (9 for the address and 2 for the command), so the odds were low of hitting a valid message by chance. 

  22. One feature of the Minuteman missile is that the missile sites themselves are uncrewed; the missile officers who launch the missiles work remotely, handling multiple missiles to reduce the personnel required. Specifically, each group of 10 missiles (called a "flight") is controlled by an underground launch control center. A squadron consists of 50 missiles. A "wing" is the largest grouping, handling 150 to 200 missiles, and attached to a particular Air Force base. At its peak, Minuteman had 1000 missiles divided among six wings in Missouri, Montana, North Dakota, South Dakota, and Wyoming, with missiles spilling across the Wyoming border into Colorado and Nebraska. 

  23. Information on the launch code mechanism is from Technical Reference Handbook D2-27524-5, "System Engineering Level Evaluation Correction Team, WS133A", chapter 2. 

  24. The Command Signals Decoder provides another layer of security. It is an electromechanical stepping decoder that blocks the first-stage rocket from igniting unless it receives the proper 27-bit code as part of an Enable command. (The Enable command (ENC) happens before the Execute Launch command (ELC); see the state diagram below.) Its operation is murky; my hypothesis is that the decoder acts much like a combination lock, with the 27 code posts raised or lowered by the input bits. If all the posts are in the proper position, the inner wheel is released, allowing it to rotate to the armed position and close the electrical firing circuit for the motor igniters. Specifically, the 27 posts have a high notch on one side and a low notch on the other, so the device is programmed by rotating each pin so the desired notch faces inward. When the device receives code bits, the wheel rotates one position for each bit and a solenoid raises or lowers the pin, depending on if it is a zero or one. If all pins are in the correct positions, the inner wheel can rotate through the notches, but if any pins are incorrect, the inner wheel will bind on that pin. The 27 bits are the "CSD(M) secure code", probably consisting of 24 code bits and three padding bits. Another Command Signals Decoder on the ground "CSD(G)" provides an interlock for ground ordnance.

    The Command Signals Decoder, from Evolution of ordnance subsystems and components design in Air Force strategic missile systems.

    I think there are two motivations behind this complicated device. First, they want an interlock that is mechanical rather than electronic, since an electronic device can be affected unpredictably by radiation, power surges, component failure, programming errors, etc. Second, they want an interlock that physically disconnects the firing circuit so there is no path that can be triggered by stray current, lightning, EMP, etc.

    The Minuteman's P92 amplifier assembly also blocks ordnance unless armed with a code. It's unclear if this is the same enable code as the Comand Signals Decoder or a different code.

    The earlier Titan missile also had a code mechanism to prevent an unauthorized launch by blocking the engine. The Titan had a butterfly valve in the fuel line with a 6-digit code. If you don't enter the right code, the fuel line stays shut and the missile simply can't take off (video). 

  25. A missile launch normally requires an Execute Launch Command (ELC) from two launch control sites, moving the missile to the "Launch in Process" mode. However, that raises the concern that there could only be one surviving site. The solution is that after receiving a single launch command, the missile starts a timer. If the "one-vote launch time" passes uneventfully, the missile is launched. However, another site can cancel a rogue launch during that time by sending an Inhibit Command (INC) message. The sites have a complex system to detect which sites are active and to determine the primary and secondary sites controlling each missile. (This is reminiscent of the Byzantine generals problem.)

    The state machine for Minuteman missile status. From Technical Reference Handbook D2-27524-5, page 2-25.

    The state machine for Minuteman missile status. From Technical Reference Handbook D2-27524-5, page 2-25.

     

  26. After detecting a nuclear blast, the Minuteman computer shuts down for an integral number of disk revolutions. When it comes back up, it double-counts the accelerometer pulses for the same number of disk revolutions to make up for the missed time (see Minuteman: A technical history pages 220 and 223). As long as not much changed during the lost time, the accuracy loss is small. Of course, this counter would need to be outside the part of the computer that gets shut down. 

  27. Missiles were aligned to such accuracy that even running a diesel generator nearby could shift the silo enough to cause alignment problems, as happened with a Titan site. (See Association of Air Force Missileers Newsletter, March 2007, page 6.) A "seismic event" could also be an earthquake; the enormous 1964 Alaska earthquake—9.2 on the Richter scale—caused Minuteman guidance systems to lose alignment with the autocollimator (See Minuteman: A technical history page 221).