Examining a technology sample kit: IBM components from 1948 to 1986

I recently received a vintage display box used by IBM to illustrate the progress of computer technology. This display case, created by IBM Germany1 in 1986 included technologies ranging from vacuum tubes and magnetic core memory to IBM's latest (at the time) memory chips and processor modules. In this blog post, I describe these items in detail and how they fit into IBM's history.

An IBM display box, showing components and board from different generations of computing. Click this (or any other photo) for a larger image.

First-generation computing: tube module

IBM is older than you might expect. It was created through merger (under the name CTR) in 1911 and produced punched card equipment for data processing, among other things. By the 1930s, IBM was producing complex electromechanical accounting machines for data processing, controlled by plugboards and relays.

The so-called first generation of electronic computers started around 1946 with the use of vacuum tubes, which were orders of magnitude faster than electromechanical systems. Appropriately, the first artifact in the box is an IBM pluggable tube module. The pluggable module combined a vacuum tube along with its associated resistors and capacitors. These modules could be tested before being assembled into the system, and also replaced in the field by service engineers. Pluggable modules were also innovative because they packed the electronics efficiently into three-dimensional space, compared to mounting tubes on a flat chassis.

Tube module from an IBM 604 Electronic Calculating Punch.

The pluggable tube module is from an IBM 604 Electronic Calculating Punch (1948). This large machine was not quite a computer, but it could add, subtract, multiply, and divide. It read 100 punch cards per minute, performed operations, and then punched the results onto new punch cards. It was programmed through a plugboard and could perform up to 60 operations per card. The IBM 604 was a popular product, with over 5600 produced. A typical application was payroll, where the 604 could compute various tax rates through multiplication.

The IBM 604 Electronic Calculating Punch behind a Type 521 Card Reader/Punch. Photo from IBM.

The 604 used many different types of tube modules. A typical module implemented an inverter, which could be used in an OR or AND gate.2 The tube module in the display box, however, is a thyratron driver, type MS-7A. The thyratron tube isn't exactly a vacuum tube since it is filled with xenon. This tube acts as a high-current switch; when activated, the xenon ionizes and passes the current. In the 604, thyratron tubes were used to drive relay coils or magnet coils in the card punch.3

A thyratron tube, type 2D21. This tube is from the pluggable module in the box.

Although the 604 wasn't quite a computer, IBM went on to build various vacuum-tube computers in the 1950s. These machines used larger pluggable tube modules that each held 8 tubes.4 The box didn't include one of these modules—probably due to their size—but I've included a photo below because of their historical importance.

A key-debouncing module from an IBM 705. Details here.

Second generation: transistors and SMS (Standard Modular System) card

With the development of transistors in the 1950s, computers moved into the second generation, replacing vacuum tubes with smaller and more reliable transistors. IBM based its transistorized computers on pluggable cards called Standard Modular System (SMS) cards. These cards were the building block of IBM's transistorized computers including the compact IBM 1401 (1959), and the larger 7000-series mainframe systems. A computer used thousands of SMS cards, manufactured in large numbers by automated machines.

The photo below shows the SMS card from the box.5 The card is a printed circuit board, about the size of a playing card, with components and jumpers on one side and wiring on the back. A typical SMS card had a few transistors and implemented a simple function such as a gate. The cards used germanium transistors in metal cans as silicon transistors weren't yet popular. I've written about SMS cards before if you want more details.

The SMS card in the technology box, type AXV.

Third generation: SLT (Solid Logic Technology)

In 1964, IBM introduced the System/360 line of mainframe computers. The revolutionary idea behind System/360 was to use a single architecture for the full circle (360°) of applications: from business to scientific computing, and from low-end to high-end systems. (Prior to System/360, different models of computers had completely different architectures and instruction sets, so each system required its own software.) The System/360 line was highly successful and cemented IBM's leadership in mainframe computers for many years.

Although other manufacturers used integrated circuits for their third generation computers, IBM used modules called SLT (Solid Logic Technology), which were not quite integrated circuits. Each thumbnail-sized SLT module contained a few discrete transistors, diodes, and resistors on a square ceramic substrate. An SLT module was capped with a square metal case, giving it a distinct appearance. Although an SLT module doesn't achieve the integration of an IC, it provides a density improvement over individual components. Each small SLT module was roughly equivalent to a complete SMS card, but much more reliable.7 By 1966, IBM was producing over 100 million SLT modules per year at a cost of 40 cents per module.6

The board below is a logic board using 24 SLT modules. These modules implement AND-OR-INVERT logic gates, the primary logic circuit used in System/360. This board was probably part of the CPU.

A logic board using SLT modules. (The display box labeled this as an MST board though.)

The photo below shows the circuitry inside an SLT module. This module has four transistors (the tiny gray squares). SLT modules typically include thick-film resistors, but none are visible in this module.

Closeup of an SLT module showing the tiny silicon dies mounted on the ceramic substrate.

The box also has an SLT card with analog circuitry (maybe for the computer's core memory or power supply). This card has one SLT module, a simple module that contains four transistors (number 361457). I don't know why this board has so many discrete transistors; perhaps they are higher-power transistors than SLT modules provided.

A card using an SLT module (the metal square in the lower left).

Integrated circuits: MST (Monolithic System Technology)

For a few years, IBM used SLT modules while other computer manufacturers used integrated circuits. Eventually, though, IBM moved to integrated circuits, which they called Monolithic System Technology (MST). An MST module looks like an SLT module from the outside, but inside it contains a monolithic die (i.e. an integrated circuit) rather than the discrete components of SLT. MST was first used in 1969 for the low-end System/3 computer.

An MST module looks like an SLT module from the outside, but has an integrated circuit die inside.

The photo above shows the box's MST module. The silicon die is the tiny shiny rectangle in the middle, connected to the 16 pins of the module. The chip was mounted upside down, soldered directly to the substrate. This upside-down mounting is unusual; most other manufacturers used ceramic or plastic packages for integrated circuits, with the silicon die connected to the pins via bond wires.

Core memory

The box contains a core memory plane; most computers from the 1950s until the early 1970s used magnetic core memory for their main memory.8 This plane holds 8704 bits and is from a System/360 Model 20, the lowest-cost and most popular computer in the System/360 line.9

Core plane from a System/360 Model 20.

In core memory, each bit is stored in a tiny magnetized ferrite ring. The ferrite rings are organized into a matrix; by energizing a pair of wires, one bit is selected for reading or writing. Multiple core planes were stacked together to store words of data. Because each bit required a separate ferrite ring, magnetic core memory was limited in scalability. This opened the door for alternative storage approaches.

Closeup of the core plane, showing the wires through the tiny ferrite cores.

Semiconductor memory

IBM was an innovator in semiconductor memory and this is reflected in the numerous artifacts in the box that show off memory technology.10 Modern computers use a type of memory chip called DRAM (dynamic RAM), storing each bit in a tiny capacitor. DRAM was invented at IBM in 1966 and IBM continued to make important innovations in semiconductor memory.

Although magnetic core memory was the dominant RAM storage technique in the 1960s, IBM decided in 1968 to focus on semiconductor memory instead of magnetic core. The first computer to use semiconductor chips for its main memory12 was the IBM System/370 Model 145 mainframe (1970). Each chip in that computer held just 128 bits, so a computer might need tens of thousands of these chips.11 Fortunately, memory density rapidly increased, as shown by the dies below. I'll discuss the 2-kilobit chip in detail; my die photos of the others are in the footnotes13.

The box includes a display with four memory dies: 2 K-Bit, 64 K-Bit, 256 K-Bit, 1 Megabit.

The photo below shows the 2-kilobit die14 under a microscope. It is a static RAM chip from 1973, not as dense as DRAM since it uses six transistors per bit. The tiny white lines on the chip are the metal layer on top of the silicon, wiring the circuitry together. Around the outside of the die are 26 solder bumps for attaching the chip to the substrate. Note that this chip is mounted upside down ("flip-chip") on the substrate, unlike most integrated circuits that use bond wires. The chip is covered with a protective yellowish film, except where the solder bumps are located.

Die photo of the 2-kilobit chip.

To increase the density of storage, four of these chips were mounted in a two-layer MST module, yielding an 8-kilobit module. The module in the box (below) has the square metal case removed, showing the silicon dies inside. These memory modules provided the main memory for the IBM System/370 models 115 and 125, as well as the memory expansion for the models 158 and 168 (1972).

The memory module has chips on two levels. This is an 8-kilobit module composed of four 2-kilobit chips.

Each memory card (below) contained 32 of these modules to provide 32 kilobytes of storage. In the photo below, you can see the double-height memory modules along with shorter modules for support circuitry. A four-megabyte main memory unit held 144 of these cards in a frame about 3 feet × 3 feet × 1 foot, so semiconductor memory was still fairly bulky in 1972.

The memory board contains regular MST modules and double-height modules that hold the memory chips.

Moving along to some different memory chips, the box includes two silicon wafers holding memory dies, a 5" wafer and a 4" wafer.

The two silicon wafers.

The smaller four-inch wafer (1982) holds 288-kilobit dynamic RAM chips, an unusual size as it isn't a power of 2.15 The explanation is that the chip holds 32 kilobytes of 9-bit bytes (8 + parity). In the die photo, you can see that the memory array is mostly obscured by complex wiring on top of the die. This wiring is due to another unusual part of the chip's design: for the most efficient layout, the memory bit lines have a different spacing from the bit decode lines. As a result, irregular wiring is required to connect the parts of the chip together, forming the pattern visible on top of the chip. Because this die is on the wafer, you can see the alignment marks and test circuitry around the outside of the chip.

Die photo of the 4" wafer.

The five-inch wafer holds 1-megabit memory chips16 that were used in the IBM 3090 mainframe17 (1985). This computer used circuit cards with 32 of these chips, providing four megabytes of storage per card, a huge improvement over the 32-kilobyte card described earlier. The 3090 used multiple memory cards, providing up to 256 megabytes of main storage. The die photo below shows how the chip consists of 16 rectangular subarrays, each holding 64 kilobits.

Die photo of the 1-megabit DRAM chip on the 5" wafer. The dark circles are dirt, not solder balls.

The photo below shows how this die is mounted upside-down on the ceramic substrate with the solder bumps connected to the 23 pins of the module. This module (not part of the box) was used in the IBM PS/2 personal computer.18 The die below looks green, unlike the die above, but that's just due to the lighting.

Construction of an IBM memory module. This module was not part of the box, but the die is the same as the 5" die. Photo courtesy of Antoine Bercovici.

The photo below compares three memory modules from the technology box. The first module is the 8-kilobit module containing four 2-kilobit chips, described earlier. The second module is a much wider 512-kilobit module, built from four 128-kilobit dies. The third module contains a 1-megabit chip (the one in the 4-chip display, not from the wafer). These megabit modules were used in the IBM 3090 mainframe's secondary storage.

Three memory modules: 8-kilobit, 512-kilobit, and 1-megabit.

Disk platter

The box contains a segment of a 14" IBM disk platter, used in disk storage systems from minicomputers to mainframes. IBM was a pioneer in hard disks, starting with the IBM RAMAC (1956), which weighed over a ton and held 5 million characters on a stack of 24" platters. IBM switched to 14" platters in 1961 and by 1980 the IBM 3380 disk system held up to 2.5 gigabytes in a large cabinet of 14" platters.19 The 14" platter was also popular in low-cost, removable disk cartridge (1965) used with many minicomputers. The 14" disk platter was finally replaced by an 11" platter with the introduction of the IBM 3390 disk drive in 1989. Nowadays, laptops typically use 2.5" platters; amazingly, disk capacity kept increasing as disk diameter steeply decreased.

Section of a 14" disk platter from the display box.

Artifacts from the IBM 3090

At the time of the box's creation, the 3090 mainframe was IBM's new high-performance computer (below), so the box has several artifacts that show off the technology in this computer. Although the IBM 3090 (1985) had top-of-the-line performance at the time, by 1998 an Intel Pentium II Xeon microprocessor had comparable performance,20 illustrating the remarkable improvements of microprocessor technology.

An IBM 3090 data center. Photo from the IBM 3090 brochure.

In 1980, IBM introduced the thermal conduction module (TCM), an advanced way to package integrated circuits at high density, while removing the heat that they generate.21 A TCM starts with a multi-chip module with about 100 high-speed integrated circuits mounted on a ceramic substrate, as shown below. This substrate contains dozens of wiring layers to connect the integrated circuits.22 To remove the heat, the ceramic substrate is packaged in a TCM, which has a metal piston contacting each silicon die. These pistons are surrounded by helium (which conducts heat better than air), and the whole TCM package is water-cooled. Finally, nine TCMs are mounted on a printed circuit board.

The hierarchy of components in the IBM 3090: chips are mounted on a ceramic substrate, which is assembled into a TCM. A board holds nine TCMs.

This incredibly complex heat-removal system was required because the 3090 used emitter-coupled logic (ECL), the same type of circuitry used in the Cray-1 supercomputer. Although ECL is a very fast logic family, it is also power-hungry and generates much more heat than the MOS transistors used in microprocessors.

The ceramic substrate for a TCM, from the box. It is fairly small, measuring 11×11.7 cm. This substrate holds 100 silicon dies; one is visible near the middle.

The photo above shows the ceramic substrate. Normally, the substrate has 100 silicon dies mounted on it, but this sample has just a single die. The box also includes a cross-section slice of the ceramic substrate (below). This shows the 38 layers of wiring inside the substrate, as well as the pins on the underside.

Cross-section of the ceramic substrate, showing the multiple layers of internal wiring.

Each TCM had 1800 pins so it could be plugged into a printed circuit board and connected to the rest of the system. Each board held 9 TCMs and was powered with an incredible 1400 amps. The box includes a PCB sample, showing its multi-layer construction (below), and the dense grid of holes to receive the ceramic substrate.

Closeup of the printed circuit board used in the IBM 3090. The routed groove shows the multi-layer construction.

Finally, here's a nice cutaway of a TCM from the detailed IBM 3090 brochure. At the bottom, it shows the silicon dies mounted on the ceramic substrate. The dies are contacted by the heat sink pistons in the middle. The connections on top are for the cooling water.

This cut-away image from IBM shows the internal construction of a TCM.

Conclusion

This technology exhibit box was created 35 years ago. Looking at it from the present provides a perspective on the history of both IBM and the computer industry. The box's date, 1986, marks the peak of IBM's success and influence,23 right before microcomputers decimated the mainframe market and IBM's dominance. What I find interesting is that the technology box focuses on mainframes and lacks any artifacts from the IBM PC (1981), which ended up having much more long-term impact..24 This neglect of microcomputers reflects IBM's corporate focus on the mainframe market rather than the PC market (which, ironically, IBM created).

In the bigger historical picture, the technology box covers a time of great upheaval as electromechanical accounting machines were replaced by three generations of computers in rapid succession: vacuums tubes, then transistors, and finally integrated circuits. In contrast to this period of rapid change, nothing has replaced integrated circuits over the past 50 years. Instead, integrated circuits have remained, but improved by many orders of magnitude, as described by Moore's Law. (Compared to the room-filling IBM 3090 mainframe, an iPhone has 1000 times the performance and 50 times the RAM.) Will integrated circuits continue their dominance for the next 50 years or will some new technology replace them? It remains to be seen.

Thanks to Cyprien for providing this amazing box of artifacts. I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed.

Notes and references

1. The box was apparently created in Stuttgart, Germany. The components are protected by a piece of plexiglass, with labels in German for all the components, such as Mehrschicht-Keramiktrager for multi-layer ceramic substrate. The labels are listed here if you're interested.

   

   The box is labeled in German: "Computertechnologie".

   The box originally included several German books on computer technology but since they are missing I had to do some research and come up with my own narrative.

    ↩

2. For more information on the pluggable tube modules, see the schematics of IBM's pluggable units (which lack the box's MS-7A module). (I suspect the MS-7A was selected for the box because it is more compact than most of the pluggable modules, having one layer of circuitry below the tube, rather than two.) ↩

3. The IBM 604 service manual says that the thyratron tube modules are designated TH, but the module in the box is designated MS-7A. I don't know why the designations don't match up. ↩

4. People sometimes think that an 8-tube module held a byte. This is wrong for two reasons. First, bytes didn't exist back then. IBM's early scientific computers used 36-bit words, while the business computers were based on characters of 6 bits plus parity. Second, 8 tubes didn't correspond to 8 bits because circuits often required multiple tubes. For instance, a tube module could implement three bits of register storage. ↩

5. The SMS card in the box is type AXV, a complementary emitter follower circuit used in the IBM 1443 printer and other systems. ↩

6. SLT was controversial, since other companies used more-advanced integrated circuits rather than hybrid modules. In typical IBM fashion, the vice president in charge of SLT was demoted in 1964, only to be reinstated in 1966 when SLT proved successful. My view is that integrated circuit technology was too immature when the System/360 was released, so IBM's choice to use SLT made the System/360 possible. However, it only took a year before integrated circuits became practical, as shown by their use in competing mainframes. I think IBM stuck with SLT modules longer than necessary. Integrated circuits rapidly increased in complexity (Moore's Law), while SLT modules could only increase density through hacks such as putting resistors on the underside (SLD) and using two layers of ceramic (ASLT). ↩

7. Curiously, this card is labeled in the box as an MST card, but checking the part numbers shows it has SLT modules. Specifically, it contains the following types of SLT modules (click for details): 361453 AND-OR-Invert, 361454 inverters, 361456 AND-OR-extender, and 361479 inverters. The SLT modules are also documented in IBM's manual.

   

   Schematic of one of the SLT modules on the board (361453 AND-OR-INVERT (AOI) gate) from the IBM manual.

   The schematic above shows one of the SLT modules. (IBM had their own symbol for transistors; T1 is an NPN transistor.) This gate is built from diode-transistor-logic, so it's more primitive than the TTL logic that became popular in the late 1960s. The "Extend" pins are used to connect modules together to build larger gates, so the modules provide a lot of flexibility. This module inconveniently requires three voltages. This SLT module contained one transistor die, three dual-diode dies, and three thick-film resistors. During manufacturing, the resistors were sand-blasted to obtain accurate resistances, an advantage over the inaccurate resistances on integrated circuit dies. ↩

8. The System/360 line was designed as a single 32-bit architecture for all the models. The Model 20, however, is a stripped-down, 16-bit version of System/360, incompatible with the other machines. (Some people don't consider the Model 20 a "real" System/360 for this reason.) But due to its low price, the Model 20 was the most popular System/360 with more than 7,400 in operation by the end of 1970. ↩

9. This core memory plane from a System/360 Model 20 is a 128×68 grid. Note that this isn't a power of 2: the plane provided 8192 bits of main memory storage as well as 512 bits for registers. Using the same core plane for memory and registers hurt performance but saved money. The computer used five of these planes to make a 4-kilobyte memory module, or 10 planes for an 8-kilobyte module. For details, see the Model 20 Field Engineering manual. ↩

10. For an extensive list of references on DRAM chips, see the thesis Impact of processing technology on DRAM sense amplifier design (1990). For a history of memory development at IBM through 1980, from ferrite core to DRAM, see Solid state memory development in IBM. ↩

11. The System/370 Model 145 was the first computer with semiconductor main memory. Each thumbnail-sized MST module held four 128-bit chips; 24 modules fit onto a 12-kilobit storage card. A shoebox-sized Basic Storage Module held 36 cards, providing 48 kilobytes of storage with parity. By modern standards this storage is incredibly bulky, but it provided twice the density of the magnetic core memory used by contemporary systems. The computer's storage consisted of up to 16 of these boxes in a large cabinet (or two), providing 112 kilobytes to 512 kilobytes of RAM.

    

    Photos showing the 512-bit memory module, the 12-kilobit memory card, and the 48-kilobyte basic storage module. Photos from IBM 370 guide.

     ↩

12. IBM had used monolithic memory for special purposes earlier, holding the "storage protect" data in the IBM 360/91 (1966) and providing a memory cache in the System/360 Model 85.  ↩

13. I wasn't able to find exact details on the 64-kilobit, 256-kilobit, and 1-megabit chips from the display, but I took die photos.

    

    Die photo of the 64k memory chip.

    The 64-kilobit chip is shown above. The solder balls are the most visible part of the chip. The article A 64K FET Dynamic Random Access Memory: Design Considerations and Description (1980) describes IBM's experimental 64-kilobit DRAM chip, but the chip they describe doesn't entirely match the chip in the box. There were probably some significant design changes between the prototype chip and the production chip.

    

    Die photo of the 256-kilobit RAM, roughly 1985.

    The 256-kilobit die is shown above. The diagonal lines on the die are similar, but not identical, to the die in A 256K NMOS DRAM (1984). That chip was designed at IBM Laboratories in Böblingen, Germany, and could provide 1, 2, or 4 bits in parallel.

    

    Die photo of the 1-megabit memory chip.

    The 1-megabit die is shown above. IBM was the first company to begin volume production of 1-megabit memory chips and the first company to use them in mainframe computers. This chip was used in the IBM 3090 mainframe, but was later replaced by the faster and smaller "second-generation" 1-megabit chip on the 5" wafer. One interesting feature of this die is the "eagle" logo, shown below.

    

    The eagle chip art on the 1-megabit RAM chip, slightly scratched.

    The box includes a 1-megabit MST module (below) that uses this chip. Because the chip's solder balls are along its center, the module omits the center three pins to make room for the connections to the chip.

    

    The 1-megabit chip mounted in an MST module.

     ↩

14. This memory card and its 2-kilobit chips are described in detail in A High Performance Low Power 2048-Bit Memory Chip in MOSFET Technology and Its Application (1976). These modules were used in the main memory of the IBM System 370 models 115 (1973) and 125 (1972) as well as upgraded memory for the models 158 (1972) and 168 (1972). The IBM System/360 Model 138 (1976) and Model 148 (1976) also used 2K MOSFET chips, presumably the same ones. The 2-kilobit chip was developed at IBM Laboratories in Böblingen, Germany; this may have motivated its inclusion in this German display box. The chip was codenamed Riesling and IBM made millions of them.

    

    Closeup of the 2-kilobit RAM chip.

    The closeup of the 2-kilobit die shows some of the decoder circuitry (left) and the storage cells (right). Two solder balls are in the lower left; the rest of the die is covered with a protective yellow film, probably polyimide. Each storage cell consists of six transistors. The chip is built with metal-gate NMOS transistors. ↩

15. The 288-kilobit chip is described in detail in A 288Kb Dynamic RAM.

    

    Closeup of the IBM 288-kilobit memory chip showing the programmable fuses.

    The closeup die photo above shows some of the memory cells (at the top and bottom), wired into bit lines. One unusual feature of this chip is that has redundancy to work around faults. In particular, four redundant word lines can be substituted for faulty ones, by blowing configuration fuses. I think the large boxes with circles in the middle are four of the fuses.

    

    The part number on the 4" die: OITETR02I IBM 032 BTV.

    The photo above shows the chip's part number; BTV refers to IBM's Burlington / Essex Junction, VT semiconductor plant where the chip was designed. This plant was acquired by GlobalFoundaries in 2015. This photo also shows the complex geometrical wiring, unlike the regular matrix in most memory chips. ↩

16. Note that there are two 1-megabit chips in the box. The chip on the 4-chip display is an older chip than the one on the 5" wafer. The 1-megabit memory chip on the wafer is described in An Experimental 80-ns l-Mbit DRAM with Fast Page Operation (1985). It uses a single 5-volt power supply. The chip is structured as four 256-kbit quadrants, each subdivided into four 64-kbit subarrays. It has two redundant bit lines per quadrant for higher yield. The horizontal solder balls through the middle of the chip are the common connections for each quadrant, while the vertical connections along the left and right edges provide the signals specific to each quadrant. This quadrant structure allows the chip to be accessed as 256K×4 or 1M×1. ↩

17. IBM's overview of the 3090 family provides details on the hardware, including the memory and TCM modules. Page 10 discusses IBM's memory technology as of 1987 and has a picture of their "second generation" 1-megabit chip, which matches the die on the 5" wafer. ↩

18. The 1-megabit memory chips were used in the IBM 3090 mainframe, but I think the faulty ones were used in IBM PS/2 personal computer. You can see the unusual metal MST packages on many PS/2 cards. Specifically, if one of the four quadrants in the memory chip had a fault, the memory chip was used as a 3/4-megabyte chip. These had four part numbers, depending on the faulty quadrant: 90X0710ESD through 90X0713ESD (ESD probably stands for Electrostatic Sensitive Device). The PS/2 2-megabyte memory card (90X7391) had 24 chips providing 2 megabytes with parity. The board used chips with alternating bad banks so the memory regions fit together. ↩

19. Since several of the artifacts in the box came from the IBM 3090 mainframe, and the 3380 disk system was used with the 3090 mainframe, my suspicion is that the platter is from the 3380 disk system, shown below.

    

    An IBM 3380E disk storage system, holding 5 gigabytes. The disk platters are center-left, labeled "E". Photo taken at the Large Scale Systems Museum.

     ↩

20. It's difficult to precisely compare different computers, especially since the 3090 supported multiple processors and vector units. I looked at benchmarks from 2001 comparing various computers on a linear algebra benchmark. The IBM 3090 performed at 97 to 540 megaflops/second for configurations of 1 to 6 processors respectively. An Intel Pentium II Xeon performed at 295 megaflops/second, a bit faster than the 3-processor IBM 3090. To compare clock speeds, the IBM 3090 ran at 69 MHz, while the Pentium ran at 450 MHz. An IBM 3090 cost $4 million while a Pentium II system was $7,000 to $20,000. The IBM 3090 came with 64 to 128 megabytes of RAM while people complained about the Pentium II's initial 512-megabyte limit. The point of this is that while the IBM 3090 was a powerful mainframe in 1985, microprocessors caught up in about 13 years, thanks to Moore's Law. ↩

21. The table below compares characteristics of the Thermal Conduction Modules used in the IBM 3081 (1980), IBM 3090 (1985), and IBM S/390 (1990) computers. The board-level technology progressed similarly. For instance, a 3081 board took up to 500 amps, while a 3090 board took 1400 amps, and an S/390 board took 3400 amps.

    

    This table of information on TCMs is from Packaging Technology for IBM's Latest Mainframe Computers (S/390/ES9000).

    The IBM 4300-series processors (1979) used a ceramic multi-chip module that held 36 chips, but it used an aluminum heat sink and air cooling instead of the more complex water-cooled TCM. The IBM 4381's smaller multi-chip module is often erroneously called a TCM by online articles, but it's a multilayer ceramic multichip module (MLC MCM). For more information about IBM's chip packaging, see this detailed web page. ↩

22. For more information on TCMs, see the EEVblog teardown. ↩

23. Desktop computer sales first exceeded mainframe computer sales in 1984. Counting the number of employees, IBM peaked in 1985 and declined until 1994 (source). 1985 was also a peak year for IBM's revenue and profits, according to The Decline and Rise of IBM. By 1991, IBM's problems were discussed by the New York Times. After heavy losses, IBM regained profitability and growth in the 1990s, but never regained its dominance of the computer industry. ↩

24. Perhaps one reason that the technology box ignores IBM's personal computers is that these computers didn't contain IBM-specific hardware that they could show off: Intel built the 80x86 processor, while companies such as Texas Instruments built the memory and support integrated circuits. The lack of IBM-specific technology in these personal computers is one factor that led to IBM losing control of the PC-compatible market. ↩

Reverse-engineering a low-power LED flasher chip

How do you make an LED blink? A vintage way is the LM3909, a chip from 1975 that can flash an LED for a year from a single flashlight battery. This chip has some surprising features, such as a charge pump that lets you power a 2-volt LED from a 1.5-volt battery. This IC was designed for simplicity, using just an LED, external capacitor, and battery. In this blog post, I reverse-engineer its silicon die.

The photo below (from Zeptobars) shows the die under a microscope. This chip is fairly simple, even for 1975, with just a few transistors and resistors. The six dark circles show where the bond wires were attached. The copper-colored lines are the metal layer on top of the silicon, wiring the components of the circuit together. The silicon itself has a purple tint in this photo. Regions with different tints are doped differently to form N-type and P-type silicon. These regions form the transistors and resistors, as I'll explain below. The large, zig-zag power transistor at the top of the chip is the most visible transistor. The part number "3909" is visible, along with various "A" alignment marks.

Die photo of the LM3909, courtesy of Zeptobars, CC BY 3.0.

How it works

The underlying idea of the IC is that the external capacitor slowly charges through the internal timing resistor. When the capacitor reaches about 1 volt, the LED is energized and lights up briefly. This discharges the capacitor and the cycle repeats. Because the LED is only powered in brief pulses, the circuit's power consumption is low, averaging about 550 µA.

The first phase, charging the capacitor, is shown in the schematic below. The red line indicates how the battery (V+) charges the positive side of the external capacitor (on pin 2) through two 400Ω resistors. The negative side of the capacitor is connected to the negative side of the battery through the 3K resistor. The flashing frequency can be slowed down by disconnecting pin 1. In that case, the capacitor charges through the additional resistance of the "Slow RC" pin.

The capacitor charging. Schematic from the LM3909 datasheet.

When the capacitor charges to about 1 volt, Q1 turns on, turning on Q2 and then power transistor Q3.1 This causes the circuit to discharge through the LED, as shown below. The tricky part is that the positive side of the capacitor is now connected to ground. Since the capacitor was charged to 1 volt, the negative side of the capacitor is now at -1 volt. (In other words, the capacitor acts as a charge pump.) The result is that the LED sees an extra volt; with a 1.5-volt battery, it receives about 2.5 volts.

The capacitor discharges through the LED, lighting it.

The LED receives a high-current pulse for about a millisecond, discharging the capacitor, and then the cycle repeats.

Resistors

The chip uses numerous resistors. Looking at the die, I'll explain how they are constructed. The first type of resistor uses a strip of doped silicon, which is conductive but has some resistance. The upper resistor in the photo has metal contacts attached to both ends of the resistive rectangle. This resistor is wide and relatively short, so it has a low resistance, 12Ω in this case. (This resistor limits current through the LED, so an external resistor isn't required.) The second resistor is long and narrow, so it has a much higher resistance, 6kΩ. This resistor winds back and forth to fit on the die.

Resistors on the die.

The second type of resistor is a "pinch resistor". An oval of P-type silicon "pinches" the underlying silicon and reduces the current flow. A pinch resistor provides a high resistance in a small space, but is less accurate than the resistors shown earlier. The two resistors below are 10kΩ and 20kΩ respectively.

High-resistance pinch resistors.

Transistors

The illustration below shows the power transistor. Since the full current to drive the LED (45 mA) passes through this transistor, it must be relatively large. The metal on top is connected to the collector (N-type silicon). The base (P) and emitter (N) are interdigitated, allowing high current flow. Note that the collector metal gets wider from left to right, while the emitter metal gets narrower. This design puts the widest metal where the current is highest; on the right, the emitter metal carries current from just one emitter "finger", while on the left the metal carries the full emitter current.

The power transistor on the die.

Two other transistors, Q1 and Q2, have an optimized layout where they are partially merged together. I'll try to explain them in the diagram below, but it's a bit tricky. I've colored the die photo to indicate P-type silicon in red and N-type silicon in blue. I'll start with Q2, a transistor with two collectors. (It may seem strange to have two collectors on a transistor, but this is common in analog integrated circuits.) Since Q2 is a PNP transistor, the emitter is P-type silicon, the base is N, and the collectors are P. Q2's emitter (P) is the small rectangle indicated in red; the silicon is connected to the metal on top. Q2's base is formed by a ring of N-type silicon around the emitter. This N-type silicon forms the large blue region over most of the diagram. Finally, Q2's two collectors are the red regions above and below the emitter. (The layers of P, N, and P silicon for the emitter, base, and collector form the layers of the PNP transistor.)k The upper collector is connected to the metal wiring. The lower collector is connected to the base by the lower metal contact.

Q1 and Q2 have a complex, optimized layout where Q1's collector and Q2's base are merged.

Now for Q1, an NPN transistor. It has a small square emitter (N, blue) connected to metal wiring. The emitter is embedded in the base region (red, P) that surrounds it. Metal wiring is connected to Q1's base. Q1's collector isn't explicitly visible; the underlying blue region that surrounds the base forms the collector. Note that this blue region also formed Q2's base. Thus Q1's collector and Q2's base are formed by the same large region of N-type silicon, implicitly wiring them together. Analog integrated circuits often optimize the layout of transistors, but this layout is trickier than most.

There's one transistor that appears on the die but not on the schematic. It is a JFET (junction field-effect transistor). JFETs are often used on integrated circuits as a compact way to provide a small current. This JFET pulls the base of the power transistor low, probably to ensure that the chip starts up correctly.

The JFET on the die. It consists of a region of N-silicon between the source and drain, pinched by the P-type gate on top.

Putting this all together, the diagram below labels the main components on the die. This can be compared with the schematic.2

The LM3909 die with pins and key components labeled. Pins 3 and 7 are not connected to the die.

Conclusion

This integrated circuit has just a few, large components, so their wiring can be traced out on the die.3 The designers came up with a clever circuit that uses just a few transistors but can flash an LED without consuming much power. One interesting feature is the use of a charge pump, so a single 1.5V battery can light an LED that requires more than 1.5V. Another nice feature is that the current-limiting resistor and the timing resistors are part of the IC, so external resistors are not required. On-chip capacitors are very limited, so the chip requires an external capacitor, though.4 Nowadays, you can get an LED with blinking built in, so a special-purpose chip is unnecessary.

Thanks to Zeptobars for the die image and thanks to Jerry Taylor for suggesting this chip. I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed.

Notes and references

1. I'll use this footnote to explain some details of the circuit. The voltage level on the capacitor is detected by Q1. A transistor turns on when the base voltage is about 0.7 volts above the emitter voltage. A typical transistor circuit modifies the base voltage. But in this case, the base voltage is fixed and the emitter voltage varies. Specifically, Q1's emitter is connected to the capacitor's low side through the 100Ω resistor, so as the capacitor charges, the emitter voltage drops.

   When Q1 turns on, current flows through Q1 from Q2. Transistor Q2 has two collectors and is wired as a current mirror: the current through the first collector is copied, creating a current through the second collector. (From the datasheet, Q3 is constructed so the output current is multiplied by 3. This is accomplished by making the upper collector three times larger; you can see on the die that it wraps around three sides of the emitter while the "input" collector is smaller, only on the bottom side.) This current flows into the base of Q3, the power transistor, turning it on. Q4 provides negative feedback to keep Q1 biased correctly. ↩

2. I think the Zener diode is in the location marked "Z", but I'm not sure since there's not a lot to see on the die for a Zener diode. The diode protects the output transistor against excessive voltage. ↩

3. For more information, see the datasheet and Application Note. The application note gives many circuits that use this chip, including unexpected possibilities such as building a radio out of it, or a logic gate. (I think the designers got a bit carried away.) ↩

4. An interesting project is the re-creation of the LM3909 from discrete components by Dillon Nichols; see the writeup and video. (I had no idea that this chip was popular enough that someone would build a clone.) ↩

Reverse-engineering an early calculator chip with four-phase logic

In 1969, high-density MOS integrated circuits were still new and logic circuits were constructed in a variety of ways. One technique was "four-phase logic", which provided ten times the speed and density of standard logic gates while using 1/10 the power.1 One notable application of four-phase logic was calculators. In 1969, Sharp introduced the first calculator built from high-density MOS chips, the QT-8D, followed by the world's smallest calculator, the handheld EL-8. These calculators were high-end products, selling for $345 (about $1800 today).

The Sharp EL-8 calculator. Note the unusual 8-segment display for the digits. Photo by Felix Maschek, CC BY-SA 3.0 DE.

Integrated circuits at the time weren't dense enough to implement an entire calculator on one chip so these calculators split the functionality across five ICs. These five chips were created for Sharp by the Autonetics division of Rockwell. Autonetics invented four-phase logic in the mid-1960s, so this logic family was a natural choice for the calculator chips.

Die photo of the NRD2256 keypad/display chip. Die photos courtesy of François Gueissaz.

In this blog post, I reverse-engineer the keypad/display chip shown above. This photo shows the tiny silicon die under a microscope. The silicon substrate has a purple tint while the doped, conductive silicon is green. The metal layer on top is white. Around the edges, thin bond wires connect the die to the 42 external pins. The chip contains roughly 500 transistors implementing 100 logic gates. While the density of this chip is absurdly low by modern standards, it illustrates the progress of MOS integrated circuits in the late 1960s.

Inside the calculator

The photo below shows the circuit board inside the calculator. The board is dominated by the four large integrated circuits with circular golden lids. These integrated circuits were packaged as 42-pin ceramic ICs with staggered pins, an arrangement that provided more room for the PCB traces. Unlike modern printed circuit boards, the traces on this board are curved, showing its hand-drawn layout.

The circuit board for the Sharp EL-8 calculator. The clock IC is the small metal-can package in the middle. Photo from Mister rf (CC BY-SA 4.0).

These four chips have different functions: an arithmetic chip, a decimal point chip, a keypad/display chip, and a control chip. This blog post focuses on the keypad/display chip (NRD2256) in the upper left. The fifth chip, is the clock chip in the small metal can that provides the four-phase timing pulses. The system clock runs at about 60 kilohertz, very slow by microprocessor standards, but fast enough for a calculator

One function of the keypad/display chip is to handle keypresses, converting a digit key into a 4-bit serial binary value. (Unexpectedly, non-digit keypresses are handled by other chips.) Its second main function is to display digits on the display. Like most calculators, this calculator multiplexes the display; it displays one digit at a time, repeated rapidly enough that the display appears uniform. It does this by activating one display tube at a time and energizing the appropriate segments to produce the desired digit.2

Display board for the Sharp EL-8 calculator. Note the special tube at the top that displays a minus sign and error dot. Photo courtesy of John Wolff's Web Museum © 2012.

The four main chips communicate serially, sending each decimal digit as four BCD (binary-coded decimal) bits. Each communication cycle consists of 8 digits plus a ninth unused spot, forming a 36-bit "packet".3 The basic timing comes from the 60-kilohertz clock chip; one bit is sent each clock cycle. The keypad/display chip produces additional timing signals keep everything synchronized. First, it divides the clock by 4, generating a "digit clock" signal that indicates each 4-bit digit. The keypad/display chip cycles through the display digits, one digit every four clocks; it transmits signals to the other chips to keep track of the current digit. Thus, as the keypad/display chip cycles through the digits of the display, it receives the binary value of each digit at the right time.

The diagram below shows the functional units in the keypad/display chip. The "digit scan" circuitry scans through the eight digit drive lines D1-D8. The "decimal point" circuitry deserializes the decimal point input "dp" and energizes the decimal point segment when the specified digit is active. The "digit serialize" circuit converts a digit keypress into four serial bits. The "wiring" section is simply wiring between the upper half of the chip and the lower half, showing how much space is wasted by signal routing. In the lower half, the "9-segment decoder" illuminates the appropriate segments to display a digit; this digit is serialized by the "digit latch" circuit. The "clk÷4" circuit divides the input clock by four to produce the digit clock. Finally, the "key encode" circuit converts a keypress (0-9) into the four-bit value used by the "digit serialize" circuit. As will be seen, these functional blocks are not very complex, consisting of maybe 20 gates each.

The die of the NRD2256 keypad/display chip with the functional blocks labeled.

PMOS Transistors

The calculator chip is built from metal-gate PMOS transistors. This type of transistor was easy to manufacture in the 1960s, but rapidly became obsolete. These transistors required large negative voltages, -25 volts for the calculator chip. (For simplicity, I will view the signals as active-low; 0V is a logical 0 and -25V is a logical 1.) Another problem with metal-gate transistors is that most of the chip was occupied by silicon and metal wiring, so the density of transistors was very low.

The diagram below illustrates a metal-gate PMOS transistor. At the bottom, two regions of silicon (green) are doped to make them conductive, forming the source and drain of the transistor The gate is formed by a metal strip between the silicon regions, separated from the silicon by a thin layer of insulating oxide. (These layers—Metal, Oxide, Semiconductor—give the MOS transistor its name.) The transistor can be considered a switch between the source and drain, controlled by the gate. To simplify the behavior, a PMOS transistor turns on when the gate is pulled negative (-25 volts), while the transistor turns off when the gate is at 0 volts.

Structure of a PMOS metal-gate transistor.

The image below shows how a transistor appears on the die. The gate is formed by the metal overlapping the doped silicon (vertical green strip). Inconveniently, a contact that connects the metal layer to the silicon looks very similar to a transistor in this chip—the metal layer in a transistor almost touches the silicon, while the metal layer in a contact touches the silicon. A contact and a transistor can be distinguished with effort; a contact is more square-shaped while a transistor is more oval-shaped and slightly blurrier. As will be explained below, four-phase logic often uses transistors where both the gate and the drain are connected to the same clock; this type of connection appears at the bottom of the diagram. By recognizing the transistors, the circuitry can be reverse-engineered.

Transistors look similar to metal/silicon contacts, but have subtle differences.

How four-phase logic works

Four-phase logic is a technique for building logic gates, such as NAND gates. At the time, the standard way of building a logic gate was called "static logic", because the output remained constant as long as the inputs didn't change. A disadvantage of static logic was that it required a large "load transistor" that continuously used current, resulting in high power consumption.

A solution to these problems was "dynamic logic". Instead of providing a steady output from the gate, the gate's output was controlled by a clock signal. The gate's value would be computed and then stored by the circuit's capacitance, instead of requiring a continuous current. Developing with dynamic logic can be tricky, however, because of its dependence on timing. (It also has the disadvantage that the output values rapidly leak away, rather than being stable as with static logic.) Dynamic logic is still used in modern CPUs, in the form of domino logic.

Four-phase logic is a specific type of dynamic logic, designed to simplify the design process. Its timing is controlled by four clock signals (below), the source of the name "four-phase".4 In the calculator, these clock signals repeated at 60 kilohertz.

This shows one cycle of the four-phase clock. The four-phase clock consists of four clock signals in this specific pattern.

The diagram below shows how an inverter is implemented in four-phase logic. In the first clock phase, φ1 is high causing the capacitor to get charged. In the second clock phase, the gate's value is determined. If the input is 0, the capacitor keeps its previous value (1). But if the input is 1, the capacitor discharges through the lower transistors so the output is 0. Thus, the circuit inverts the input.5 The capacitor holds the output for the remainder of the clock cycle, so the gate also acts as a latch. (This is an important feature of four-phase logic, simplifying many circuits.)

Operation of a four-phase inverter. The gate is first precharged. In the evaluation step, the gate either remains charged (if the input is 0) or is discharged (if the input is 1).

More complex gates are built in a similar manner. For a NAND gate, multiple input transistors are put in series; if all inputs are 1, the capacitor will discharge and the output will be 0. For a NOR gate, multiple input transistors are put in parallel; any 1 input will yield a 0. As will be seen later, complex gates can be created with a mixture of series and parallel transistors.

The gate described above only uses two phases,6 so why four-phase logic? The problem with the above circuit is that if you connect two gates together, during step 2 the output of the first gate will be changing while the second gate is using this value. This could cause the second gate to erroneously discharge, yielding the wrong answer. The solution is for the second gate to wait until the first gate is stable. Specifically, the first gate operates during time periods 1 and 2, while the second gate operates during time periods 3 and 4. The second gate can then be safely connected to another gate operating during time periods 1 and 2. A circuit that alternates the two types of gates will operate safely.7

The diagram below shows how a four-phase inverter appears on the die. The schematic is the same as before, but the circuit is stretched vertically, with a layout that is tall and skinny. The inverter consists of a doped silicon line (green) running vertically, crossed by metal wiring. The gate is implemented by three small transistors. The large capacitor in the middle holds the output voltage. Dynamic logic is often built to use the stray capacitance of the wiring, but this chip uses many large capacitors (perhaps due to leakage or the slow clock speed).

An inverter on the die of the calculator chip.

Implementation of the calculator circuits

In the next sections, I'll describe how some of the calculator IC's circuits are implemented using four-phase logic.

Shift register

This chip uses shift registers to convert a serial input signal into a parallel binary value. One shift register is used for the decimal point position input while another shift register handles the digit to be displayed. The basic implementation of the shift register is a chain of inverters with two inverters per stage. Because four-phase logic is clocked, a bit will advance through the two inverters every clock cycle. (One inverter during Φ1/Φ2 and the second inverter during Φ3/Φ4.) This is an advantage of four-phase logic; standard logic requires a flip-flop at each stage to hold the bits, making the circuit much more complex. Each stage has an additional inverter to output the uncomplemented value. To keep both outputs synchronized, these inverters use special timing, precharging on Φ3 and reading on Φ1.7

A shift register, built from inverters.

The diagram below shows how the shift register for the decimal point position is implemented on the die. It shows nine inverters, implemented with 27 transistors. Each vertical green line of doped silicon is one gate, while the white metal wiring is mostly horizontal. Note that this circuitry, just nine gates, takes up a large fraction of the die. While the gates are tightly packed side-to-side, they are very tall, so the die holds just two rows of gates. The density of transistors is very low, with most of the area consumed by wiring. Even so, four-phase logic was considered a dense way of creating gates, since other techniques were even worse. (A couple of years later, microprocessors used an additional layer of polysilicon wiring, which made signal routing much easier and greatly increased the density.)

A shift register as implemented on the die.

Examples of transistors and capacitors are indicated on the diagram. At the bottom, the arrow shows one of the connections between two inverters. The short horizontal wire is connected to the inverter on the left, and forms the gate of the inverter on the right. Other wires are longer as they connect inverters to other parts of the circuitry.

Binary encoding

The chip converts each digit keypress into a binary encoding, using the NAND gates shown below. The calculator's buttons are magnets, closing reed switches. These switches are connected to the inputs on the right. When a key is pressed, the input goes low and the circuit generates the corresponding 4-bit binary output at the bottom.

The key encoder uses NAND gates to convert key presses into the binary encoding. The circles are probably mask alignment marks.

Each vertical green line corresponds to a NAND gate. (These gates are tall like the previous ones, but I'm only showing the interesting part.) The interesting thing about the encoder is that the binary representation is visible in the transistor pattern. For instance, the "1" bit output is connected to alternating inputs, while the "4" bit output is activated by keys 4 through 7. The unlabeled lines are used to determine if any key is pressed.

Segment decoder

The desktop QT-8D calculator uses an unusual 9-segment display with curved segments, while the handheld EL-8 used an 8-segment display (omitting segment i, which provided a tail on the 4). These produce curved digits, unlike the blocky 7-segment digits seen in most calculators. The zero is particularly unusual: it is half-height. The calculator doesn't suppress leading zeroes, so the half-height zeros are less obtrusive. (1234, for instance, appears as oooo1234.)

The 9-segment vacuum fluorescent display tube used in the QT-8D calculator. The vertical line down the middle is the heated cathode and the hex mesh is the grid.

The role of the segment decoder is to take a binary value and drive the appropriate segments, labeled a through i. The circuit below is the interesting part of the decoder circuit. The bit values and their complements enter on the right from the shift register. Most of the segments are decoded by AND-NOR gates; an AND-NOR gate consists of several AND terms with the results NOR'd together. An AND-NOR gate is implemented in four-phase logic as a single gate with a separate vertical strip for each AND term. The strips are tied together at the top and bottom so if any strip is activated, the gate is discharged; this provides the NOR action. As a result, the physical structure of the gate maps directly to its logical structure.

Part of the segment decoder circuitry.

The gate for segment f is indicated on the diagram by an arrow. It has two vertical strips, so two AND terms. Studying the transistor connections, this gate implements: bit1 NOR (bit3 AND bit2). Evaluating this expression shows that f will be active for the digits 4, 5, 8, and 9. Looking at the display, you can verify that these are the digits that use segment f. Similar expressions are used to generate the other segments. For instance, segment h has four AND terms.

Segment i is activated by a NOR gate, which has two parallel vertical segments with three transistors in between. If any transistor is activated, it will connect the segments and discharge the gate, providing the NOR action. NOR gates are rare on the chip, probably because they require twice the width of a NAND gate. Segment i is NOR(bit0, bit2, bit1), so it is activated only for the number 4; this segment provides a short tail on the displayed 4.

Decimal point decoding

One of the tasks of this chip is to display the decimal point, which is more complex than you might expect. The decimal point is encoded as a 4-bit value, transmitted serially to the chip. Three bits indicate the position of the decimal point (0 to 7), while the fourth bit enables or disables the decimal point. A shift register (described earlier) converts the serial bits to a 4-bit value. A remarkably complex gate (below) is used to determine when the active digit matches the specified decimal point position. At that time, the decimal point segment is activated, causing the correct decimal point to light up.

A complex gate decodes the decimal point.

The circuit is implemented in four-phase logic as a single gate. The gate can be viewed as an 8-to-1 multiplexer that selects one of the eight digit (D) lines based on the bit value. This gate also includes a latch to hold the multiplexed value. Note that if the digit clock is 0, the AND gate at the bottom will cycle the output value (through an inverter, not shown), holding the value. When the digit clock is 1 (i.e. a digit has been read in), a new value from the multiplexer tree will be read. The branching tree structure is visible in the silicon structures above.

Diagram showing the complex gate that decodes the decimal point.

Other circuits

I won't describe the remainder of the circuits on the chip in detail. They were implemented using similar techniques, in particular shift registers. The keypress is converted to serial data with a latch and shift register, built from AND-NOR gates. The digit scan circuit is also a latch and shift register, with a gate to start a 1 value. This shift register is triggered by teh digit clock, so it shifts every 4 cycles. The circuit that divides the clock by 4 is a shift register to count four cycles.

Conclusion

Although Sharp managed to fit the calculator circuitry onto five chips, it was soon overshadowed by single-chip calculators. In a few years, calculators shrank from the handheld but blocky Sharp EL-8 to credit-card-sized. The calculator market was highly profitable for a short time until the "calculator wars" caused calculator prices to drop from hundreds of dollars to a few dollars. Most of the hundreds of calculator manufacturers left the market, leaving Texas Instruments, Hewlett-Packard, Sharp, and Casio as the dominant manufacturers.

As for four-phase logic, its success peaked in the 1970s. Most notably, the company Four-Phase Systems created a 24-bit desktop computer in 1971 using four-phase logic, and Motorola bought the company in 1982. For the most part, though, microprocessors of the 1970s used static NMOS logic rather than four-phase logic. I haven't been able to find an explanation of why four-phase logic wasn't more widely used. My suspicion is that improvements in semiconductor technology in the early 1970s reduced the benefits of four-phase logic, specifically the introduction of depletion-load NMOS logic.

The Sharp EL-8 calculator. Photo by Daniel Sancho (CC BY 2.0).

I plan to analyze the remaining three calculator chips so follow me on Twitter @kenshirriff for updates. I also have an RSS feed. Thanks to François Gueissaz for doing all the hard work of obtaining the calculator ICs, decapping them, and providing me with die photos and other information.

Notes and references

1. The advantages of four-phase logic are discussed in a talk by Lee Boysel, an early proponent of MOS circuitry and four-phase logic. He founded the company Four-Phase Systems, which build a powerful desktop computer using four-phase logic. His interesting video on MOS history is here. ↩

2. The calculator display uses vacuum fluorescent display (VFD) tubes, developed as a lower-cost alternative to Nixie tubes to avoid paying patent royalties to Burroughs. Nixie tubes are similar to neon bulbs; there are 10 cathodes, each shaped like a digit, and applying 170 volts to a cathode causes the digit to light up with a neon glow.

   The multi-segment VFD was invented in 1967 by Noritake Itron Corp. VFD tubes are vacuum tubes, sort of a cross between a triode and a low-voltage CRT. Unlike the "cold cathode" of Nixie tubes, the VFD's cathode is heated, causing electrons to boil off. These electrons are accelerated toward an anode by applying 25 volts to the anode, and cause a phosphor to light up when they hit the anode. A grid between the cathode and anode controls the electron flow; this is how a single tube is selected for multiplexing. The voltage in a VFD is much lower than a CRT, 25 volts instead of 25,000 volts. Another difference is that a CRT deflects the electron beam with deflection coils to create a pattern on the screen, while the VFD uses individual anodes that light up separately for each segment.

   These Sharp calculators were the first calculators to use VFD tubes. The EL-8 calculator uses eight-segment Itron type DG10L tubes while the QT-8D calculator uses nine-segment DG10B tubes. The driver board has nine driver integrated circuits to interface between the calculator chips and the display tubes. ↩

3. I'm skipping over a bunch of details of the calculator. For instance, some signals are active-high, while others are active-low, and some signals are shifted by half a clock. (The design is optimized to minimize the hardware, rather than being conceptually clean.) In this blog post, I'm describing the concepts of the circuitry rather than the cycle-exact details. ↩

4. I haven't found many publications explaining four-phase logic. One is the article Four-phase logic is practical (1977). The 1969 master's thesis Basic design of MOSFET, four-phase, digital integrated circuits has a lot of information. The book MOS integrated circuits and their applications (1970) has a chapter on four-phase logic. See also Low-power VLSI implementation by NMOS 4-phase dynamic logic, published at the surprisingly late date of 2000.  ↩

5. Note that the gate is powered only by the clock; there are no power or ground connections. Although the four-phase gate are powered through the clock, the chip does have connections for power (-25V) and ground. Power and ground are used by the output pins so they can provide static signals with more substantial current. Ground is also used for the gate capacitors. ↩

6. Most of the classic 1970's microprocessors used a two-phase clock. They used dynamic circuitry, typically for temporary data storage and timing, but the logic was typically static. The Intel 8086 used dynamic logic in a few places, such as the ALU, probably for performance reasons. ↩

7. In most cases, four-phase circuitry alternates between φ1φ2 gates and φ3φ4 gates. A problem arises, however, if one path to a gate has an odd number of gates and another has an even number of gates. The solution is two more types of gates, one that precharges on phase 1 and samples on phase 3, and one that precharges on phase 3 and samples on phase 1. These gates are slower, but can interface between the earlier two types. Thus, four-phase logic has four types of gates, distinguished by the clock phases they use. Following the simple interconnection rules ensures that the circuit operates correctly.

   The four types of four-phase gates are illustrated in A mathematical model characterizing four-phase MOS circuits for logic simulation. (1968) and Four-phase logic is practical (1977). (I'm pretty sure the second article has some errors in Figure 2 though.)

   

   The four types of four-phase gates.From A mathematical model characterizing four-phase MOS circuits for logic simulation.

   Only certain combinations of four-phase gates can be connected. The diagram below shows that, for instance, the output from a type 1 gate can connect to the input of type 2 or type 3. A typical circuit alternates between Type 1 and Type 3. The calculator chip uses a few Type 2 gates and Type 4, for example when an extra inversion is required.

   

   Connections between four-phase gates must satisfy certain rules.

    ↩↩