Energizing a vacuum-tube flip-flop module from a 1948 IBM system
Energizing a vacuum-tube flip-flop module from a 1948 IBM system
In 1948, IBM introduced the 604 Electronic Calculating Punch.<br>This machine was a programmable calculator, about the size of a double refrigerator.<br>It was not quite a computer, but was programmed by plugging wires into a plugboard.<br>This machine read numbers from a punch card, performed up to 60 calculations on these numbers,<br>and then recorded the results by punching holes in the card.1<br>It processed 100 cards per minute—over one card per second—and IBM advertised it<br>as the equivalent of 150 engineers.<br>The machine rented for $550 a month, making it very popular, with over 5600 units produced.2
The IBM 604 Electronic Calculating Punch. Photo from Ed Thelen's IBM 604 page.
The IBM 604 came out just after the transistor was invented, too early to use transistors.<br>At the time, calculators and computers were<br>moving from slow electromechanical components to fast vacuum tubes.<br>One of the innovations of the 604 was to combine a vacuum tube and its associated circuitry into a<br>pluggable module.<br>Along the left side of the photo above, you can see rows of these modules with the handles sticking out,<br>making it easy to replace a faulty module.<br>More modules are behind the silver metal covers.<br>In total, the IBM 604 used about 1300 vacuum tubes.
The photo below shows a pluggable tube module, with a vacuum tube underneath the insulated handle.<br>The nine pins at the bottom of the module plugged into a socket in the 604, with the sockets connected by backplane wiring.<br>The vacuum tube was also socketed, so a bad tube could be quickly replaced. At the left, the resistors and capacitors are mounted<br>on insulating wafers.<br>Modules provided a dense way to implement circuits, packing components into three dimensions.
The TR-3 trigger module from the IBM 604 Electronic Calculating Punch.
Each pluggable tube module implemented a specific function, such as an inverter, amplifier, or power driver.<br>The module above is a "trigger" module, type TR-3.<br>A trigger is a circuit with two states—on and off—and can be<br>switched from one state to the other, providing one bit of temporary storage.<br>(In modern terminology, this is called a flip-flop.)<br>Triggers were important building blocks in the 604,<br>generating timing signals and storing pulses.<br>Arithmetic in the IBM 604 was implemented with decimal counters, built from TR3 triggers.
In this article, I describe the circuitry of the TR-3 trigger module.<br>(I recently wrote about a thyratron module in the 604; this is a different module.)<br>After reverse-engineering the module, I powered it up.<br>The video above shows the module in operation. By pressing buttons, I switch the trigger from<br>one state to the other. Glowing orange neon bulbs show the state of the trigger module.<br>The fundamental feature of the trigger is that it stays in a state until I push the other button.<br>This might appear trivial, but the ability to store information is vitally important for<br>computation.
How a vacuum tube works
The trigger module uses a common type of vacuum tube called a triode, which amplifies a<br>weak signal to control a stronger signal.<br>The diagram below shows the construction of a triode vacuum tube.<br>The heater is a filament, similar to an incandescent light bulb, that heats the cathode to roughly 750 ºC.<br>At this high temperature, the cathode emits electrons.<br>If a large positive voltage (say, 150 volts) is put on the plate, the negatively charged electrons are attracted to the plate.<br>The stream of electrons from the cathode to the plate causes a current to flow through the tube.<br>Since air would block the electrons, the fragile glass envelope holds a vacuum, giving the<br>vacuum tube its name.<br>The current is controlled by the grid: if a small negative voltage is placed on the grid, it repels the negative<br>electrons, preventing them from reaching the plate and blocking the current through the tube.3<br>Thus, a small signal on the grid controls the large current through the tube.
The components of a triode vacuum tube. From IBM 604 Customer Engineering manual.
The advantage of vacuum tubes was that they could switch on and off millions of times per<br>second, phenomenally faster than electromechanical devices such as relays.<br>The clock speed of the IBM 604 was 50 kilohertz, much below what a tube could handle,<br>but three orders of magnitude faster than the 50 hertz pulses in an electromechanical<br>accounting machine like the contemporaneous IBM 407.
The tube that I used in the module is called a 2033.6<br>This tube is a dual triode, combining two triodes into one physical glass tube.<br>Dual triodes were very popular because they doubled the density of the circuitry.<br>In the photo below, the two vertical black structures are the plates of the<br>two triodes; the other structures are not visible as they are inside the plates.
The 2033 dual-triode vacuum tube.
This tube is a "miniature" vacuum tube, about 5 cm...