Reverse-engineering the audio amplifier chip in the Nintendo Game Boy Color

The Nintendo Game Boy Color is a handheld game console that was released in 1998. It uses an audio amplifier chip to drive the internal speaker or stereo headphones. In this blog post, I reverse-engineer this chip from die photos and explain how it works.1 It's essentially three power op-amps with some interesting circuitry inside.

Die photo of the audio amplifier chip in the Nintendo Game Boy Color. Click this (or any other image) for a larger image. Photo courtesy of John McMaster.

The photo above shows the chip's silicon die as it appears under a microscope. The white lines are the chip's metal layer, connecting the components. The silicon itself appears greenish and is underneath the metal. The black circles around the outside are the bond wire connections, where tiny wires connected the silicon die to the chip's package. Regions of the chip are treated (doped) to change the electrical properties of the silicon. The next sections explain how components are created from these different types of silicon.

NPN transistors

The amplifier chip is built from transistors known as NPN and PNP bipolar transistors, different from the low-power MOS transistors used in processors. These transistors have three connections: the emitter, the base, and the collector. The magnified photo below shows one of the transistors as it appears on the chip. The slightly different tints in the silicon indicate regions that have been doped to form N and P regions, with dark lines separating the regions. The bubbly silverish areas are the metal layer of the chip on top of the silicon—these form the wires connecting to the collector, emitter, and base.

An NPN transistor in the amplifier chip. The collector (C), emitter (E), and base (B) are labeled, along with N and P doped silicon.

Underneath the photo is a cross-section drawing illustrating how the transistor is constructed. The emitter (E) wire is connected to N+ silicon. Below that is a P layer connected to the base contact (B). And below that is an N+ layer connected (indirectly) to the collector (C). If you look at the vertical cross-section below the 'E', you can find the N-P-N layers that form the transistor.

The photo below shows one of the large output transistors used to drive the speaker. These transistors must produce a high-current output, so they are much larger than the regular transistors and have a different structure. Note the multiple interlocking "fingers" of the emitter and base, surrounded by the large collector. If you look back at the die photo, you can see two of these transistors filling the upper left part of the die.

A large, high-current NPN output transistor in the chip. The collector (C), base (B) and emitter (E) are labeled.

PNP transistors

The chip also uses PNP transistors, which have an entirely different construction, as shown in the diagram below.2 The PNP transistor has a small square emitter (P-silicon), surrounded by a square base region (N-silicon), which in turn is surrounded by the collector (P-silicon). (The emitter metal covers both the emitter and the base, but is only connected to the emitter.) These regions form a P-N-P sandwich horizontally (laterally), unlike the vertical structure of the NPN transistors. Note that although the base region physically surrounds the emitter, the metal connection to the base is further away; the base signal passes through the N and N+ regions, underneath the collector, to reach the base region.

A PNP transistor in the chip. Connections for the collector (C), emitter (E) and base (B) are labeled, along with N and P doped silicon. The base forms a ring around the emitter, and the collector forms a ring around the base.

How resistors are implemented in silicon

Resistors are an important component of analog chips. The photo below shows a long, zig-zagging resistor, connected to metal wiring at the bottom of the photo. (The resistor passes under the metal layer at several points.) The resistor is formed as a strip of P silicon. The resistance is proportional to the length of the resistor, so large-value resistors have a zig-zag shape to fit in the available space. Because resistors are relatively large and inaccurate, chip designs try to minimize the number of resistors required. Even so, an analog chip like this one requires numerous resistors.

A resistor inside the chip, along with the part number. The resistor is a zig-zagging strip of P silicon between two metal contacts. Parts of other resistors are visible at the left and right.

Capacitors

This chip has three large capacitors, one for each amplifier. The photo below shows one of the capacitors. The capacitors are simply a layer of metal over the underlying silicon, separated by a thin insulating oxide layer. In this chip, capacitors are used to ensure the stability of the amplifiers. Because they are large, the three capacitors are easy to spot in the chip die photo.

A capacitor on the chip.

The chip and the Game Boy Color

The role of the audio chip is to take the sound generated by the CPU and amplify it, either for the internal speaker or for external headphones. The photo below shows how the chip appears on the Game Boy motherboard. It also shows the speaker, headphone jack, and the volume control that adjusts the input levels to the amplifier chip.

The Game Boy Color motherboard with key components labeled. Photo from Evan-Amos.

The chip contains three audio amplifiers: one for the speaker and two for the headphones (because they have left and right channels). The design of these three amplifiers is almost identical, except the speaker amplifier uses larger transistors for more output power. The amplifiers use an op-amp, a type of amplifier that uses negative feedback to control the level of amplification. (The feedback resistors are internal to the chip, but it uses external capacitors for filtering.4)53

IC circuits: The current mirror

There are some subcircuits that are very common in analog ICs, but may seem mysterious at first. The current mirror is one of these. The idea is you start with one known current and then you can "clone" multiple copies of the current with a simple transistor circuit, the current mirror. A common use of a current mirror is to replace resistors. As explained earlier, resistors inside ICs are both inconveniently large and inaccurate. It saves space to use a current mirror instead of a resistor whenever possible. Also, the currents produced by a current mirror are nearly identical, unlike the currents produced by two resistors.

The following circuit shows how a current mirror implemented with PNP transistors.6 A reference current "I" passes through the transistor on the left. (In this case, the current is set by the resistor.) Since all the transistors have the same emitter voltage and base voltage, they source the same current, so the currents through each transistor match the reference current on the left. In this mirror, the three transistors on the right are connected so the total output is 3I. Thus, by using multiple transistors, currents can be generated with precise ratios.

Current mirror circuit. The transistors on the right each copy the current on the left.

Six transistors form a current mirror in the chip.

The photo above shows how that current mirror is implemented on the chip with six PNP transistors. Their bases are all connected (top thin metal strip) as are their emitters (wide central middle strip). The leftmost transistor has its base and collector connected, so it controls the current mirror.

IC component: The differential pair

The second important circuit to understand is the differential pair, the most common two-transistor subcircuit used in analog ICs. 7 The differential pair is the basis of an op-amp: it takes two voltages, computes their difference, and amplifies the result. The schematic below shows a simple differential pair. The resistor at the top provides a fixed current I, which is split between the two input transistors. If the input voltages are equal, the current will be split equally into the two branches (I1 and I2). If one of the input voltages is a bit higher than the other, the corresponding transistor will conduct more current, so one branch gets more current and the other branch gets less. The load resistors at the bottom produce an output voltage depending on the current.

Schematic of a simple differential pair circuit. The current source sends a fixed current I through the differential pair. If the two inputs are equal, the current is split equally.

To improve performance, a differential pair is implemented as shown below. A current mirror at the top provides the fixed current. The two load resistors at the bottom of the differential pair have been replaced by load transistors. The output is taken from one branch of the differential pair and fed into a transistor for more amplification. The output then goes to the amplifier's high-current output stage (not shown). A compensation capacitor stabilizes the circuit.

A differential pair as implemented in the chip.

The diagram below shows the implementation of a differential pair in silicon, corresponding to the schematic above. The circuit has three larger PNP transistors above and three smaller NPN transistors. By following the metal, it can be seen how the circuit corresponds to the schematic.

A differential pair in the headphone amp.

Layout of the chip

The diagram below shows the main functional blocks of the chip. The upper-left part of the chip has the two large driver transistors for the speaker output (one to pull the signal low and the other to pull the signal high). The remaining circuitry for the speaker amplifier includes the differential pair, current mirrors, and other circuits. The headphone amplifier consists of two nearly-identical blocks: one for the left channel and one for the right. The circuitry for the current sources and current mirrors is shared by both headphone channels. The lower-left of the chip contains digital logic to enable the speaker amp or the headphone amp, depending if a headphone is plugged into the jack and depending on the enable pin.

The chip with pins and key functional blocks labeled.

Zooming in on the upper-right corner shows the amplifier circuitry for one of the headphone channels. The input signal goes through the differential stage (discussed earlier) and amplification, before going to the output stage, which consists of multiple transistors. Although the speaker amp uses large output transistors, the headphone amp uses 10 regular transistors in parallel; one set to pull the output high and the second to pull the output low. Resistors are used to generate the negative feedback signals for the amplifier. Note that power and ground use much thicker metal traces to support the necessary current.

The headphone amplifier, right channel.

I created a complete schematic of the chip here. I won't explain it in detail here, since its op-amps use a standard architecture, but I'll point out some highlights.9 The headphone amplifiers and the speaker amplifier have very similar designs, but there are a few differences. Most notably, the speaker transistors are larger because the speaker requires more current: not just the output transistors, but many of the other transistors in the circuit. The current mirrors are also structured slightly differently between the headphone amplifiers and the speaker.8 Unlike many amplifier chips, this chip doesn't appear to have any protection if the output is short-circuited.

Part of the reverse-engineered schematic for the AMP-MGB chip. Click here for the full schematic.

Conclusion

This amplifier chip from 1998 has about 100 transistors and is simple enough that the circuitry can be traced out under a microscope. (In comparison, a Pentium II processor from the same time had 7.5 million transistors.) The chip illustrates important analog design functions such as the differential pair and current mirror, and how they can be combined to build an amplifier. People have reverse-engineered many Nintendo chips to help build Nintendo emulators. I don't think knowing the audio chip circuitry helps with emulation, but it's interesting to see how it is constructed.

I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed. My KiCad files for the schematic are on Github. Thanks to John McMaster for providing the chip photos; his page is here.

Notes and references

1. The audio chip is labeled AMP MGB, presumably for "amplifier, Mini-Game Boy". The part number on the 18-pin chip is IR3R53N.

   

   The IR3R53N chip. Photo courtesy of John McMaster.

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2. On this chip, the NPN transistors and PNP transistors look superficially similar, but the PNP transistors are considerably larger. The PNP transistors can also be distinguished by the wide base ring under the square emitter metal. ↩

3. One interesting thing about the chip is that it has three ground pins (1, 2, and 11), and two power pins (4 and 14). By examining the chip, we can why there are multiple pins. Most of the chip uses the pin 1 ground. The pin 2 ground is used solely for the speaker output transistor. The pin 14 ground is used by the headphone driver circuitry. The separate grounds prevent transients from the high-current output transistors from affecting the rest of the chip. For the power pins, most of the chip uses pin 4, while pin 14 feeds the various current sources. This ensures the current sources remain stable. ↩

4. I believe the three external filter capacitors implement a high-pass filter for each channel. ↩↩

5. The excerpt from the Game Boy Color Schematic below shows how the audio chip is connected. The Game Boy CPU chip provides left and right audio channels to the audio chip inputs (LIN and RIN). The chip provides a single-channel speaker output SPKOUT. It also provides two-channel headphone output: HPLOUT and HPROUT. Each channel has an external capacitor attached for filtering: SPKBC, HPLBC, and HPRBC.4 When headphones are plugged in, this signals the SW pin, causing the chip to switch from the speaker output to the headphone outputs. The SD pin allows the chip to be disabled, but is unused.

   

   Schematic showing the audio chip's role in the Game Boy Color. From Consoles TechWiki.

   On the left, the chip receives the audio inputs from the CPU, via a volume control. On the right, the chip is connected to the speaker and headphone jack. The filter capacitors are also connected on the right. The SW input is connected to a switch in the headphone jack; it is normally grounded, but disconnected when headphones are inserted into the jack. ↩

6. For more information about current mirrors, check Wikipedia or chapter 3 of Designing Analog Chips. ↩

7. According to Analysis and Design of Analog Integrated Circuits differential pairs are "perhaps the most widely used two-transistor subcircuits in monolithic analog circuits" (p214). For more information about differential pairs, see Wikipedia or chapter 4 of Designing Analog Chips. ↩

8. The headphone amp or speaker amp are disabled by shutting down their respective current mirrors. Some of the current mirrors remain partially powered, rather than shutting down completely. ↩

9. The amplifiers use a fairly complex scheme to bias and drive the two output transistors. I'll explain my understanding of it; follow along with the schematic. A standard approach is to use diodes to achieve the biasing. However, this chip uses a complex current mirror setup. Looking at the speaker amplifier circuit, transistor Q128 provides the main amplification. The current sunk by this transistor controls the output. The output pull-up transistor Q126 receives base current from current sources Q118 and Q119. This base current can instead flow through Q124 and Q128 if Q128 is conducting, shutting off Q126. At the same time, if Q128 is conducting, the current through it will be (partially) mirrored by Q122, causing current flow through Q121 to turn on pull-down output transistor Q125. To turn off Q125, this current will flow through Q123 instead. To summarize, if Q128 is conducting, Q125 turns on and the output is pulled low. If Q128 is not conducting, Q126 turns on and the output is pulled high. In between, the output will be linear. (I couldn't find references to this approach anywhere, so please let me know if you have more details about this amplifier configuration.) ↩

A circuit board from the Saturn V rocket, reverse-engineered and explained

In the Apollo Moon missions, the Saturn V rocket was guided by an advanced onboard computer system built by IBM. This system was built from hybrid modules, similar to integrated circuits but containing individual components. I reverse-engineered a circuit board from this system and determined its function: Inside the computer's I/O unit, the board selected different data sources for the computer.

A circuit board from the Saturn V LVDA. (Click this image (or any others) for a larger version.) This board was partially disassembled when I received it and some chips are missing.

This post explains how the board worked, from the tiny silicon dies inside its hybrid modules to the board's circuitry and its wiring in the rocket. This board was first studied by Fran Blanch in The Apollo Saturn V LVDC Project. Then EEVblog made a video about it. Now it's my turn to analyze the board.

The Launch Vehicle Digital Computer (LVDC) and Launch Vehicle Data Adapter (LVDA)

The race to the Moon started on May 25, 1961, when President Kennedy stated that America would land a man on the Moon before the end of the decade. This mission required the three-stage Saturn V rocket, the most powerful rocket ever built. The Saturn V was guided and controlled by the Launch Vehicle Digital Computer (below), from liftoff into Earth orbit, and then on a trajectory towards the Moon.1 In an era when most computers ranged from refrigerator-sized to room-filling, the LVDC was very compact and weighed just 80 pounds since it was mounted inside the rocket. The downside was that it was very slow, performing 12,000 instructions a second.

The LVDC mounted in a support frame for testing. Behind the operator is a test system called ACME (Aerospace Computer Manual Exerciser). The ACME paper tape reader is visible at the back. Photo from IBM.

The LVDC worked in conjunction with the Launch Vehicle Data Adapter (LVDA, below), which provided the input/output functions for the computer. All communication between the computer and the rocket went through the LVDA, which converted the rocket's analog signals and 28-volt control signals to the serial binary data the computer required. The LVDA contained buffers (implemented with glass delay lines) and control registers for its various functions. The LVDA had analog-to-digital converters to read data from the inertial measurement unit's gyroscopes and digital-to-analog converters to provide control signals to the rockets. It also processed telemetry signals that were sent to the ground and received ground-based commands for the computer. Finally, power to the LVDC was provided by redundant switching power supplies in the LVDA.

The Saturn V LVDA was a 176-pound box that provided I/O for the LVDA. It had 21 round connectors for cables to other parts of the rocket. From System Description and Component Data.

Because the LVDA had so many different functions, it was almost twice the size of the LVDC computer. The diagram below shows the circuitry crammed into the 176-pound LVDA.2 It had two sections filled with circuit boards called "pages": the front logic section and the back logic section. (The board I examined was from the front logic section.) The power supplies and filters were in the central section. A methanol coolant solution flowed through channels in the LVDA to keep it cool. The LVDA was wired to the LVDC and other parts of the rocket through the 21 round connectors on the ends.

Exploded diagram of the LVDA, from NASA.

Diode-Transistor Logic

There are many different ways to build logic gates. The LVDC and LVDA used a technique called Diode-Transistor Logic (DTL) that builds a gate from diodes and a transistor. This was more advanced than the Resistor-Transistor Logic (RTL) used by the Apollo Guidance Computer, but inferior to Transistor-Transistor Logic (TTL), which became very popular in the 1970s.

The standard logic gate in the LVDC was an AND-OR-INVERT gate3 that implements a logic function such as (A·B + C·D)'. It gets its name because it ANDs together sets of inputs, ORs them, and finally inverts the results. The AND-OR-INVERT gate was powerful because it could be built with many inputs, e.g. (A·B + C·D·E + F·G·H)'. While the AND-OR-INVERT gate may seem complex, it only required one transistor which was important in an era when every transistor counted.

If you want to understand how the gate works internally, look at the diagram below. It shows a four-input AND-OR-INVERT gate with two AND terms. First consider inputs A and B, which are both set to 1 (high). The pull-up resistor4 pulls the AND value high (red, 1). In comparison, in the lower AND gate, input C is 0, so current flows through input C, pulling the AND value low (blue, 0). Thus, the diodes and the pull-up resistor implement an AND gate. Next, look at the OR stage. Current from the top AND (red) pulls the OR stage high (1). Finally, this current turns the transistor on, pulling the output low (blue, 0) and providing the inversion. If both AND stages were 0, the OR stage wouldn't be pulled high. Instead, the pull-down resistor would pull the OR value low (0), turning off the transistor and causing the output to be pulled high (1).

An AND-OR-INVERT gate computing (A·B + C·D)'. Since inputs A and B are both high, the output is pulled low.

An AND-OR-INVERT gate could be built with more resistors or diodes to provide as many inputs as required, potentially many inputs to each AND, and many blocks ORed together. You might expect that AND-OR-INVERT gate would be implemented on a single chip, but the LVDC used multiple chips for each gate, as will be shown below. Different chips had various combinations of diodes, resistors, and transistors that were wired up in flexible ways to form the desired logic gate.

Unit Logic Devices (ULD)

The LVDC and LVDA were built with an interesting hybrid technology called ULD (Unit Logic Devices).5 Although they superficially resembled integrated circuits, ULD modules contained multiple components. They used simple silicon dies, each implementing just one transistor or two diodes. These dies, along with thick-film printed resistors, were mounted on a .3-inch-square ceramic wafer. These modules were a variant of the SLT (Solid Logic Technology) modules used in IBM's popular S/360 series of computers. IBM started developing SLT modules in 1961, before integrated circuits were commercially viable, and by 1966 IBM produced over 100 million SLT modules a year.

ULD modules were considerably smaller than SLT modules, as shown in the photo below, making them more suitable for a compact space computer. ULD modules used flat-pack ceramic packages instead of SLT's metal cans, and had metal contacts on the upper surface instead of pins. Clips on the circuit board held the ULD module in place and connected with these contacts. The LVDC and LVDA used more than 50 different types of ULDs.

ULD modules (right) are smaller than SLT modules or more modern DIP integrated circuits (left). An SLT module was about 0.5" on a side, while a ULD module was 0.3" on a side and much thinner.

Internally, a ULD module contained up to four tiny square silicon dies. Each die implemented either two diodes or one transistor. The photo below shows the internal components of a ULD module, next to an intact ULD module. On the left, the circuit traces are visible on the ceramic wafer, connected to four tiny square silicon dies. While this looks like a printed circuit board, keep in mind that it is much smaller than a fingernail. Thick-film resistors were printed on the underside of the module, so they are not visible.

A ULD of type "INV" opened to show the four silicon dies inside. The upper-right die is a transistor, while the other three dies are dual diodes. The module was protected by pink silicone, which has been removed to show the circuitry. Photo courtesy of Fran Blanche.

The microscope photo below shows a silicon die from a ULD module that implements two diodes. The die is very small; for comparison, grains of sugar are displayed next to the die. The die had three external connections through copper balls soldered to the three circles. The two lower circles were doped (darker regions) to form the anodes of the two diodes, while the upper circle was the cathode, connected to the substrate. Note that this die is much less complex than even a basic integrated circuit.

Photo of a two-diode silicon die next to sugar crystals. This photo is a composite of top-lighting to show the die details, with back-lighting to show the sugar.

The schematic below shows the circuitry inside the "INV" module shown earlier.7 The left side forms an AND-OR-INVERT gate with a single input. A gate with a single input may seem pointless, but additional AND inputs can be attached to pin 1 and additional OR gates can be attached to pin 3. The right side of the schematic provides components that can be used as additional inputs.

Schematic of the "INV" inverter module. Based on Saturn V Guidance Computer, Semiannual Progress Report, page 2-37. Pins 7 and 14 switched from original, which didn't match the actual circuitry.

The board also uses AND gate modules (types "AA" and "AB"), shown below. Keep in mind that these aren't independent gates, but components that can be wired to an INV chip to provide more AND or OR inputs.6 These modules can be wired up in many flexible ways; there are no specific inputs and outputs. One common configuration is to use half of an AA chip as a three-input AND gate. Part of an AB chip can provide two more inputs if needed.

Internal schematics of the type "AA" and type "AB" AND gates. From Laboratory Maintenance Instructions for LVDA, Vol 1.

The photo below shows the semiconductors (dual diodes) inside an AA gate. You can match up the components with the schematic above if you wish; pins 1 and 5, the common pins, are most interesting. Note that the pin numbering does not match the standard IC scheme.

A ULD of type "AA" opened to show the four silicon dies inside. The four dies are dual diodes with the cathodes connected. Original photo courtesy of Fran Blanche.

The board's circuitry

To determine what the board did, I tediously beeped out the connections between chips with a multimeter to create wiring diagrams. (Shortly after I finished, LVDA manuals with schematics turned up8 making my reverse-engineering effort unnecessary.) The board forms a 7-input multiplexer, selecting one of 7 input lines and storing the value in a latch. With 1960s technology, this simple function required a whole board of chips.

The schematic below is a simplified diagram of the board. At the left, the board receives 7 inputs; six of them are 28-volt signals that need to be buffered to generate logic signals, while the seventh is already a 6-volt logic signal. One of the seven select lines is energized to select the corresponding input, which is then stored in the latch.9 (The main simplification is that there are multiple select lines for each input. The full schematic is in the footnotes.10) When the "reset multiplexer" signal and the "multiplexer address" are energized, the latch is reset.

Simplified schematic of the board. It is a multiplexer that selects one of the six inputs and stores the value in the latch.

While the schematic shows many logic gates, it is implemented with just two AND-OR-INVERT gates. The yellow gates form one large AND-OR-INVERT gate, while the blue gates form a second. (The two yellow OR gates merge into one.) The two gates are implemented across eight chips: two chips of type INV, four AA, and two AB. This illustrates the flexibility and expandability of the AND-OR-INVERT logic model, but it also shows that circuits use many chips. Note that there are only two transistors in the logic circuit (one in each INV chip); almost all of the logic is implemented with diodes.

The buffer circuitry

Of the 26 chips on the board, 18 of them were analog chips that buffered and processed the input signals. The inputs were 28-volt signals, while the logic requires 6-volt signals. Each input (except #7) passes through a "Discrete Interface Circuit" that converts the input to a logic signal. The diagram below shows the circuit, built from chips of types 321, 322, and 323.11 The photos show the contents of each chip. Since the 321 chip only consists of resistors (on the underside), the chip appears empty from the top. The 322 chip contains a single diode, while the 323 chip contains two transistors. (The dies are missing from the 323 photo; they are small squares as in the 322.)

Discrete Input Circuit, type A (DIA). The published "322" pinout is wrong, showing two pins 5. From Laboratory Maintenance Instructions for LVDA, Vol 1, Figure A-15. 321 and 322 photos courtesy of Fran Blanche.

The diagram below summarizes the structure of the board. The eight logic chips in the middle are outlined in green. Each of the six input buffers consists of three chips (321, 322, and 323). The signal flow through these chips is shown with the blue arrows. The board has 35 spots for chips, of which 26 were used. By putting chips in the empty locations, the same circuit board could be reused for slightly different functions.13

The circuit board with input paths in blue and logic circuitry in green. Original photo courtesy of Fran Blanche.

The board's role in the LVDA

This board was part of the multiplexer in an LVDA subsystem called the "System Data Sampler" that selects signals and sends them either to the computer or to the ground for telemetry. The System Data Sampler consists of a multiplexer that selects one of eight signals, and the Serializer-Selector that converts the 14-bit data to serial form. The multiplexer has several data sources: the RCA-110 ground computer that was connected to the rocket before launch;14 the "command receiver" that received computer commands from the ground after the rocket had launched; the "control distributor" box that provided various discrete signals;12 "spare discrete inputs"; feedback from the "switch selector", a relay box that the computer used to control the rocket; telemetry from the Digital Data Acquisition System (DDAS); and real-time data.

Physically, many of these data sources were large boxes in the Instrument Unit. For instance, the "control distributor" was a 35-pound box next to the LVDA, connected by a thick cable. The LVDA's "command receiver" input came from the "command decoder", a 7.5-pound box connected to other boxes that provided radio input and output. Because the LVDA was cabled to many different devices in the Instrumentation Unit, it required 21 connectors.

The locations of the LVDA, LVDC, Command Decoder, and Control Distributor in the Instrument Unit. Also shows the electronic assembly (ST-124-M3) that interfaces the inertial measurement unit to the LVDA. From the Saturn V Flight Manual page 7-8.

The board's physical structure

The circuit boards in the LVDA and LVDC used interesting construction techniques to withstand the high accelerations and vibrations of the rocket and to keep the circuitry cool. The board I examined was damaged and missing its mounting frame but the photo below shows an intact unit called a "page". The page's frame is made from a magnesium-lithium alloy that combines light weight, strength, and good heat transfer properties. Heat from a board flowed through the frame to the LVDA or LVDC's chassis, which was liquid-cooled via methanol flowing through channels drilled in the chassis.

A page including the metal frame. This board implemented voting circuitry in the LDVC. Photo from Dmitris Vitoris via Virtual AGC.

Each page could hold two circuit boards, one on the front and one on the back. The printed circuit board has 12 layers, which is a remarkably high number for the 1960s. (Even in the 1970s, commercial PCBs typically had just two layers.) The page has a 98-pin connector, with 49 connections to each PCB. The two boards were connected by 30 "thru pins" at the top of the board. The top of each board also has 18 test connections; these allowed signals to be probed while the boards were installed. (IBM reused this page construction in its System/4 Pi aerospace computers.15)

The board I examined had been forcibly separated from the other board in the page. The photo below shows the back of the board. The thru-pins are visible at the top; they would have been connected to the other board. At the bottom, the 49 connections from the connector to the missing board are visible. Some of the board's insulation has been removed, showing the 12 vias at each ULD module position. These provide a connection from a chip pin to any of the 12 layers of the circuit board.

Back of the LVDA board. A second board was mounted on this side originally, but has been removed.

Conclusion

This small circuit board illustrates several stories about computing in the 1960s.

The board used hybrid modules rather than still-new integrated circuits. While this technology may seem backward, it was a key to IBM's success with the IBM System/360 line. Introduced almost exactly 56 years ago (April 7, 1964), these computers used hybrid SLT modules with AND-OR-INVERT logic. These computers dominated the market for years, and the System/360 architecture is still supported by IBM's mainframes.

The LVDC and LVDA also led to IBM's System/4 Pi line of aerospace computers, announced in 1967. These computers used the same "page" design and connectors as this board, even though they abandoned ULD modules for flat-pack TTL integrated circuits. The System/4 Pi line of computers evolved into the AP-101S computers used on the Space Shuttle.

Finally, the board shows the remarkable improvements in technology since the 1960s. Each ULD module contained up to 4 transistors, so even a basic circuit like a multiplexer took a whole board of modules. Now, an iPhone processor has over 8 billion transistors. It's amazing that such simple technology was enough to get to the Moon.

I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed. This work builds on Fran Blanche's Apollo Saturn V LVDC Project. Thanks to Fran for providing photos, Ben Krasnow for passing the board along to me, and Mike Stewart for documentation. For more information on the LVDC, see the Virtual AGC project's LVDC page. I recently wrote about the core memory stack in the Saturn V LVDC.

Notes and references

1. The LVDC was one of several computers onboard the Apollo mission. The better-known Apollo Guidance Computer (AGC) guided the spacecraft to the Moon's surface. (I recently helped restore an Apollo Guidance Computer to running condition.) The Command Module had an AGC while the Lunar Module had a second AGC. The Lunar Module also contained the backup Abort Guidance System computer. The LVDC/LVDA was connected to the Flight Control Computer, a 100-pound analog computer mounted in the Instrument Unit.

   

   Multiple computers were onboard an Apollo mission. The Launch Vehicle Data Adapter (LVDA) is discussed in this blog post.

   The LVDA and LVDC were mounted in the rocket's Instrument Unit, a ring between the rocket stages and the payload, the Apollo spacecraft. The Instrument Unit contained the guidance and control systems for the Saturn V rocket as well as extensive telemetry systems sending hundreds of parameters to the ground.

   

   The Saturn V Instrument Unit under construction. The LVDC (Launch Vehicle Digital Computer) and LVDA (Launch Vehicle Data Adapter) are silver boxes. For scale, note the engineer sitting on the left. Photo from NASA.

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2. The detailed block diagram of the LVDA below is from the IBM Study Report. (Click the image for a larger version.) This diagram shows that the LVDA has many different functions, registers, and circuits, with many connections to the LVDC (left) and the Instrument Unit (top and bottom). The board I examined is part of the "Digital Input Multiplexer", highlighted in yellow. Note the various data sources feeding into the multiplexer.

   

   Block diagram from IBM Study Report.

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3. IBM's use of diode-based AND-OR logic goes back to vacuum tube computers from the 1950s. The large 700-series computers primarily used AND-OR diode networks for their logic, with vacuum tubes for amplification instead of transistors. The photo below shows an 8-tube module. Note the large number of diodes (black components with white stripes) in the module below. I think the role of semiconductor diodes is largely ignored in the era of vacuum tube computers. The IBM 709, for instance, used 2000 vacuum tubes and 14,500 diodes in its arithmetic unit.

   

   Tube module from an IBM 700-series computer in the 1950s. Note the many diodes, especially in the lower left.

    ↩

4. One unusual feature of the LVDC's gates is that the pull-up resistor often isn't connected to the positive voltage source, as you'd expect. Instead, it is connected to a clock signal. When the clock is high, the AND gate functions normally, but when the clock is low, the AND gate is disabled. This has two benefits. First, the pull-up acts as an additional input, ANDing the clock into the result. Second, this reduces power consumption, since there is no current through the pull-up resistor when the clock is low. ↩

5. Dr. Wernher von Braun wrote an interesting article about the use of ULD modules for Apollo: Tiny Computers Steer Mightiest Rockets (Popular Science, Oct 1965). ↩

6. The ULD logic chips exist in a liminal space, a transition between individual components and integrated circuits. They are not arbitrary components, but neither are they logic gates with defined functions. Instead, they are sets of components that can be pieced together into gates in flexible ways. ↩

7. While the ULD chips have 14 pins, the numbering doesn't match normal 14-pin integrated circuits. The top contacts are numbered 1 through 7 (left to right), and the bottom contacts are 8 through 14 (left to right). (Note that The Apollo Saturn V LVDC Project does not use the IBM numbering.) In addition, the circuit board can only use 12 of the pins because of the 12 vias at each position; contacts 4 and 11 (the middle ones) are not connected. ↩

8. There is very little documentation available for the LVDC and even less for the LVDA. The Virtual AGC document library is the best source that I found. In particular, the strangely-named "Laboratory Maintenance Instructions for LVDC" volume 1 and volume 2 provide detailed explanations and schematics. The recently-uncovered "Laboratory Maintenance Instructions for LVDA" volume 1 and volume 2 provide similar detail for the LVDA. The System Description and Component Data has photos of the Instrument Unit components and brief descriptions. The Saturn V Flight Manual discusses the LVDC and LVDA at a high level. The IBM Apollo Study Report has more high-level information on the LVDC and LVDA and some nice diagrams. To get more information the LVDC and LVDA, I'll need to visit the US Space and Rocket Center in Huntsville, Alabama, but currently travel is off the table. ↩

9. The latch is a circuit to store a single bit; it is a standard SR NOR latch, built by cross-coupling two NOR gates. ↩

10. The schematic for the board is below. (Click for full-size.) Each box corresponds to a logic element, part of a chip. The top line "A", "I" shows the element type (AND, INVERT) while the bottom line ("A31") shows the chip position on the board. ("NU" indicates "Not Used"; the board is wired with the circuitry but the chip is not installed.) The left side of the schematic is the input buffers, while the right side is the logic.

    

    Schematic of the board. From Laboratory Maintenance Instructions for LVDA, Volume II, page 10-114.

     ↩

11. Most of the chips in the LVDA/LVDC have descriptive alphabetic codes such as INV (invert), DLD (delay line driver), or ED (error detector). However, the analog chips on the board have numbers instead: 321, 322, 323, and 324. It looks like instead of coming up with descriptive names for these chips, they just took the last three digits of the part number, e.g. "323" has part number "6000323". I also noticed that on the 6000322 parts, the last "2" has been retouched on the chips; I'm not sure what significance that has. ↩

12. The "discretes", the binary inputs to the LVDA/LVDC, consisted of high-level signals such as "Liftoff", "S-IB Outboard Engine Out", "S-IVB Engine Manual Cutoff", or "S-IB Stage Separation". I was surprised that the hundreds of measurements throughout the rocket are ignored by the computer; it only cares about the major state transitions such as the engine stopping and a stage separating. (As well as the inertial guidance data, which was key to the computer's navigation.) ↩

13. The board has nine empty positions where modules aren't installed, but these positions are wired into the circuitry. The purpose of this is that the same circuit board can be used for multiple functions based on which chips are installed. Specifically, the multiplexer used 13 boards of which 4 were identical to the one I examined, 8 had a few different chips, and 1 was entirely different. The reason for this is that the multiplexer was 14 bits wide, while the inputs were of varying widths. For instance, there were 8 Discrete Input Spares and 10 Telemetry Scanner bits. Thus, some of the boards didn't use some of the inputs and those chips could be omitted, saving a small amount of weight and cost. The diagram below shows the missing chips that can be added.13

    

    The circuit board with the missing chips filled in. The chip with an X could be replaced by the 321 below it. Original photo courtesy of Fran Blanche.

    The board had two unused inputs; to use these, additional 321/322/323 chips were installed. The board also had one input wired up so it could use either a 324 input chip (as in the board I examined) or a 321 input chip. The 321 chip was used for a discrete input that used standard 28-volt signaling, while the 324 chip was used for a signal that was either grounded or floating. The 324 chip included a diode and pull-up resistors. By putting the necessary chip in the appropriate spot, the same PCB could be used for either type of input.

    Two of the boards included an extra logic gate separate from the multiplexer (the INV and AA chips). These gates generated the signals to switch the command input between the RCA-110 mainframe when on the ground, and the radio command decoder after liftoff. In other words, when the umbilical cable pulled out of the Instrument Unit during launch, the signal ("ICS") from the ground computer was lost. Through these two gates, the multiplexer switched the command input from the ground computer to the command decoder, enabling radio commands for the LVDC. ↩↩

14. The RCA-110A computer that communicated with the rocket was in the mobile launch platform, complete with card reader, keypunch, and line printer. In other words, they were moving a whole computer room on the crawler out to the launch pad, with the rocket mounted on top. (In the photo below, the computer room is at the front left of the blue launch platform, under the launcher-umbilical tower.) It communicated with a second RCA-110A computer in the firing room. For details on the mobile launcher and swing arms, see Apollo Maniacs or the book Rocket Ranch. To summarize the wiring, cables went from the RCA-110A computer room near the rocket nozzles, up the tower and across swing arm 7, through the umbilical panel, and to the LVDA. One bit of these signals went to the multiplexer board I examined.

    

    Apollo 11 Saturn V on the mobile platform, July 1, 1969. Swing arm #7 (marked with arrow) is connected to the Instrument Unit and the top of the S-IVB stage. Photo from NASA.

     ↩

15. IBM's 4 Pi series aerospace computers in the 1960s used the same mechanical board structure as the LVDC, with two multi-layer boards mounted on a "page" mounted in a metal frame. The 4 Pi boards were also double-width or triple-wide compared to the LVDC boards, using two or three of the same 98-pin connections. (Compare the board below with the board that I examined.) The circuitry was entirely different though; the 4 Pi boards used flat-pack TTL integrated circuits instead of ULD modules. The 4 Pi architectures and instruction sets were also entirely different from the LVDC. These early 4 Pi systems were used in aircraft such as the A-7E, F-111 and space missions such as Skylab. The 4 Pi series led to the AP-101 computer used on the Space Shuttle.

    

    An IBM 4 Pi page. From Technical Description of IBM System 4 Pi Computers (1967).

     ↩

Repairing a vintage 40-kilovolt xenon lamp igniter

What do xenon lamps and the invention of radio have in common? The box below is a 1960s German high voltage unit that CuriousMarc obtained as part of an auction. After some research, we determined that it is an Osram1 igniter2, which generates a 40-kilovolt pulse3 to ignite a xenon arc lamp. The unit didn't work, so I opened it up, figured out its circuitry, and fixed it, so we could generate some sparks. The circuit turned out to be very similar to a Tesla coil, although the sparks are much smaller.

The igniter, producing a nice 40 kV spark.

A xenon arc lamp generates light by producing a high-temperature plasma of ionized xenon between two electrodes. It produces bright white light that has a spectrum similar to daylight and is useful for movie projectors, searchlights, and laboratory uses. Although the lamp is powered by a low-voltage, high-current DC power supply, a high-voltage spark is required to start the arc, and that is the role of this 40 kV igniter.

Closeup of a 4 kW Osram xenon arc lamp for a movie theater. Image by Hyperlight, CC BY-SA 2.5.

I searched for information on this ignitor. The only thing I found was a 1964 paper titled A Spectrofluorophosphorimeter that described an experimental setup for measuring fluorescence and phosphorescence spectra. The experiment used a 450-W Osram xenon arc lamp, ignited by a Z2201 igniter, the same as this one. The research was done at SRI (Stanford Research Institute), just a few miles away, so there's a good chance that Marc obtained the exact unit that was used in this research.

The igniter's output is on a cone sticking out of the box. It also has five screw terminals for the 220V input, ballast, and ground. Photo courtesy of Marc Verdiell.

We opened up the unit and I examined the unusual components inside. A large 220V to 7kV transformer is at the right of the photo below. The output transformer is the reddish flat cylinder at the back left; this transformer's output is the connection pillar on the front of the unit. In front of this transformer is a dark yellowish disk, a 1000pF 20kV capacitor. The most unusual component is the ceramic cylinder in the front.

Inside the igniter, showing the transformers, capacitors, and spark gap.

I traced out the circuitry of the unit6. It is a high-voltage circuit that is also sometimes used in Tesla coils (details). The way it works is that the high voltage transformer raises the 220 V input to 7 kV. This charges the high-voltage "tank" capacitor until it has enough voltage to break down the spark gap, causing a spark across it. When the spark gap fires it conducts at low resistance. This creates a high-frequency resonant circuit between the tank capacitor and the output transformer's primary. Energy is transferred to the secondary, at a much higher voltage, producing the 40 kV output. As energy shifts back and forth between the primary and secondary, it is dissipated, until the spark gap stops conducting and the process repeats, thousands of times a second.5

Schematic of a Tesla coil circuit. This is a less popular topology for a Tesla coil, but is the circuit used in the igniter. (The igniter has an output, not a torus, of course.) Schematic from Omegatron.

So where is the spark gap in this unit? It turns out to be the ceramic cylinder. I opened up the cylinder and found a stack of eight metal disks with (maybe) carbon electrodes in the center. The disks are separated by mica washers to leave 0.33 mm gaps between each pair. This forms a series of 7 tiny spark gaps.

The spark gap disassembled, showing the stack of contact disks and mica insulators inside the ceramic tube.

This type of spark gap is known as a "quenched spark gap". Spark gap transmitters were the first form of radio transmitter, used from 1887 to 1920. They used a spark to transmit Morse code via radio waves (details). The quenched spark gap was one type of spark gap used in these transmitters, as shown in the diagram below. By combining multiple small gaps, the quenched spark gap could cool off efficiently.

Diagram of a quenched gap, from Telegraph Office.

Repair

We cautiously hooked the igniter to 220V to test it, but nothing happened. I checked various parts of the circuit and everything seemed fine. In the photo below, notice the pink block at the left that looks like a Lego piece. This is a safety interlock that disconnects the 220 V input if the case is removed; the case has prongs that mesh with the interlock to close the circuit. Eventually, we figured out that the safety interlock had some loose screws that weren't making contact. This was tricky to find because when the case was open, the safety interlock was (of course) open.

Inside the igniter. The output transformer (reddish round unit) is at the top with the yellowish tank capacitor above it. The ceramic spark gap is the cylinder in the middle. The pink Lego-link block is the safety interlock. The HV power transformer is at the bottom (label visible). T.

After tightening all the screws, the igniter worked. Since we didn't have a xenon arc lamp, we used the unit to generate sparks instead. Marc attached a strip of copper to the center output and a white wire to the ground, bending them to form a small gap. He pulsed the power switch to produce brief sparks, as seen in the video below. (Since the text on the unit indicates the unit should be powered for under 0.5 seconds, we kept the sparks brief to prevent overheating.) Although the repair was anticlimactic, at least we got some nice sparks.

Conclusion

Spark gaps generate radio waves across a wide spectrum;5 inventor David Hughes first noticed this interference in 1878. Marconi experimented with spark-gap transmitters in the 1890s, discovering how to transmit telegraph signals across short distances and then between continents. This work won Marconi the Nobel Prize for inventing radio. The CuriousMarc video below explains in more detail how the spark gap generator led to radio. Vacuum tubes made spark-gap transmitters obsolete by the 1920s, but these spark-gap circuits live on, igniting xenon arcs in modern headlights.

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Notes and references

1. You might know Osram as the maker of headlights4 and other lights. The story starts with the Austrian chemist Carl Auer von Welsbach, who discovered four elements as well as inventing the gas mantle (used in Coleman lamps) and the metal flint used in lighters. He registered Osram as a trademark in 1906; the name was a combination of osmium and wolfram (tungsten), two elements he used in incandescent lamp filaments. In 1919, the Osram company was formed in Germany. ↩

2. The document Osram guidelines for control gear and igniters discusses the properties of xenon arc lamps, how to power them, and the characteristics of igniters. ↩

3. The front of the unit is shown below. Siemens-Schuckertweke AG is a German engineering company that I think owned Osram at the time. Under that are the warnings "Vorsicht! Hochspannung" (Danger! High voltage) and a circle labeled "In diesen Zone keine Metallteile" (No metal parts in this zone). At the center of the circled zone is a pillar with a screw terminal; this is the connection for the 40 kV output. At the bottom are connections for 220V / 50 Hz, which can be applied for a maximum of 0.5 s, as well as "zum Vorschaltgerät" (to the ballast).

   

   Front view of the igniter. The black text is hard to read under the brown front.

   The label on the back of the unit (below) says ZX 501, Höchstzulässiger Lampenstrom 25 A (Maximum lamp current 25 A), Zündkreis (Ignition circuit) 220V/50Hz, Zündsp. ca. 40 kV (Ignition voltage approximately 40 kV), OSRAM - Best. - Nr. (Order number) Z2201. "

   ↩

   The label on the back of the unit. Photo courtesy of Marc Verdiell.

4. Xenon headlights are also known as HID (high-intensity discharge) headlights. These headlights produce most of their light from an arc through vaporized metal halides, such as scandium iodide. However, it takes seconds to minutes for the light to heat up enough to vaporize these halides. During this startup time, a xenon arc provides the headlight's illumination. In other words, the xenon arc is just to provide light temporarily until the metal halides kick in. HID headlights require an igniter/ballast circuit to provide the high voltage (25 kV) for ignition and the regulated voltage (e.g. .41A, 85V) to power the light. These automotive circuits use modern switching power supply techniques and are much smaller than our igniter. ↩

5. We measured the output from the igniter and found that it produces 2000-4000 very short spikes a second. The spikes decay very rapidly so they are about 1µs long, and are random noise in the tens of megahertz. This random noise has a very wide bandwidth showing that spark gap generators produce radio noise across a wide spectrum.

   

   Oscilloscope trace pickingup electrical noise from the igniter over the air. Image from CuriousMarc's video.

    ↩↩

6. I traced out the circuitry of the unit and made the rough schematic below. The unlabeled rectangle is the ceramic spark gap cylinder. The circuit is essentially the same as the Tesla coil schematic earlier, except there are two capacitors and an external ballast resistor on the output side to limit current. (We did not use a ballast resistor, but shorted the two connections.)

   

   Schematic of the spark generator.

    ↩