IBM's vacuum tube computers of the 1950s were built from pluggable modules, each holding eight tubes and the associated components. I recently came across one of these modules so I studied its circuitry. This particular module implements five contact debouncing circuits, used to clean up input from a key or relay. When you press a key, the metal contacts tend to bounce a bit before closing, so you end up with multiple open/closed signals, rather than a nice, clean signal. The signal needs to be "debounced" to remove the extra transitions before being processed by a computer. (Perhaps you have used a cheap keyboard that sometimes gives you duplicated letters; excessive key bounce causes this.)

Front of an IBM tube module. This module contains five key debouncing circuits.

The module is apparently from an IBM 705, a powerful business computer introduced in 1954.1 The 705 was a big computer, weighing 16 tons. It contained 1700 vacuum tubes and consumed 70 kilowatts of power, with 40 tons2 of air conditioning to take away the heat from the tubes. A typical system cost $1,640,000 ($15 million in 2017 dollars), but was normally rented monthly for $33,500 ($300,000 in 2017 dollars). A few dozen 705 systems were built, mostly used by large companies and the US government. 8

The CPU of an IBM 705 was a massive unit. Photo from Farmers Insurance , showing their first computer.

IBM introduced multi-tube pluggable modules in 1953 as part of the 700 series of computers.3 Pluggable modules were an innovation that simplified manufacturing and maintenance, as well as providing a way to pack circuitry densely. Computers were built from hundreds of pluggable modules of many different types. The photo below shows how the 8-tube modules were packed in the IBM 709.

Closeup of tube modules in the IBM 709. Note that the modules are installed "sideways" with the tubes horizontal.

IBM hoped that they could manufacture a small set of common pluggable modules, but this didn't work out since the computers required many different types of modules. Instead, IBM settled for modules that were built from standardized design rules and circuits. The main circuit classes were the inverter, the flip flop (which IBM called a "trigger"), diode logic,6 the multivibrator (pulse generator), and the cathode follower (buffer).75 Unfortunately even this level of standardization didn't work too well. For instance, they ended up with dozens of different types of inverters for special cases, everything from an "Open-filament neon inverter" to a "Line capacity yanker". Thus, each computer ended up with hundreds of different types of tube modules, built from a multitude of circuits.4

A single tube module could contain a couple dozen of these simple circuits. You might wonder how eight tubes could support so many circuits, but there were two factors. First, the tubes were typically dual triodes in one package, so the module had the equivalent of 16 simple tubes. Also, AND and OR logic gates were implemented in the module with compact semiconductor diodes, so complex Boolean logic could be implemented almost for free.

I've seen claims that an 8-tube module represents an 8-bit byte, but that's not the case. The eight tubes don't generally map onto eight of anything since functions can take more or less than one tube. For instance, the module I examined contains five circuits. I have another module that stored three bits. Also, the byte wasn't a thing back then. These computers used 36-bit words (scientific models), or 6-bit characters (business models) so 8 bits was irrelevant.

Vacuum tubes

This module was built from a type of vacuum tube known as a triode. In a triode, electrons flow from the cathode to the plate (or anode), under the control of the grid. The heater, similar to a light bulb filament, heats the cathode to around 1000°F, causing electrons to "boil" off the cathode. The anode has a large positive voltage (e.g. +140V), which attracts the negatively-charged electrons. The grid is placed between the cathode and the anode. If the grid is negative, it repels the electrons, blocking the flow to the plate. Thus, the triode can act as a switch, with the grid turning on and off the flow of electrons. The module I examined used dual triode tubes, combining two triodes into one tube for compactness. 9

Schematic symbol for a triode tube

Vacuum tubes required inconveniently large voltages; this module used -130V. -60V, +140V and 6.3VAC. Tubes also had high power consumption—note the many large (1 watt) resistors in the module. The filament was hot enough to glow, using a couple watts per tube. In total, each tube module probably used dozens of watts of power.

A 5965 vacuum tube from the module. The pins plug into a socket on top of the tube module. This tube is a dual-triode; the two plate structures are visible.

The module's circuitry

I closely examined the tube module and found that it consisted of five copies of the same circuit. I traced out one of these circuits, which was a bit inconvenient because the module has components on both the front and the back. I came across a manual of 700-series circuits, and found a circuit that was an exact match, down to the values of the resistors. The circuit is called a "Contact-Operated Trigger" and was used to interface a mechanical input (key, relay, or cam) to electronic circuits.1

Schematic of one "trigger" circuit of the tube module.

The schematic above shows one of the trigger stages. The basic idea of this circuit is a resistor-capacitor filter (left) smooths out the input, removing any short glitches. This signal goes through two inverter circuits, creating a sharp output. Both inverters are part of the same 6211 tube and are wired as a Schmitt trigger.12 To understand how the inverter works, when the input (pin 2) is high, the current through the tube pulls the plate (pin 1) low. Conversely, when the input is low, the electron flow is blocked and the resistors pull the plate high.10 The output from the first inverter (plate, pin 1) is fed into the second inverter (grid, pin 7). This inverter works similarly, with its output on pin 6. The final output is buffered by a "cathode follower" circuit (not shown) to drive other modules. It also drives a neon bulb for a status indicator,

The tube module showing how the five debounce triggers are divided among the tubes.

The tube module contains five of the above debounce circuits. The diagram above shows how these circuits map onto the eight tubes. Each cathode follower (CF) uses half a tube so one tube implements two cathode followers.11

One amusing component I found in the tube module was "Vitamin Q" capacitors. Presumably this is related to the quality factor or Q factor in radio, and these capacitors were designed to improve the Q.

A Vitamin Q capacitor in an IBM 705 tube module.

A brief history of vacuum tube computing, with a focus on IBM

It took longer to get from the invention of the vacuum tube to the development of electronic computer than you might expect. The triode vacuum tube was invented in 1907 and started being used for radio in 1912. The vacuum tube flip-flop, able to store one bit of data was invented by Eccles and Jordan in 1918, but it wasn't until about 1937 that flip flops were connected together into binary counters.

IBM started investigating vacuum tube counters for calculations in 1941 and built an experimental vacuum tube multiplier in 1942. In 1948, IBM introduced the IBM 604 "Electronic Calculating Punch", which implemented high-speed arithmetic operations for punch-card systems by using vacuum tubes. It was IBM's first product with pluggable tube units, using simple modules with one or two vacuum tubes in each module (as seen below).14

Meanwhile, vacuum tube computers were being invented, with Atanasoff's linear equation solver built in 1942 and the Colossus codebreaking system in 1943. But it wasn't until the programmable ENIAC computer was announced in 1946 that the vacuum tube computer revolution really started, leading to the development of numerous computers by the early 1950s.

In 1952, announced the 701, IBM's first commercial scientific computer. The 701 introduced multi-tube pluggable modules, the focus of this article. These modules were used in the 700-series computers until they were obsoleted in 1958 by IBM's transistorized 7000-series computers, which used germanium transistors. The transistorized computers were built from printed circuit boards the size of a playing card, called SMS cards.13 Thus, although vacuum tube computers were highly influential, their lifespan was short. (Transistorized computers also had a short lifespan of a bit over a decade. Integrated circuits, on the other hand, have been powering computers for more than 50 years, with no signs of being replaced.)

A tube module implements more logic than an SMS card, but the cards are much smaller and use less power. Photo shows a " DGT " SMS card implementing four AND gates on top of a different tube module.

How the modules are built

One important advantage of the tube module is the circuitry is somewhat three-dimensional, compared to components soldered on a circuit board. This allows the circuitry to more efficiently fill the volume of the computer. The module has four layers of contact boards with terminal strips attached, allowing four vertical layers of components (resistors, capacitors, etc) to be soldered to the terminal strips.5 The solder terminals are connected together by thin horizontal metal strips, which can be cut. Thus, neighboring tabs can be electrically connected or not, as required by the circuit. Jumper wires are used to provide additional connections as needed.

The module has four levels of components between the tubes and the connector. In addition, there are components on the front and back of the module.

The tube module has 64 metal contact tabs at the bottom, providing connections for power and signals. The tube module doesn't simply plug into the socket as you might expect, but uses a two step mechanism. First, the module is placed in the socket but the pins don't make contact. You may have noticed the rod sticking up from the top of the module. To lock the module into place, the rod is rotated 180 degrees with a special tool, causing a circular cam at the bottom of the module to rotate. The force of the cam against the socket causes the module to slide sideways so the pins mesh with the socket contacts. This reduces the force required to insert the module and minimizes the risk of the module coming loose.

The module has 64 metal tabs that plug into the socket. The circular cam (rusted) rotates to slide the module (and the tabs) sideways into the socket.

The contacts are carefully positioned to establish the ground connection first, then the negative-voltage connections and finally the positive connections. This allows hot-swapping, replacing a tube module while the system is powered up. (The manual does say, "However, it is wise to drop DC voltages before removing or installing pluggable units.") Interestingly, USB connectors use the same idea. If you look at a USB connector, you'll see that the middle two contacts (data) are shorter than the outside contacts (power and ground), so power and ground connections are established first.

Patent 2,754,454 describes the tube module mechanism in detail.

The tube module has locking tabs that holds the module into place after it slides sideways. I noticed that these tabs are broken off on the module I examined; compare with the intact module below. Probably the rusted cam couldn't be moved, so the module had to be forced out of the socket, breaking the tabs.

When the tube module slides sideways into the connectors, the locking tabs engage to keep it from being pulled out. In this tube module, the locking tabs are broken off.

More evidence of the module's hard life is there are at least four resistors missing from the module, and two resistors with broken solder joints. (This made it considerably harder to figure out what the module did.) Positions where two components are soldered to the same tabs seem especially prone to having a component knocked off. The photo below illustrates one of the missing resistors; one trigger circuit has a 91K (white-brown-orange) resistor soldered across the capacitor, while it is missing in the second circuit.

The arrow shows one of the missing resistors in the tube module.

Conclusion

The tube module is an interesting artifact from the era of vacuum tube computers. Studying it revealed its function, five contact debouncing circuits for use with keys or relays. Part II of this blog post will describe powering up the module and getting it to work. For a sneak preview, you can watch CuriousMarc's video:

Thanks to Carl Claunch for providing the module.

Follow me on Twitter or RSS to find out about my latest blog posts.

Notes and references