Reverse-engineering the LM185 voltage reference chip and its bandgap reference

Many circuits, such as a computer power supply or a phone charger, require a stable voltage reference, but it's harder than you might expect to keep a voltage stable when the temperature changes. One integrated circuit that does this is the LM185.1 I looked at the die of this chip and found some interesting features. The same silicon die is used for three different integrated circuits, using tiny internal fuses to change its functionality. The chip uses a special circuit called the bandgap reference to keep the voltage stable even if the temperature changes. In this blog post, I'll discuss the circuitry of the LM185 and its implementation in silicon.

Composite die photo of the LM185. Click this (or any other) image for a larger version.

The photo above shows the LM185 die under the microscope, a tiny square of silicon. The underlying silicon is blue-gray, while the metal wiring on top is orangish. Regions of the silicon are doped with various impurities to form the transistors, resistors, and other devices on the chip. The variations in doping are visible as slight color changes in the silicon. At the top is the National Semiconductor logo.

The LM185 is available in three variants. The LM185-ADJ is the adjustable voltage reference. It has three pins: one is a feedback pin that controls the voltage. The LM185-1.2-N is a two-pin device, called a "micropower voltage reference diode". It is similar to a Zener diode providing 1.235V, but with better performance. (Lower power consumption, less noise, and better stability.) Finally, the LM185-2.5-N provides a 2.5V reference. The three variants are based on the same silicon die. The latter two have the feedback wired internally to provide a fixed voltage rather than an adjustable voltage.

The next sections describe how the various components of the chip are fabricated from silicon, and how they appear on the die.

NPN transistors

The photo below shows a closeup of one of the transistors in the LM185. The black lines and slightly different tints in the silicon indicate regions that have been doped to form N and P regions. The whitish areas are the metal layer of the chip on top of the silicon—these form the wires connected to the collector, emitter, and base.

Structure of an NPN transistor on the die. I edited the transistor layout so a cross-section would work.

Underneath the photo is a cross-section drawing illustrating how the transistor is constructed. There's a lot more than just the N-P-N sandwich you see in books, but if you look carefully at the vertical cross-section below the 'E', you can find the N-P-N that forms the transistor. 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).

The output transistor (below) is much larger than the other transistors and has a different structure in order to support the chip's high-current output. It has multiple interlocking "fingers" for the emitter and base, surrounded by the large collector.

A large, high-current NPN output transistor in the LM185 chip. The collector (C), base (B) and emitter (E) are labeled.

PNP transistors

You might expect PNP transistors to be similar to NPN transistors, just swapping the roles of N and P silicon. But for a variety of reasons, PNP transistors have an entirely different construction. They consist of a small circular emitter (P), surrounded by a ring-shaped base (N), which is surrounded by the collector (P). This forms a P-N-P sandwich horizontally (laterally), unlike the vertical structure of the NPN transistors.

The diagram below shows one of the PNP transistors in the LM185, along with a cross-section showing the silicon structure. Note that although the metal contact for the base is on the edge of the transistor, it is electrically connected through the N and N+ regions to its active ring in between the collector and emitter.

A PNP transistor in the LM185 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.

Resistors

Resistors are a key component of analog chips. Unfortunately, resistors in ICs are large and inaccurate; the resistances can vary by 50% from chip to chip. Thus, analog ICs are designed so only the ratio of resistors matters, not the absolute values, since the ratios remain nearly constant. The photo below shows two paralleled resistors. Other resistors have a zig-zag shape to fit a longer resistor into the available space.

A resistor inside the LM185 chip. The resistor is a strip of P silicon between two metal contacts.

Capacitors

A capacitor consists of a metal plate on top of silicon, separated by a thin oxide layer that acts as a dielectric. Capacitors are fairly large on integrated circuits; they are the most visible components on this die. The capacitor below contains multiple circular patterns. These may be doped silicon regions, where the junction between two regions provides additional capacitance.

A capacitor on the die.

Fuses

Fuses allow the circuitry of the chip to be changed after manufacturing. The LM185 uses fuses for two reasons. First, fuses can add or remove resistance, allowing the circuit to be tuned for higher performance. Second, a fuse changes the feedback circuitry between the LM185-1.2-N and LM185-2.5-N variants. (The LM185-ADJ version requires more changes than are supported by fuses, so it needs some changes to the metal layer. For instance, it has three pads connected instead of two.)

A fuse has two metal pads attached. Before the chip is packaged, probes can contact the pads and apply a high current to blow the fuse. The first type of fuse is implemented with a tiny strip of metal that is vaporized to break the circuit, just like a large-scale fuse. The second type of fuse is an "antifuse", which has the opposite behavior: it does not conduct until a high current is applied, at which point it becomes conductive. The antifuse can be built from a Zener diode, and the process of shorting it out is called a "Zener zap". The high current forms metal spikes through the junction, causing it to permanently conduct. The diagram below shows a fuse and an antifuse as they appear on the die.

A fuse and an antifuse on the die (I think). The contacts originally had more metal, but I used acid to clean gunk off the die and it dissolved some of the metal.

IC circuit: 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.

The following circuit shows how a current mirror is implemented with three identical transistors.2 A reference current passes through the transistor on the right. (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 on the left match the reference current.

Current mirror circuit. The currents on the left copy the current on the right.

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 multiple resistors whenever possible. Also, the currents produced by a current mirror are nearly identical, unlike the currents produced by two resistors.

Interactive chip explorer

To illustrate how the components form the chip, the die photo and schematic below are interactive. Click on a component in the die or schematic, and a brief explanation of the component will be displayed.

Click the die or schematic for details...

Because the three variants of the LM185 are slightly different, I had to combine three schematics to form the schematic above. Red components are only in the LM185-ADJ, green components are in the LM185-1.2-N, blue components are in the LM185-2.5-N, and cyan components are in the latter two chips. Note that the primary difference is the feedback circuit, but there are additional differences as well.

How a bandgap reference works

The main problem with producing a stable voltage from an IC is that the chip's parameters change as temperature changes. The bandgap voltage reference is commonly used to create a temperature-independent voltage reference.5 The trick is that it has one voltage that goes down with temperature and another than goes up with temperature. If you combine them correctly, you get a voltage that is stable with temperature.

To create a voltage that goes down with temperature, you can put a constant current through the transistor and look at the voltage between the base and emitter, called V_be. The graph below shows how this voltage drops as the temperature increases. At the left, the line hits the bandgap voltage of silicon, about 1.2 volts; this will be important later.

Vbe vs temperature for a transistor

If you set up a second transistor this way but with a lower current3, you get the same effect but the voltage V_be curve drops faster. This may not seem helpful since we need a voltage that goes up with temperature. But here's the trick: if you subtract the two V_be voltages, the difference increases as temperature increases, since the lines get farther apart. The difference is called ΔV_be. The graph below shows the V_be curves for two different transistors, and you can see how the difference ΔV_be between the curves increases with temperature, even though both curves decrease with temperature.

Voltages in a bandgap reference: Vbe for two transistors as temperature changes.

The final step to a bandgap reference is to combine V_be and ΔV_be in the right ratio so the result is constant with temperature. It turns out that if the values sum to the bandgap voltage of silicon (approximately 1.2 volts), the drop in V_be and the increase in ΔV_be cancel out. In the graph below, adding 10 copies of ΔV_be is the right ratio; the exact ratio depends on the particular transistors. The important thing to notice in the graph below is that as the temperature changes, V_be+nΔV_be remains constant - the top of the blue ΔV_be line remains at the bandgap voltage.

By adding multiples of ΔVbe to Vbe, the bandgap voltage is reached regardless of temperature.

In the LM185, the key transistors are Q10 and Q11, where Q10 has 10 emitters in parallel, so each has 1/10 the current. Thus, if you feed the same current into both transistors, Q10 has a lower V_be voltage than Q11 as described above. Note that Q10 is split in two: one half above Q11 and one half below Q11. This layout minimizes potential error due to a temperature gradient across the die. Half of Q10 will be hotter than Q11 and half will be cooler, so the difference will cancel out.

Transistors Q10 and Q11 are the key to the bandgap reference. Q10 has 10 emitters, so each has 1/10 the current as Q11.

The diagram below shows how the bandgap reference is implemented in the LM185. Transistors Q10 and Q11 have different V_be voltages due to their relative sizes. The difference in these voltages (ΔV_be) is developed across R7. Since the same current flows through R6, R7, and R8, the voltage across R6 will be 4ΔV_be and the voltage across R8 will be 6ΔV_be by Ohm's law. Thus, the combination of R6, R7, and R8 multiply ΔV_be by 11. Meanwhile, Q14 has its own V_be.

The bandgap circuit in the LM185.

Summing the voltages along the right gives V_be + 11ΔV_be, which is designed to match the temperature-stabilized bandgap voltage of 1.2 volts. Thus, the circuit will be balanced4 if the voltage between the feedback input and V+ is 1.2 volts. If the voltage is not 1.2 volts, Q10 and Q11 will pass different amounts of current. Since the current mirror (Q12 and Q13) attempts to feed the same current into Q10 and Q11, any discrepancy will appear as current at the error output. This error current is amplified and controls the output transistor, adjusting the voltage until the feedback voltage is brought back into compliance. Thus, the circuit maintains the desired voltage, stabilized even if the temperature changes.

Conclusion

Well, that turned into a longer blog post than I was expecting. Although the LM185 doesn't contain many components by modern standards, it provides a stable, regulated voltage reference. It has some interesting features such as the use of fuses both to improve performance and to sell variant chips. It also illustrates the principle of the bandgap voltage regulator.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed. Thanks to Mitch Wright for supplying the chip.

Notes and references

1. The LM185, LM285, and LM385 are the same chip, but with different temperature ranges. The LM185 is rated for the military temperature range: -55°C to 125°C. The LM285 is rated for the automotive temperature range: -40°C to 85°C. The LM385 is rated for the standard temperature range: 0°C to 70°C. I believe the chips are identical except for testing. For the purposes of this post, you can treat the three chips as identical. ↩

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

3. When building a bandgap reference, what really matters for V_be is the current density through the transistors - the current divided by the area of the emitter. Decreasing the current through the transistor decreases the current density. The second way to decrease current density is to use a larger transistor with a larger emitter. Often five or ten identical transistors in parallel will be combined to form this large transistor to ensure the large transistor and the small transistor are exactly matched. ↩

4. One tricky thing about the bandgap circuit is that it is implemented "backward", taking the voltage as an input. The chip's block diagram6 shows that the reference generates 1.2 volts and this is compared to the input voltage. But in reality, the input voltage is fed into the bandgap circuit. If the input is 1.2 volts, the circuit is balanced. But if the input is too high or too low, the bandgap circuit will be unbalanced with more current through one transistor than the other. This "error" signal is amplified and used as feedback to adjust the input voltage until it matches 1.2 volts. In other words, there's no 1.2-volt reference inside the chip. Instead, the chip and its external input form a feedback loop that generates 1.2 volts. ↩

5. I've written before about bandgap references, specifically the 7805 voltage regulator and the TL431. ↩

6. The TL431 is a popular voltage reference, used in many power supplies. The main difference is that the LM185 regulates the voltage relative to the positive side, while the TL431 regulates the voltage relative to the negative side.

   

   Comparison of the LM185 and TL431 block diagrams, from the datasheets.

    ↩

Reverse-engineering a mysterious Univac computer board

The IBM 1401 team at the Computer History Museum accumulates a lot of mystery components from donations and other sources. While going through a box, we came across the unusual circuit board below. At first, it looked like an IBM SMS (Standard Modular System) card, the building block of IBM's computers of the late 1950s and early 1960s.1 However, this board is larger, has double-sided wiring, the connector is different, and the labeling is different.2

The circuit board is about 15cm×7.3cm.

I asked around about the board and Robert Garner identified it as from the Univac 1004, a plugboard-controlled data processing system from 1963.4 The Univac 1004 was marketed as a "Card Processor" rather than a computer,3 designed for business applications that read punch cards and producing output, but still required calculation and logical decisions. Typical applications were payroll, inventory, billing, or accounting.

Photo of the Univac 1004. From bitsavers.

The most unusual feature of the Univac 1004 was that it was programmed by a plugboard (below) instead of a stored program. The system was programmed by plugging patch cords into a plugboard to indicate the desired action for each of the 31 program steps. While earlier electromechanical accounting machines used plugboards, they were pretty much obsolete by 1963, so I was a bit surprised to see plugboards still in use.

A plugboard for the Univac 1004. This board was used for payroll consolidation from 1965 to 1972. From Museums Victoria Collections, Copyright Museums Victoria / CC BY 4.0.

The computer's "program" consisted of 31 steps. The operations for each step were specified by plugging wires into the board. For instance, a data field could be moved from a punch card to memory, a value could be added or subtracted, or a line of output could be configured for the printer.5 The system even supported conditional branches. The diagram below shows the structure of the plugboard. The highlighted wire shows a subtraction operation, activated by the wire in the "algebraic minus" position.

Part of a program in the plugboard. Click for a larger version. From the Reference Manual.

The computer had a small memory of 961 6-bit characters. Like most computers of the era, it used magnetic core memory, storing each bit by magnetizing a tiny ferrite ring. Note that since the computer was programmed through a wiring panel, none of the memory was used for program code.

A plane of magnetic core storage, from the Reference Manual.

While the Univac 1004 was primitive for its time compared to even a low-end business computer like the IBM 1401, it had a few advantages. First, it rented for $1900 a month, compared to $2500 a month for the IBM 1401 (about $18,000 vs $23,000 a month in current dollars). Second, the Univac computer was compact (by 1960s standards), weighing 2500 pounds. Finally, many customers found plugboard programming easier than programming with code, both because they were more familiar with it and because it is visual and direct.

The Univac 1004 could be extended with peripherals such as tape drives, a card punch, or disk storage. The photo below shows the Unidisc cartridge, which held one million characters. Although it looks like an absurdly-large floppy disk, it was a removable hard disk.

The Unidisc cartridge is 15¾ inches square and ⅝-inch thick. (source).

Reverse-engineering the board

The function of the board wasn't immediately obvious and we had various theories of what it might do. To find out, I reverse-engineered the board by tracing out the circuitry.6 The board has 32 diodes, which seems like a lot, as well as resistors, transistors, and capacitors. The transistors are not silicon transistors, but germanium PNP transistors.

A closeup of the circuit board showing resistors and diodes.

The board turned out to be a logic board implemented with AND-OR-INVERT logic.7 That is, various inputs are ANDed together, the AND results are then ORed together, and finally the result is inverted. The board is implemented with diode-transistor logic. One layer of diodes implements the AND gates and the second layer of diodes implements the OR gates. Finally, a transistor amplifies the result, inverting it in the process. Diode-transistor logic (DTL) performed better than earlier resistor-transistor logic (RTL), but was soon replaced by transistor-transistor logic.

The diagram below explains how the AND-OR-INVERT logic was implemented. This circuit has four inputs: two AND gates that are then ORed together and inverted. (It's a bit confusing because the circuit uses active-low logic, so the voltage levels are all inverted.) If the AND gates all have a 0 (high) input, a diode in the first stage will conduct and pull the AND node high. This blocks the diodes in the second stage (which have the opposite orientation), so the OR node is also high. In the INVERT stage, the +20V resistor will pull the transistor's base high, which turns it off (since it is PNP). Finally, the -8V resistor will pull the output low (i.e. 1), providing the desired AND-OR-INVERT logic.

The AND-OR-INVERT logic producing a 1 output.

The diagram below shows that if the first AND gate's inputs are 1 (low), the first diodes are blocked, so the -30V resistor pulls the AND node low (1). Now the second-stage diode conducts, pulling the OR node low (1). This allows base current to flow through the PNP transistor, turning it on. This pulls the output high (0). (Note that ground is a high output compared to the low output of -8V.) The gates on the board have more inputs, but use the same principle.

The AND-OR-INVERT logic producing a 0 output.

After tracing out the board's logic, I recognized that it implemented a full adder.8 That is, it adds two input bits along with a carry-in, producing a sum bit and a carry-out. By connecting four full-adders in series, a 4-bit value can be added, allowing one decimal digit to be added. Thus, the computer probably has four one-bit adder boards similar to this, along with circuitry to convert the output from binary to binary-coded decimal.10

The board has a few additional circuits along with the full adder circuit. It includes an inverter circuit. The board also has 4 inputs that are ANDed, subject to the carry value. Finally, the board also has a disable input that blocks the outputs.9 Without knowing more about the circuitry, I can't determine the role of these circuits.

Conclusion

The mystery circuit board turned out to be from the Univac 1004. Although this computer was produced in the 1960s, its technology occupies an interesting location between the electro-mechanical accounting machines of the 1940s and the electronic business computers of the late 1950s. The Univac computer used transistors and core memory, but it kept the earlier plugboard programming of the accounting machines, rather than moving to stored-program computing (introduced in 1948). Even though the Univac 1004 was technologically backward for 1963, businesses flocked to it, making it the second-most popular computer at the time with 3400 installations.4

This shows that progress isn't as linear as you might expect; "obsolete" technologies can continue to thrive long after the introduction of "superior" alternatives such as stored-program computing. Instead, new systems can still be developed with supposedly-obsolete technologies, depending on the tradeoffs involved.

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Notes and references

1. The Computer History Museum links to a similar board. ↩

2. The photo below compares the Univac board to a smaller IBM SMS board.

   

   Comparison with an IBM SMS card.

    ↩

3. The Univac 1004 computer came in two versions. The "80" read standard IBM 80-column punch cards. The "90" read Univac's 90-column cards (details), which held 90 characters per card instead of 80. The 90-column card was introduced in 1930 by Remington Rand. It had round holes instead of IBM's rectangular holes. The card stored two characters per column by using a denser, binary code. Despite the superior capacity of the 90-column card, IBM's 80-column cards dominated the market. (Even IBM couldn't displace the 80-column card, although they tried with the 96-column card that they introduced in 1969.)

   

   A 90-column punch card. From Marcin Wichary, (CC BY 2.0).

    ↩

4. Robert Garner discusses the Univac 1004 briefly in his article on Early Popular Computers. More information is in the 1964 BRL report as well as on Bitsavers. A related board from the Univac 1040/1050 is described here. ↩↩

5. The plugboard supported conditionals and looping, so I think the system was Turing-complete, although you couldn't do a lot in 31 programming steps. You could implement multiplication or division with a short shift and add (or subtract) loop. ↩

6. To reverse-engineer the board, I took photos of both sides, flipped the image of the back in GIMP so the two sides were aligned visually, arranged the components on a schematic in EAGLE, and connected the components to match the circuit board. Then I moved the components around until the layout made sense.

   

   The underside of the circuit board.

   The back of the circuit board is shown above. Note that the edge connectors are completely different on the two sides of the board.

    ↩

7. AND-OR-INVERT logic was also used in the IBM System/360 computers, although it was built from hybrid SLT modules instead of discrete components. ↩

8. I suspected the board was an adder when I saw that it had three inputs and was combining them symmetrically. The full adder is implemented in AND-OR-INVERT logic as follows. If the two bits are A and B and the carry-in is CIN, then a carry-out (COUT) is generated if at least two input bits are set. This is computed by the AND-OR logic "(A and B) or (A and CIN) or (B and CIN)". The sum bit is set if there is a single 1 input or three 1 inputs. The sum bit was computed by "(A and not COUT) or (B and not COUT) or (CIN and not COUT) or (A and B and CIN)" As a result of the AND-OR-INVERT circuit, the output is inverted. The inverter circuit on the board was probably used to un-invert it. ↩

9. The full reverse-engineered schematic is below.

   

   Reverse-engineered schematic of the board. Click for a larger version.

    ↩

10. The computer uses excess-three encoding for digits, adding 3 to the value before converting to binary. For example, 6 is represented as binary 1001. The advantage of this encoding is that flipping the bits yields the 9's-complement decimal value, simplifying subtraction. For example, flipping the bits of 6 yields binary 0110, which is 3 in excess-3 notation. Excess-3 representation also handles carries correctly; if you add two numbers that sum to 10, the excess-3 values will sum to 16, causing a binary carry. To convert the sum to excess-3, The value 3 must be added (if a carry) or subtracted (if no carry).

    To see how addition works with excess-3, 2 + 4 in excess-3 is binary 0101 + 0111 = 1100. Subtracting 3 yields 1001, which is 6 in excess-3. But 2 + 9 is binary 0101 + 1100 = 10001, generating a carry out of the 4 bit value. Adding 3 yields 0100, which is 1 in excess-3. Considering the carry-out, this is the desired result of 11. ↩

Inside the Apple-1's shift-register memory

Apple's first product was the Apple-1 computer, introduced exactly 46 years ago, on April 11, 1976. This early microcomputer used an unusual type of storage for its display: shift register memory. Instead of storing data in RAM (random-access memory), it was stored in a 1024-position shift register. You put a bit into the shift register and 1024 clock cycles later, the bit pops out the other end. In the early days of random-access memory chips, shift-register memory was cheaper so many systems used it.1 The downside, of course, is that you had to use bits as they became available, rather than access arbitrary memory locations.2

Die of the Signetics 2504 shift register chip. Click this image (or any other) for a larger version.

The photo above shows the chip under the microscope. The underlying silicon is grayish, with white metal wiring on top. The thickest metal wiring provides power to the chip. The chip also has wiring and transistors constructed from a type of silicon called polysilicon; the polysilicon appears red in the photo. Most of the die is occupied by the shift register, arranged in rows that snake back and forth. The squares around the edge of the die are bond pads, where bond wires connect the die to the chip's external pins.3

The Apple-1's display

The Apple-1 displayed 24 lines of forty characters on a television monitor. Like most computers at the time, the Apple-1 stored characters rather than pixels to reduce memory requirements. A character-generation ROM converted each character into a 5×7 matrix of pixels as it was displayed. To reduce memory even more, the display didn't store full bytes, but 6-bit characters, supporting upper-case letters, numbers, and some symbols.

The Apple-1 computer was sold as a circuit board. The user had to supply a keyboard, power supply, display, and case. Photo by Cynde Moya, CC BY-SA 4.0.

The six-bit display characters were held in six 1024-bit shift registers. A seventh shift register tracked the cursor position.4 The diagram below shows the shift registers and the clock driver on the Apple-1 circuit board. These chips are in 8-pin packages, so two chips fit into the space of a regular TTL chip.5

Apple-1 circuit board, showing the 1024-bit shift register chips and the clock driver chip. Original image from Achim Baqué, CC BY-SA 4.0.

The image below shows how the 2504 shift register chips are represented on the Apple-1 schematic. The chips use just 6 pins. Each chip has a single connection for bits coming in and a connection for the bits coming out. The remaining pins provide the two clock signals and the ±5 volt power supplies. Unlike RAM chips, these chips do not take an address.

Detail of the Apple-1 schematic showing two of the shift register chips.

PMOS integrated circuits

This shift register chip was created around 1970, an interesting time in the development of MOS integrated circuits. Early integrated circuits used a type of transistor known as bipolar. However, the metal-oxide-semiconductor (MOS) transistor had the potential to make cheaper, high-density integrated circuits. The first commercial MOS integrated circuit was a 20-bit shift register, created in 1964 by a company called General Microelectronics.

The diagram below shows the structure of a MOS transistor. At the bottom is the silicon, which is doped with impurities to form p-type silicon. The two conductive p-type regions are called the transistor's source and drain. The channel acts as a switch between the source and drain, turned on by voltage in the metal gate above. A thin insulating oxide layer separates the metal gate from the underlying silicon. These three layers—metal, oxide, semiconductor— give the MOS transistor its name. In the late 1960s, chips started to use gates made of polysilicon, a special type of silicon that produced better transistors than metal gates. This is the technology used by the 2504 shift register: the "P-MOS silicon gate process".

Structure of a P-type MOSFET.

By the mid-1970s, however, integrated circuits changed in two more ways. First, P-MOS transistors were replaced by N-MOS transistors, which had better performance. Second, the introduction of ion implantation machines allowed transistor characteristics to be adjusted, with "depletion-mode" transistors8 leading to faster, lower-power circuitry. These changes ushered in the age of popular microprocessors such as the Zilog Z80, MOS Technology 6502, and Intel 8085. These had much better performance than earlier PMOS processors such as the Intel 8008.7 The 6502, of course, was the processor in the Apple-1 (and Apple II).

The shift register

Next, I'll look at the details of how the shift register was constructed. The idea of a shift register is that bits are passed from stage to stage, controlled by clock pulses. With 1024 stages, the shift register can hold 1024 bits. Each shift register stage uses two transistors and two inverters as shown below. During the first clock phase, the first transistor turns on, allowing the input bit to pass through it and the first inverter. During the second clock phase, the second transistor turns on, allowing the inverted value to pass through it and the second inverter, producing the output. Thus, a bit takes two clock phases to move through the shift register stage.

In the first clock phase, the input passes through the first transistor. In the second clock phase, the input is held by the gate capacitance and passes to the output.

This circuit is a dynamic shift register, which works due to the circuit's capacitance. When the first transistor turns off, the value remains at the input to the first inverter, held by the capacitance of the circuit. (And likewise for the second transistor.) Because the gate of a MOSFET uses almost no current, the bit value will remain for a couple of milliseconds or so before it drains away. (This is the same principle used by DRAM, holding bits through capacitance.) As long as the clock keeps going, the bit gets refreshed by each stage.

Each inverter is implemented using two MOS transistors. The concept is shown on the left, below. A high input turns on the transistor, which pulls the output low. A low input turns off the transistor allowing the pull-up resistor to pull the output high. Thus, the circuit inverts its input.

Conceptually, the inverter uses the circuit on the left. The implementation uses the circuit on the right.

The circuit is actually implemented with a transistor in place of the resistor, as shown on the right, because transistors are more compact than resistors. A high input to the upper transistor turns it on, causing the pull-up current to flow. In a standard inverter, the transistor would be connected to be always on.9 However, the output of the inverter is only used during one clock phase. To reduce power consumption, the transistor is wired to the clock so it only acts as a pull-up when needed.

A shift-register stage on the die

The diagram below shows how shift-register stages are physically constructed on the die. The first part of the image shows how the circuitry appears under the microscope, a complicated jumble of silicon, polysilicon, and metal circuitry. In the middle, I've highlighted the doped silicon in green and the polysilicon in red. A transistor gate (yellow) is formed where polysilicon crosses silicon, with the source and drain on either side. (The horizontal metal wiring should be clear without highlighting.) Note the complex, optimized shapes of the polysilicon and the transistors. Finally, a black dot indicates a contact that connects two layers. In the bottom half of the image, bits are shifted to the right, while in the top half, bits are shifted to the left.

One stage of the shift register.

In the lower right, one stage of the shift register is represented by a schematic on top of the underlying circuitry. The stage is implemented with six transistors as described earlier. Note that the pull-up transistors to Vdd are long and skinny, reducing their current. The inverter transistors to Vcc, on the other hand, are wide, so they provide a lot of current. The circuitry in the top half of the image is the same, but rotated 180°. Note that the two rows of shift registers share the clock phase lines and Vdd, making the layout more efficient.6

Topology of the chip

You might expect the chip to consist of 1024 shift-register stages arranged into a chain. However, the chip had an unusual topology that allowed it to operate at double speed: one bit per clock phase instead of one bit per complete clock cycle. It accomplished this with a simple trick: it was really two 512-bit shift registers operating in parallel. The first operated on clock phase 1, phase 2, phase 1, ..., while the second was the opposite: phase 2, phase 1, phase 2, ... The result was that one half would produce bits in phase 1, while the other would produce bits in phase 2. The output circuit merged these together into a single output stream. From the outside, it looked like a 1024-bit shift register that operated twice as fast.

Another complication is that Signetics produced three 1024-bit shift register chips from the same silicon layout: the 2502 (organized as four 256-bit shift registers), the 2503 (512×2), and the Apple-1's 2504 (1024×1). The different chips were created by changing the metal wiring of the chip during manufacturing, which was much easier than building completely different chips. To support this, the shift register was broken into eight 128-bit segments, shown below. In the 1024×1 chip, two chains of four 128-bit segments ran in parallel (on opposite clock phases) to produce a 1024-bit shift register. The first chain used the light-colored segments A, B, C, D, while the second chain used the dark-colored segments. The segments are connected by the metal wiring along the side of the die. The chip's pads around the edges are labeled; the grayed-out ones are not used in this chip. The large block of circuitry above the output pin combines the two chains into one output.

The chip consists of 8 shift-register chains, each 128 bits long. They are connected in different ways to form different shift register chips.

The other variants of the chip wire the shift-register segments differently and use additional input and output pins. The 512×2 2503 chip used four chains of 256 bits along with two input and output circuits. The 2502B chip used all eight 128-bit chains in parallel to form a 256×4 shift register, with four input and output circuits.10

The image below shows one of the unconnected outputs: the red polysilicon wire isn't connected at one end. With a small change to the metal layer, the metal wiring between two segments can be broken and the segment wired to this output instead. The other changes between chip versions are similar.

The polysilicon wire in the middle is disconnected.

The clock driver

I'll wrap up with a brief mention of the clock driver chip that drives the shift registers. Shift-register memory chips required clock pulses with high current and unusual voltages due to the PMOS circuitry: from +5 volts to -11 volts. These pulses were provided by a special chip, the DS0025 Two-Phase MOS Clock Driver. The die photo below shows this chip. The die is dominated by four power transistors that produced 1.5 amp pulses. I wrote a blog post about the clock driver chip if you want more details.

Die photo of the DS0025 clock driver chip.

Conclusion

The Apple-1 is now a collector's item, with boards selling for hundreds of thousands of dollars. However, when it was introduced in 1976, it wasn't a particularly important computer, with about 200 sold at the price of $666.66. The Apple II, which came out a year later in 1977, was a much more influential computer, selling millions to become one of the archetypical home computers of that era. The Apple II used RAM chips for all its storage, illustrating that shift-register memory had rapidly become obsolete.

The shift-register chip illustrates the amazing decline in memory prices, as reflected by Moore's Law. This 1-kilobit shift register cost about $60 (in current dollars), while a 16-gigabit DRAM chip now costs about $6. Thus, memory is about 160 million times cheaper now, an amazing drop.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed. Thanks to @TubeTimeUS for supplying the chip. I've written about the Intel 1405 shift register memory if you want to know more about this type of storage.

Notes and references

1. The first reference to the Signetics 2504 that I could find was in 1970, when each chip cost $11.05 in quantities of 100 (about $60 in current dollars). Looking in an old Byte magazine from 1976, a 1-kilobit shift register chip cost $9 ($34 in current dollars), while a 4-kilobit DRAM chip cost $20 ($75 in current dollars). Thus, it appears that even by the time the Apple-1 was released, DRAMs had become cheaper than shift registers.

   The Apple-1 used 4-kilobit RAM for data and program storage. It's possible, though, to build a computer that uses shift-register storage for its main memory. The Datapoint 2200 is one example. If memory is accessed sequentially, shift-register storage is efficient since the bits are provided sequentially. However, if you access memory out of sequence, the processor has to wait while the memory cycles around, until the desired bits become available. In a way, shift-register memory is a throwback to very early computers such as EDSAC (1949), which used mercury delay lines for main storage. ↩

2. The behavior of shift-register memory was a good match for video circuitry, since characters are displayed on the screen in a fixed, repeating order (left to right and top to bottom). The IBM 2260 video display terminal (1965) used a technique similar to shift registers: it stored data in a sonic delay line, sending torsional pulses through a 50-foot nickel wire. But unlike the Apple-1, this delay line stored pixels, not characters. For more about this system, see my blog post. ↩

3. The die was encased in an epoxy package. To expose the die, Eric (@TubeTimeUS) tediously sanded through the plastic package until the die was visible. There are a few scratches on the die from this process, especially in the upper left. ↩

4. The display circuitry has some additional complexity. Characters can't be taken directly from the display shift register: since each character is made up of eight scan lines; a line of character must be processed eight times. To handle this, a second shift register (six 40-bit registers) buffers a line of characters and feeds each character into a display ROM. Another 1024-bit shift register keeps track of the cursor position. For more details, see stackexchange. ↩

5. The Apple-1 display has a lot of similarity with the popular TV Typewriter, a hobbyist video terminal kit from 1973. The TV Typewriter used shift-register memory for its 32×16 display, but had a complex 5-board design. Wozniak's design for the Apple-1 was much simpler. ↩

6. The schematic of the chip is shown below. Notice the upper and lower shift registers, which run on opposite clock phases. Apart from the 6-transistor shift-register stages, the only circuitry is the output stage that merges the two results and drives the output pin.

   

   Schematic of the chip. Click for a larger image. From the 1972 databook.

    ↩

7. Another major improvement in integrated circuits was the introduction of CMOS, which used NMOS and PMOS transistors together, with much lower power consumption. By the 1980s, processors such as the Intel 80386 (1985) and Motorola 68030 (1987) used CMOS. CMOS is still used in modern integrated circuits. ↩

8. In the mid-1970s, ion implantation technology allowed the creation of depletion-mode transistors. These transistors could be used as pull-up elements, called depletion loads. Since depletion-load transistors could operate faster and with less current, they rapidly became a standard part of MOS integrated circuits, until replaced by CMOS in the 1980s. The Zilog Z80 and Intel 2102 SRAM were two early chips that used depletion loads. ↩

9. You might think that the inverter circuit will result in a short circuit between Vdd and Vcc when both the input and the clock are high. However, the pull-up transistor is designed to produce a weak current, so the other transistor can still pull the output low. This current results in relatively high power consumption for PMOS or NMOS circuitry, a problem that is fixed by CMOS. ↩

10. The 256×4 2502B chip required a 16-pin package, rather than the 8-pin package of other chips, due to the additional input and output pins. ↩

Inside the Apple-1's unusual MOS clock driver chip

Apple's first product was the Apple-1 computer, introduced in 1976. This early microcomputer used an unusual type of storage for its display: shift register memory. Instead of storing data in RAM (random-access memory), it was stored in a 1024-position shift register. You put a bit into the shift register and 1024 clock cycles later, the bit pops out the other end. Since a shift-register memory didn't require addressing circuitry, it could be manufactured more cheaply than a random-access memory chip.1 The downside, of course, is that you had to use bits as they became available, rather than access arbitrary memory locations. The behavior of shift-register memory was a good match for video circuitry, though, since characters are displayed on the screen in a fixed, repeating order (left to right and top to bottom).2

The Apple-1 was sold as a bare board, so users needed to make a case for it, or mount it in a briefcase as shown here. Note the cassette drive used for mass storage. Photo cropped from Binarysequence, CC BY-SA 4.0.

Shift-register memory chips required clock pulses with high current and unusual voltages: from +5 volts to -11 volts. These pulses were provided by a special chip, the DS0025 Two-Phase MOS Clock Driver. This chip, introduced in 1969, was the first monolithic (i.e. integrated circuit) clock driver. In this blog post, I look inside the chip and explain how it was implemented.

Die of the DS0025 clock driver. Click this image (or any other) for a larger version.

The photo above shows the silicon die under the microscope. This chip is very simple, containing four large NPN transistors, four diodes, and four resistors. The silicon appears blue-gray in this image, while the metal layer on top appears speckled white. Around the outside of the die, are six dark rectangles, the pads where golden bond wires connected the die to the chip's external pins.

The die was encased in an epoxy package. To expose the die, Eric (@TubeTimeUS) tediously sanded through the plastic package until the die was visible. Some bits of epoxy remained, caught in the bond wires, so I cleaned up the die with a few drops of boiling sulfuric acid.

The Apple-1's display

The Apple-1 displayed 24 lines of forty characters on a television monitor. Like most computers at the time, the Apple-1 stored characters rather than pixels to reduce memory requirements. A character-generation ROM converted each character into a 5×7 matrix of pixels as it was displayed. To reduce memory even more, the display didn't store full bytes, but 6-bit characters, supporting upper-case letters, numbers, and some symbols.

The six-bit display characters were held in six 1024-bit shift registers. A seventh shift register tracked the cursor position.3 The diagram below shows the shift registers and the clock driver on the Apple-1 circuit board. These chips are in 8-pin packages, so two chips fit into the space of a regular TTL chip.

Apple-1 circuit board, showing the 1024-bit shift register chips and the clock driver chip. Original image from Achim Baqué, CC BY-SA 4.0.

Transistors

Next, I'll discuss the components of the chip. Because the chip generates high-current pulses, it uses large NPN transistors, with a different construction from most integrated circuit transistors. Each transistor consists of 24 emitters, paralleled in two groups. (You can consider it one large transistor, 2 transistors, or 24 small transistors.) The transistor is structured vertically with the collector (made of N-doped silicon) underneath, a thin P-type base in between, and the N-type emitters embedded in the top, forming the N-P-N layers of the transistor. The doped silicon regions are faintly visible with black lines around their boundaries.

Half of a transistor, with 12 emitters.

In the photo above, you can see the metal wiring for the transistor's collector, base, and emitter. The collector wiring is on the outside, with base wiring in between. The collector and emitter wiring is tapered: at one end, the wiring needs to support the full current load, while at the other end it only handles 1/12 of the current. The tapered approach saves space, since it is thicker only where it needs to be thick.

Resistors

The resistors are formed from silicon doped to have higher resistance. The doped silicon rectangle is faintly visible in the die photo. At each end of the resistor, a contact connects the silicon to the metal layer on top. The 1000Ω resistor on the left is longer than the 250Ω resistor on the right, giving it more resistance.

Two resistors as they appear on the die.

"Tunnels"

The chip has a single layer of metal wiring, which poses a problem if two signals need to cross. The solution is to put one signal in the silicon layer so it can pass under the metal layer. In essence, a low-valued resistor is used to pass under the metal layer. The image below shows how a tunnel appears on the die.

The conductive silicon strip at the top connects the metal regions on either side. The conductive strip at the bottom doesn't fulfill a wiring need, but ensures that both paths encounter the same resistance.

One problem is that the silicon has relatively high resistance compared to metal, so the tunnel adds resistance. The chip is carefully designed so both "sub-transistors" encounter the same resistance, to avoid one transistor turning on before the other. You can see that the input path in the upper left has a tunnel to pass under the metal wiring, while the path in the lower right has a tunnel of identical dimensions that doesn't go under any metal. While the second tunnel appears pointless, it assures that both paths have the same resistance.

The chip's circuit

The shift register requires a two-phase clock, that is two clock signals in alternation that step the bits through the circuit. To support this, the clock driver chip has two identical driver circuits. The schematic below shows one of the circuits. When the input goes high, it turns on transistor Q1, pulling its collector low. This pulls the output low through diode CR2. When the input drops, Q1 turns off. This lets R2 provide a current to the base of Q2, turning it on, and pulling the output high. Thus, the circuit is essentially an inverter, but one that can provide up to 1.5 amps of output.4

Schematic of the DS0025, from the application note.

The image below shows the various components of the schematic as they appear on the die. Most of the chip is occupied by the large power transistors. Although the chip is mounted in an 8-pin package, only six pins are used; the corresponding pads are labeled below. The chip consists of two identical mirror-image drivers; one is labeled. There are a few blackened regions in the transistors; we suspect this is where the chip failed.

Die with the components labeled. Note: diodes are labeled CR (crystal rectifier) in the schematic but D here.

Conclusion

This chip provides an interesting view of computer technology in the 1970s. The Apple-1 used shift-register memory, a technology that rapidly became obsolete as RAM prices dropped. Shift-register memory required a specialized clock driver integrated circuit, a chip that contained just four large transistors. With billions of transistors in modern integrated circuits, it's hard to imagine that it was once worthwhile to build a chip that was this simple. The Apple II, introduced just a year later in 1977, used RAM chips for all its storage, making shift-register memory a thing of the past.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed. Thanks to @TubeTimeUS for supplying the chip. I've written about the Intel 1405 shift register memory if you want to know more about this type of storage.

Notes and references

1. Looking in an old Byte magazine from 1976, a 1-kilobit shift register chip cost $9 ($34 in current dollars), while a 4-kilobit DRAM chip cost $20 ($75 in current dollars). Thus, it appears that even by the time the Apple-1 was released, DRAMs had become cheaper than shift registers. (This also illustrates the amazing drop in memory prices since the 1970s, as described by Moore's Law.)

   The Apple-1 used 4-kilobit RAM for data and program storage. It's possible, though, to build a computer that uses shift-register storage for its main memory. The Datapoint 2200 is one example. If memory is accessed sequentially, shift-register storage is efficient since the bits are provided sequentially. However, if you access memory out of sequence, the processor has to wait while the memory cycles around, until the desired bits become available. In a way, shift-register memory is a throwback to very early computers such as EDSAC(1949), which used mercury delay lines for main storage. ↩

2. The IBM 2260 video display terminal (1965) used a technique similar to shift registers: it stored data in a sonic delay line, sending torsional pulses through a 50-foot nickel wire. But unlike the Apple-1, this delay line stored pixels, not characters. For more about this system, see my blog post. ↩

3. The display circuitry has some additional complexity. Characters can't be taken directly from the display shift register: since each character is made up of eight scan lines; a line of character must be processed eight times. To handle this, a second shift register (six 40-bit registers) buffers a line of characters and feeds each character into a display ROM. Another 1024-bit shift register keeps track of the cursor position. For more details, see this post. The Apple-1 schematic is in the Operation Manual. ↩

4. The large current is required because of the design of the shift-register memory. The clock line snakes through the chip, providing a clock signal to each stage of the shift register. As a result, the clock line has a fairly high capacitance, about 150 picofarads. This clock line must be switched between +5 volts and -11 volts at a 1 megahertz rate. The combination of large capacitance and large voltage swing with the fast rate requires a high current. ↩

Reverse-engineering the waveform generator in a 1969 breadboard

How hard could it be to fix a vintage solderless breadboard that doesn't quite work? The "elite 2 circuit design test system" below combined a solderless breadboard with some supporting circuitry: power supplies, a waveform generator, a pulse generator, switches, and lights. CuriousMarc found one of these breadboards on eBay, but the function generator didn't work, so we set out to repair it.

The E&L Instruments elite 2 solderless breadboard has a variety of supporting circuitry.

I figured that the waveform and pulse generators would be simple circuits, but they turn out to be implemented with a board crammed full of components, including over 40 transistors. I reverse-engineered the circuitry and found some interesting circuits inside, including op-amps implemented from discrete transistors. This complexity probably explains the shockingly high price of this breadboard: $1300 in 1969 (equivalent to $10,000 in current dollars).1

The circuit board for the function/pulse generator is crammed full of components. The upper part holds the waveform circuitry while the lower part holds the pulse generator.

The waveform generator

The breadboard has a waveform generator that produces triangle, square, and sine waves over a wide frequency range, up to 1 megahertz. These waveforms are generated through a complex circuit that charges and discharges an integrator to produce the triangle wave. A comparator turns the triangle wave into a square wave. Finally, a sine-wave shaping network produces a sine wave from the triangle wave.

Triangle-wave generator

The oscillator's frequency is selected by resistors and capacitors. The faster the capacitor charges through the resistor, the higher the frequency. Thus, increasing the capacitance and resistance slows the oscillation. The frequency range knob turns a vintage wafer rotary switch to select one of seven different resistors and capacitors, allowing frequencies over a wide range from 1 Hertz to 1 megahertz. A potentiometer adjusts the frequency within the range to provide the exact desired frequency.

The triangle wave is generated from an op-amp integrator circuit, using the approach below. This circuit uses the capacitor to integrate the input voltage, producing the output voltage. The result is that the square wave input increases or decreases the output linearly, yielding a triangle wave. (The op-amp keeps that right side of R1 at ground, so the current that charges the capacitor is proportional to the input voltage. In contrast, in a simple R-C charging circuit, the capacitor charges exponentially; the charging current drops as the capacitor's voltage increases.)

A simplified op-amp integrator, based on image by Rutujadeshpande, CC BY-SA 3.0.

To make an oscillator, the other piece is a comparator to provide the square wave input to the integrator. The comparator reverses direction at the top and bottom of the triangle wave, as shown below. When the input to the integrator is positive, the integrator output climbs linearly. This output is fed into the comparator, along with a limit level (red dots). When the output exceeds the upper limit at "A", the comparator output becomes negative. Since the comparator output is used as the integrator input, the integrator now discharges and the signal drops. When the signal drops below the limit at B, the comparator switches on and the process repeats. A hysteresis circuit changes the comparator level (dotted red line) at A and B, setting the upper and lower limits of the triangle wave.2

The triangle-wave generation process.

The comparator is a 710HC, a simple differential comparator integrated circuit introduced by Fairchild around 1965. This is one of just two integrated circuits on the board.3 The IC is packaged in an 8-pin circular metal can. This package was common at the time for analog integrated circuits, as the metal can provided shielding.

The 710HC comparator is packaged in a round metal case.

Op-amp

The op-amp is a key component of the integrator. Although integrated-circuit op-amps date back to 1963, this board builds op-amps out of discrete components. The integrator op-amp consists of seven transistors, along with a bunch of resistors and capacitors, as shown below. The heart of the op-amp is the differential pair (Q30 and Q32), a standard analog circuit. A fixed current is fed into the differential pair transistors. If one transistor has a slightly higher input than the other, that transistor turns on and most of the current will go through that transistor. Thus, the differential pair amplifies the difference between the inputs, the key function of an op-amp. Additional amplification is provided by Q33, while Q34 and Q35 buffer the dual outputs.

Implementation of an op-amp in the waveform generator.

The output amplifier for the waveform circuit uses another discrete op-amp. This one has two stages of differential pairs for additional amplification, followed by power transistors to produce a high-current output. Another op-amp circuit is used in the sine-wave shaper, discussed below.

Sine-wave shaper

The board uses a surprising technique to generate sine waves: it synthesizes a sine wave from the triangle wave. Specifically, a resistor-diode network shapes the sine wave using piecewise-linear segments. The idea is to use diodes as switches that turn on as the signal level crosses various points. This adds resistance into the circuit, changing the slope. The result is a sine wave with less than 1% distortion.

This diagram explains the sine-wave shaping network. From HP Journal, Nov 1965.

The sine-wave shaper appears to be inspired by the similar circuit in the HP 3300A Function Generator, introduced in 1965. The schematic below shows the HP 3300A's sine-wave shaper; the breadboard's network is similar. The resistances are carefully chosen to achieve the sine wave. Similar resistor-diode networks were also used in analog computers to implement arbitrary functions, sometimes with user-adjustable resistances to change the function.

The sine-shaping circuit from the HP 3300A is very similar to the circuit in the breadboard. The resistor-diode network is highlighted. The surrounding circuitry biases the network and amplifies the output. From the Service Manual Fig 6-2.

Pulse generator

The pulse generator produces pulses from 100 ns to 100 ms wide. These pulses can be triggered by the waveform generator, an external trigger input, or a "one-shot" pushbutton. The pulse width is controlled by a switch-selectable resistor-capacitor network.

The most unusual part of the pulse generator is how the output circuit adjusts the pulse amplitude. Instead of simply adjusting the amplification, the circuit changes the voltage that powers the output amplifier. This variable voltage is produced by an LM305 voltage regulator IC, adjusted by the amplitude knob on the breadboard. A four-transistor circuit produces the matching negative voltage.4 These voltages power a fairly complex output stage with two circuits. One circuit produces positive pulses, while the other produces negative pulses.

The LM305A integrated circuit is in an old-fashioned metal can.

Conclusion

A prototyping breadboard may seem like a simple product, but everything becomes more complicated when built with 1969 technology. This breadboard includes a precision waveform generator and power supplies, designed for high accuracy, so it was almost like having test equipment included. But these features came at a steep price, equivalent to $10,000 today.

After I reverse-engineered the board,5 CuriousMarc used the schematic to fix the problems. The breadboard turned out to be in bad shape with a broken wire, a bunch of bad transistors, and a failed bridge rectifier in the power supply.6 It's unclear why the board had so many problems, more than you'd expect from age alone. Maybe the power supply over-voltaged the components at some point? My full schematic for the board is here.7

Some of the bad transistors that needed to be replaced.

CuriousMarc now has a video about the breadboard, so check it out:

Follow me on Twitter @kenshirriff for more posts. I also have an RSS feed.

Notes and references

1. The breadboard is described in a 1971 brochure. ↩

2. The hysteresis circuit is critical to the stability of the triangle wave. If the comparator input doesn't immediately switch to the lower level at "A", the comparator will switch again as soon as the integrator output drops slightly. Then the integrator will start rising, causing the comparator to switch again. The result is undesired high-speed oscillations (around a megahertz) with the triangle wave remaining stuck. This happened to us while attempting to repair the circuit when we replaced the hysteresis transistor with one that was a bit too slow. ↩

3. One unusual feature of the comparator chip is its asymmetrical power supplies. The positive supply (Vcc+) can go up to 14 volts, while the negative supply (Vcc-) is limited to -7 volts. This is inconvenient for the breadboard, which uses ±12 volt supplies internally. The solution is that the breadboard uses a 6.2-volt Zener diode to reduce the negative supply to the chip. R-C filters in the supply lines to the chip reduce noise. ↩

4. The negative-voltage circuit produces a negative voltage to match the user-selected positive voltage. In essence, it uses feedback from a resistor voltage divider between the positive and negative rails. When the two voltages are equal (and opposite), the voltage divider will yield 0 volts. If the voltages don't match, the signal from the voltage divider provides feedback to increase or decrease the negative rail as necessary. ↩

5. To reverse-engineer the board, I used a process that I've developed recently that works well. I took photos of both sides of the board and used the GIMP software to mirror one image and then align the images using the perspective tool. Next, I created a schematic in EAGLE, putting the symbols in their approximate locations. I wired up the schematic by drawing connections in EAGLE and marking the traces in GIMP as I handled them. The result of this was a "physical" schematic, approximately matching the board's layout.

   The next step was to rearrange the components in EAGLE to create a more logical "functional" schematic. (I find that EAGLE works better than KiCad for this.) This schematic helped me understand how the circuitry was implemented, and we used it to trace through the circuitry and diagnose its problems. ↩

6. The breadboard has five different power supplies: three user-adjustable power supplies, and two supplies for internal use, providing unregulated ±32 volts and regulated ±12 volts. Modern systems typically use compact switching power supplies, but the breadboard uses two large and heavy power transformers. Five large power transistors provide regulation, along with massive capacitors. Overall, the power supply illustrates how much power supplies have improved since the 1960s. ↩

7. Disclaimer: the schematic isn't completely accurate. In particular, I didn't look up component numbers, so I'm kind of guessing on NPN vs PNP transistors and there are definitely errors. I also didn't record resistor and capacitor values. (The purpose of the schematic was to guide the repairs, not to completely document the device.) ↩