Fixing the Ethernet board from a vintage Xerox Alto

A Xerox Alto system on the East coast had Ethernet problems, so the owner sent me the Ethernet board to diagnose. (After restoring our Alto, we've heard from a couple other Alto owners and try to help them out.) This blog post describes how I repaired the board and explains a bit about how the Alto's groundbreaking Ethernet worked.

The Alto was a revolutionary computer designed at Xerox PARC in 1973, introducing the GUI, high-resolution bitmapped displays and laser printers to the world. But one of its most important contributions was Ethernet, the local area network that is still heavily used today. While modern Ethernets handle up to 100 gigabits per second, the Alto's Ethernet was much slower, just 3 megabits per second over coaxial cable. Even so, the Alto used the Ethernet for file servers, email, network boot, distributed games and even voice over Ethernet.

An extender board made it easy to probe the Ethernet card in the Xerox Alto. The chassis is pulled out to access the boards.  Above the chassis is the disk drive, which stores just 2.5 megabytes on a 14-inch disk pack.

An extender board made it easy to probe the Ethernet card in the Xerox Alto. The chassis is pulled out to access the boards. Above the chassis is the disk drive, which stores just 2.5 megabytes on a 14-inch disk pack.

The Alto's chassis slides out of the cabinet, allowing access to the circuit boards (see above). To test the Ethernet board, I plugged it into the Alto we've restored. The Ethernet board is hanging out the right side of the chassis because I used an extender board.

The board had a couple straightforward problems—a chip with a bent pin (which I straightened) and another chip with broken pins. (While some people recommend re-seating chips on old boards, this can create new problems.) The broken chip was a 74S86, a quadruple XOR gate that has been obsolete for years (if not decades). I replaced it with a 74LS86, a similar but non-obsolete chip.1

When I powered up the Alto, I didn't get anything from the Ethernet board: it was not sending or receiving successfully. This double failure was a bit surprising since malfunctions usually affect just one direction. But I quickly discovered a trivial problem: when I pulled out the Alto's cabinet to access the circuit boards, the Ethernet connector on the back came loose. After plugging the connector in, I saw that the Alto was sending packets successfully, but still wasn't receiving anything.

Oscilloscope trace from the Ethernet board. Yellow is input data, green is R-C filtered, blue is detected edges. The blue trace is a bit of a mess with runt pulses.

Oscilloscope trace from the Ethernet board. Yellow is input data, green is R-C filtered, blue is detected edges. The blue trace is a bit of a mess with runt pulses.

I started probing the Ethernet board's input circuit with the oscilloscope. The board was receiving the input okay, but a few gates later the signals looked kind of sketchy, as you can see above. The yellow trace is the input Ethernet signal. The board applies an R-C filter (green) and then the signal edges are detected (blue). While the input (yellow) is a clean signal, the green signals only go up to about 2 volts, not the expected 4-5 volts. Even worse, the blue waveform is irregular and has "runt" pulses—short irregular signals that don't go all the way up. With waveforms like this, the board wouldn't be able to process the input.

Manchester encoding and the decoding circuit

I'll take a detour to explain the Ethernet's Manchester encoding, before getting into the details of the circuitry.

The Alto's Ethernet encoded data over the coaxial cable using Manchester encoding.32 In this encoding, a 1 bit is sent as a 1 followed by a 0, while a 0 bit is sent as a 0 followed by a 1. The diagram below shows how the bits "0110" would be encoded. A key reason to use this encoding is that simplifies timing on the receiver, since it is self-clocking. Note that there is a transition in the middle of each cell that can be used as a clock. (The obvious scheme of just sending the bits directly has the problem that in a long string of 0's, it's hard to know exactly how many were transmitted.)

An example of Manchester encoding, for the bits 0110.

An example of Manchester encoding, for the bits 0110.

To decode the Ethernet signal arriving at the Alto, the first step is to extract the clock signal that indicates the timing of the bits. To extract the clock, the transitions in the middle of each bit cell must be detected. The trick is to ignore transitions at the edges of cells. The diagram below shows how the clock is extracted. First, edge pulses are generated for each transition edge of the input. Then, a clock pulse is generated for each edge pulse. The clock pulse about 75% of the width of the bit cell, and any second pulse that happens while the clock is still high is blocked. (The edge circled in red is an example of an ignored edge.) The result is the steady clock signal, synchronized to the input.4 This clock is used to latch the input signal value into a shift register, which converts the serial Ethernet stream into a word value usable by the Alto.

Extracting the clock signal from a Manchester-encoded Ethernet input. Edges are extracted from the input signal and used to trigger the clock. An edge too close to the previous (red) is dropped.

Extracting the clock signal from a Manchester-encoded Ethernet input. Edges are extracted from the input signal and used to trigger the clock. An edge too close to the previous (red) is dropped.

The schematic5 below shows the circuit that performs this clock extraction. There are a few tricks in this interesting semi-analog circuit. When the Ethernet input signal changes, the low-pass R-C (resistor capacitor) filter output will be slower to change. This will cause the edge detect XOR gate to go high briefly until the R-C signal catches up. Then the clock is generated by a "one-shot", a chip that generates a fixed-width pulse for each input pulse. The pulse width is set to about 75% of the bit cell width by another resistor and capacitor. The one-shot is wired to ignore any additional pulses that occur while the clock output is high.

Schematic of the clock extraction circuit in the Xerox Alto's Ethernet interface.

Schematic of the clock extraction circuit in the Xerox Alto's Ethernet interface.

Since the timing in this circuit was controlled by multiple resistors and capacitors, I thought that a capacitor might have gone out of tolerance. But first I decided to do the simpler test of swapping the XOR chip with the one from our working Alto board. Surprisingly, this fixed the problem, and the Ethernet board now functioned flawlessly. Thus, while the 74LS86 has input characteristics similar to the 74S86, they weren't close enough, and substituting the more modern chip messed up the R-C filter's behavior. (If the gate draws too much input current, the capacitor won't charge as fast.) Marc dug through is collection of old chips and found another 74S86 so we could get both boards operating.

Oscilloscope trace showing the extracted clock (pink), edges of the Ethernet input (yellow) and the R-C filtered input (green).

Oscilloscope trace showing the extracted clock (pink), edges of the Ethernet input (yellow) and the R-C filtered input (green).

The oscilloscope trace above shows the nice, steady clock (pink) after replacing the XOR chip. With the right XOR chip, the RC-filtered signal (green) rises to 4 volts, unlike before when it just went to 2 volts. The edge signal (yellow) is nice and even, without the "runt pulses" seen earlier.

An overview of the Ethernet board

I'll wrap up by explaining a bit about the Alto's Ethernet board (shown in the photo below). The large connector at the left plugs into the Alto's backplane, providing communication with the rest of the system. The small connector at the right connects to the Ethernet, via a transceiver. Some of the key components of the Ethernet interface are labeled on the diagram below. With this generation of chips, even a whole board of chips doesn't give you a lot of functionality so much of the Ethernet functionality is implemented in software and microcode.

The Xerox Alto's Ethernet board, showing major functional blocks.

The Xerox Alto's Ethernet board, showing major functional blocks.

Most of the chips are simple TTL logic, but the board uses a few interesting and obscure chips. Input and output data goes through a 16-word FIFO buffer built from four Intel 3101A RAM chips; each one holds just 16×4 bits. (The 3101 was Intel's first product.) Commands to the Ethernet board are decoded by three Intel 3205 decoder chips. Buffer chips interface between the card and the Alto's bus. Shift registers for the input and output convert between data words and serial data over the Ethernet. The CRC error checking is done by a specialized Fairchild 9401 CRC chip. Surprisingly, the Manchester encoding is implemented with a PROM-driven state machine, built from two Intel 3601 PROMs. Finally, the XOR chip is the chip that caused all the problems.5

The board has TTL inputs and outputs, so it doesn't connect directly to the Ethernet cable. Instead, a transceiver box is the interface between the Alto and the cable. In the photo below, you can see the transceiver clamped onto the coaxial cable. The transceiver converts the Alto's logic signals to the signals on the cable and injects the signals through an isolation transformer. In addition, the transceiver detects collisions if the Alto tries to send at the same time as another machine on the network.

The transceiver injects the Xerox Alto's signals into the Ethernet coaxial cable and reads signals from the cable.

The transceiver injects the Xerox Alto's signals into the Ethernet coaxial cable and reads signals from the cable.

You might wonder how the Alto can communicate with a modern system, since the Alto's 3 Mb/s Ethernet isn't compatible with modern Ethernet. I am building a gateway based on the BeagleBone (below) that uses the BeagleBone's microcontroller (the PRU) to "bit bang" the Ethernet signals. The BeagleBone also runs an implementation of IFS (Interim File System), the server side software to support the Alto. This is a C# implementation of IFS written by the Living Computers Museum.

I'm building an interface to the Alto's 3Mb/s Ethernet, using a BeagleBone. The cable on the right goes to the Alto's Ethernet port, while a standard Ethernet plugs into the left.

I'm building an interface to the Alto's 3Mb/s Ethernet, using a BeagleBone. The cable on the right goes to the Alto's Ethernet port, while a standard Ethernet plugs into the left.

Conclusion

Bob Metcalfe invented Ethernet at Xerox PARC between 1973 and 1974, with a 3 megabit per second Ethernet for the Alto. Since then, Ethernet has become a ubiquitous standard, and has increased in speed more than 4 orders of magnitude. Unfortunately, as with many other technologies, Xerox failed to take advantage of Ethernet as a product. Instead, Bob Metcalfe left Xerox in 1979 and formed the company 3Com, which sold Ethernet products and became one of the most successful technology firms of the 1990s.

I tracked down a problem in an Alto Ethernet board—my replacement for a broken chip was almost the same, but incompatible enough to make the circuit fail. The problem showed up in the circuit that extracted the clock from the incoming Ethernet signal. This circuit illustrates some clever techniques used by the Alto designers to implement Ethernet with simple TTL chips.

The Ethernet card plugged into the Xerox Alto's chassis. The disk drive is above the chassis, with a white 14" disk pack visible inside.

The Ethernet card plugged into the Xerox Alto's chassis. The disk drive is above the chassis, with a white 14" disk pack visible inside.

The Alto I've been restoring came from YCombinator; the restoration team includes Marc Verdiell, Carl Claunch and Luca Severini. My full set of Alto posts is here and Marc's extensive videos of the restoration are here. Thanks to the Living Computers Museum+Labs for the extender card and the IFS software.

You can follow me on Twitter here for updates on the restoration.

Notes and references

  1. The 7400 family of TTL chips has many subfamilies with different characteristics. The 74S components use Schottky diodes for faster performance than standard TTL. The 74LS (low-power Schottky) chips use much less power (1/8 in this case), but are slightly slower and draw slightly different input currents. 

  2. Ethernet started with Manchester encoding and continued to use this encoding up to 10 Mb/s speed. However, Manchester encoding is somewhat inefficient since each bit can have two transitions, doubling the bandwidth requirement. 100 Mb/s Ethernet solved this by using MLT-3 encoding, which uses three voltage levels. To avoid the problem of losing the clock on a long string of 0's, each group of 4 bits is sent as a particular 5 bit pattern that eliminates long repetitive sequences. Ethernet gets progressively more complex as speed increases. At 100 Gb/s Ethernet, blocks of 64 bits are encoded with 66 bit patterns and data is split across multiple lanes. 

  3. Manchester encoding was developed for magnetic drum storage for the University of Manchester's Mark 1 computer. The Mark 1 was an influential early computer from 1949, one of the earliest stored-program computers, also introducing index registers and Williams tube storage. 

  4. One potential problem with the clock extraction is if you have a steady stream of bits (1111... or 0000...) the Ethernet signal will be a steady stream of transitions, and you can't tell where to start the clock. The solution is that each Ethernet transmission starts with a sync bit to get the clock started properly. 

  5. The schematics for the Alto's Ethernet board are at Bitsavers. The Ethernet is described in more detail in the Alto Hardware Manual

Identifying the "Early IBM Computer" in a Twitter photo: a 405 Accounting Machine

The photo below of a "woman wiring an early IBM computer" has appeared on Twitter a bunch of times, and it stoked my curiosity: what was the machine in the photo? Was it really an early IBM computer? I was a bit skeptical since the machine in the photo is much smaller than IBM's first room-filling computers, and there aren't any vacuum tubes visible. I investigated this machine and it turned out to be not a computer, but an IBM 405 "Alphabetic Accounting Machine" from 1934, back in the almost forgotten pre-computer age of tabulating machines.1

A common photo on Twitter shows a woman wiring an early IBM computer.

A common photo on Twitter shows a woman wiring an early IBM computer.

The image is from photographer Berenice Abbott who took many scientific photographs from 1939 to 1958. She photographed everything from basic magnetic field and optics experiments to research on early television tubes, and many of these photos were used in physics textbooks. Her photos (including IBM photos) were published in the book Documenting Science. I had hoped that the book would identify the computer in the photo, but it merely said "IBM Laboratory: Wiring an early IBM computer". Surprisingly for an art book, it didn't even give a date for the photo.

The diagram below shows the back view of an IBM 403 accounting machine, which IBM introduced in 1948.2 (An accounting machine (also called a tabulator) summed up and printed records from punched cards for applications such as payroll or inventory.) Note the similarities with the Abbott photo: the thick laced wire bundles, the vertical wire bundles in the middle for the counters, and the hinged doors that swing open.

Back view of an IBM 403 accounting machine. From the IBM 402/403 Field Engineering Manual.

Back view of an IBM 403 accounting machine. From the IBM 402/403 Field Engineering Manual.

A second Berenice Abbott photo shows the machine from a slightly different angle.3 The "Card Feed Circuit Breaker Unit" in the upper right looks like a perfect match between the IBM 403 and the Abbott photo. The dangling cables from the counters in the middle look right, as well as the thick cable between the counters and the circuit breaker unit. The 403 diagram above shows a large printing carriage on top, while the Abbott photo just shows a base, presumably because the carriage hadn't been installed yet.

A second photo of "Woman wiring an early IBM computer" by Berenice Abbott.

A second photo of "Woman wiring an early IBM computer" by Berenice Abbott.

Although the machine in the Abbott photo looks very similar to the IBM 403, there are a few differences if you look carefully. One clear difference is the IBM 403 had caster wheels attached directly to the frame, while the machine in the photos has stubby curved legs. In addition, the doors of the IBM 403 were hinged at a different place. In the Abbott photos, the doors are attached just to the left of the counters and to the right of the card feed circuit breaker unit. But the IBM 403 has some bulky components to the left of the counters such as the "Bijur pump" (an oil pump), and components on the right such as the drive motor. Overall, the machine in the Abbott photos has a narrower cabinet than the IBM 403. Additionally, the thick cable snaking down between the IBM 403's circuit breaker units appears to go straight down in the photos. Thus, although the machine in the photos is very similar to the IBM 403, it's not an exact match.

After more research into IBM's various accounting machines, I conclude that machine in the photos is the IBM 405, an IBM accounting machine introduced in 1934 (earlier than that 1948 IBM 403 despite the larger model number).4 The IBM 405 (below) had curved legs that match the Abbott photos. In addition, the 405 has a narrower main cabinet than the 403, with bulky additional components attached to the left and right, outside the legs. This matches the narrower cabinet in the Abbott photos. (The 403 was an improved and modernized 405, explaining the overall similarity between the two machines.)

An IBM 405 accounting machine. Photo courtesy of Columbia University Computing History.

An IBM 405 accounting machine. Photo courtesy of Columbia University Computing History.

Punched cards were a key part of data processing from 1890 until the 1970s, used for accounting, inventory, payroll and many other tasks. Typically, each 80-column punched card held one record, with data stored in fixed fields on the card. The diagram below shows a typical card with columns divided into fields such as date, vendor number, order number and amount. An accounting machine would process these cards: totaling the amounts, and generating a report with subtotals by account and department, as shown below.

Example of a punched card holding a 'unit record', and a report generated from these cards. The accounting machine can group records based on a field to produce subtotals, intermediate totals, and totals. From Manual of Operation.

Example of a punched card holding a 'unit record', and a report generated from these cards. The accounting machine can group records based on a field to produce subtotals, intermediate totals, and totals. From Manual of Operation.

Punched-card data processing was invented by Herman Hollerith for the 1890 US census, which used a simple tabulating machine to count census data, stored on punched cards. Tabulating machines steadily became more complex, becoming feature-laden "accounting machines" that could generate business reports. Businesses made heavy use of these electromechanical accounting machines and by 1944, IBM had 10,000 tabulating and accounting machines in the field.

Accounting machines were "programmed" with a removable plugboard. By switching the plugboard, an accounting machine could be rapidly reconfigured for different tasks. Each wire corresponded to one character or digit. Wires plugged into the plugboard connected columns on the input card to adders. Each column on the printer had an associated wire controlling what got printed. Other wires had control functions. (I explained the tax preparation plugboard below in detail in this article.)

Plugboard to generate a tax report on an IBM 403 accounting machine. Courtesy of Carl Claunch.

Plugboard to generate a tax report on an IBM 403 accounting machine. Courtesy of Carl Claunch.

The IBM 405 was IBM's first "Alphabetic Accounting Machine," able to print text as well as numbers. It had more complexity than you might expect from the 1930s, able to generate three levels of subtotals, intermediate totals, and grand totals. It could process up to 150 cards per minute; that's remarkably fast for an electromechanical system, reading and summing more than 2 cards per second. The 405 was IBM's flagship product for many years, with IBM manufacturing 1500 of them per year. Like most IBM machines, the 405 was usually rented rather than purchased; it cost over $1000 a month (equivalent to about $15,000 per month in 2017 dollars). Renting out these machines (and selling the punch cards) was highly profitable for IBM, with the IBM 405 accounting machine called "the most lucrative of all IBM's mechanical glories".5

Amazingly, although accounting machines were designed for business purposes, they were also used for scientific computation in the 1930s and 1940s, before digital computers existed. They solved everything from differential equations and astronomy computations to nuclear bomb simulations for the Manhattan Project.6

How does an accounting machine work and what are all those parts?

Accounting machines (also called tabulators) were built from electromechanical components, rather than transistors or even vacuum tubes. The main components in accounting machines were electromechanical counters and relays. Everything was synchronized to rotating driveshafts that ran the counters, card reader and printer. In a way, accounting machines were more like cars than computers, with a motor, clutches, driveshafts, an oil pump, gears and cams.

Counters

The heart of the accounting machine was the mechanical counter, a digit wheel somewhat like an odometer. Each wheel stored one digit, with the value indicated by the rotational position of the wheel. To add a number, say 3, to the counter, a clutch was briefly activated, causing the drifeshaft to rotate the counter three more positions. Since these counters were adding 2 1/2 numbers per second, they were spinning rapidly with the clutches engaging and disengaging with precision timing. By combining multiple counters, numbers of up to 8 digits could be handled. The counter kept a running total of the numbers fed into it. Since it accumulated these numbers, the counter was known as an accumulator, a term still used in computing.

A counter unit from an IBM accounting machine. The two wheels held two digits. The electromagnets (white) engaged and disengaged the clutch so the wheel would advance the desired number of positions.

A counter unit from an IBM accounting machine. The two wheels held two digits. The electromagnets (white) engaged and disengaged the clutch so the wheel would advance the desired number of positions.

The photo above shows a board with two counters: the two wheels on the left stored two digits. The counters are more complex than you might expect, with electromechanical circuits to handle carries (including fast carry lookahead). The clutch is underneath the wheel and is engaged by the metal levers in the photo, controlled by electromagnets. A gear underneath the clutch connects the counter to the driveshaft. The electrical connections on the right control the clutch and allow the values from the counters to be read out. Since the IBM 405 had 16 accumulators, with up to 8 digits, many counters were required, resulting in the mass of counter wires in the photos.

Relays

Another key component of the accounting machine was the relay, an electromagnetic switch. The control logic of the accounting machine was implemented with hundreds of relays, which would turn on and off to direct the various components of the accounting machine. Example relay functions are switching on when punched cards are in the input hopper, selecting addition or subtraction for a counter, generating the final total when all cards are done, or printing a credit symbol for a negative balance.

The back of the IBM 403 accounting machine shows numerous relays, used to control the machine.

The back of the IBM 403 accounting machine shows numerous relays, used to control the machine.

The relays were mounted on swing-out panels. The photo above shows an IBM 403 with the panels closed. In the Abbott photos the relay panels are opened and you can see the extensive wiring that connected the relays to the rest of the system.

Circuit breakers

The final component I'll explain is the "circuit breaker," which has nothing to do with the circuit breakers in your house. Instead, these are cam-controlled switches that turned on and off (breaking circuits) as the drive shafts rotated. Dozens of circuit breakers provided the timing signals to the accounting machine, ensuring all operations in the machine were synchronized to the drive shaft. (Every 18° of drive shaft rotation corresponded to reading one row on a punched card, moving one character position on a printer typebar, or advancing a counter wheel by one position.)

Conclusion

The woman in Abbott's photos illustrates the large, but mostly ignored role that women played in electrical manufacturing. Women formed the majority of workers in the 1920s radio manufacturing industry, and their presence in electrical manufacturing increased even more when World War II led many women to take industrial jobs. The famous ENIAC computer (1946) also illustrates this: most of the "wiremen" assembling the ENIAC computer were in fact women, probably recruited from the telephone company or radio assembly.8

The photos also provide a glimpse into the era before digital computers, when businesses used electromechanical accounting machines and tabulators for their data processing. Although you'd expect a machine from 1934 to be primitive, the IBM 405 accounting machine in the photos was an advanced piece of technology for its time, containing 55,000 parts and 75 miles of wire.5 These punched card machines were also capable of performing complex scientific tasks, even contributing to the Manhattan Project. In the 1960s, businesses gradually switched from accounting machines to stored-program business computers such as the IBM 1401. Even so, IBM continued marketing accounting machines until 1976.

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Notes and references

  1. By the "era before computers", I mean before digital electronic computers came along in the 1940s. There's a long history of computing machines, analog computers, and human computers (as shown in Hidden Figures) before digital computers came along. Two interesting books that cover this history are The First Computers—History and Architecture or The Computer: From Pascal to von Neumann

  2. IBM's 402 and 403 accounting machines were the same except the 403 could print three-line addresses. This feature was called MLP (multi-line printing) and was useful for printing addresses on invoices, for instance. So when I refer to the IBM 403 accounting machine, I'm also including the IBM 402. 

  3. I was surprised to realize that there are two different, but nearly identical photos of the woman wiring the IBM machine floating around. In first photo, the woman's right leg is straight, there's a screwdriver in front of the bench and she's wiring the left side of the machine. In the second, the woman's left leg is straight and she's wiring the middle of the machine. 

  4. I couldn't find any manuals for the IBM 405 or photos of the back of a 405, so I can't totally nail down the identification. There's a possibility that the Abbott photos show an IBM 401 accounting machine (below), which was similar to the 405 but introduced a year earlier. The IBM 401 and IBM 405 both had the same basic shape and arrangement of components. The main difference is 405 had a full cabinet in front while the 401 was empty in the front with a bar bracing the front feet. The Abbott photos seem to show a full cabinet like the 405, rather than the open 401. Also, since the 405 was a much more popular machine than the 401—the 405 was the "flagship of IBM's product line until after World War II"—the photos are most likely the 405.

    IBM 401 accounting machine (1933). Photo courtesy of Computer History Museum.

    IBM 401 accounting machine (1933). Photo courtesy of Computer History Museum.

  5. While I couldn't find detailed manuals for the IBM 405, several sources provide information. One source is Computing before Computers (online) p144. IBM has a FAQ with a short overview. Columbia's Computer History page on the 405 has a longer discussion. Also see IBM's Early Computers, pages 18-22 for information on the IBM 405. Computer: Bit Slices from a Life has some hands-on stories of the IBM 405 (online). Computer: A history of the information machine page 51 has some information on the IBM 405. 

  6. See Punched Card Methods in Scientific Computation, 1940 for details on how accounting machines were used for scientific computation. The book is by W. J. Eckert, confusingly unrelated to the ENIAC's J. Presper Eckert. The use of IBM accounting machine by the Manhattan Project is described by Feynman in Los Alamos from Below, p25; and in this page. The Manhattan Project used a 402 accounting machine, several IBM 601 multiplying punches for multiplication, and other card equipment (reference, with more details in Early Computing at Los Alamos). 

  7. I was unable to find documentation on the IBM 405 specifically, but Bitsavers has manuals for the later 402/403 accounting machines. The operation of accounting machines is discussed in detail in IBM 402, 403 and 419 Accounting Machines: Manual of Operation. For a thorough discussion of how the machine works internally, see IBM 402, 403, 419 Field Engineering Manual of Instruction. For an overview of how plugboard wiring for IBM's works, see IBM Functional Wiring Principles

  8. Women and Electrical and Electronics Manufacturing provides a detailed discussion of the history of women in technological manufacturing in the 20th century. The book ENIAC in Action describes how most of the wiring of the ENIAC was done by women. It also has a detailed discussion of the role of women as programmers for early computers such as the ENIAC.