Logic Gates

The entire vacuum tube computer is essentially built from collections of logic gates. OR and NOR gates are used the most extensively, however there are a few instances in which NAND gates are also used. AND, XOR and XNOR are built from combinations of OR, NOR and NAND. The primary driving force behind mostly using OR and NOR gates is that they can be quickly and easily built using diodes.

Solid State vs. Hollow State

The entire design of the computer could be built using either vacuum tube diodes or semiconductor diodes. Semiconductor diodes were used due to their small size and price, helping to reduce total tube count. However, while this may seem a little like cheating at first, semiconductor diodes, notably germanium diodes, were used extensively in vacuum tube computers through the late 1950s and early 1960s. The biggest inspiration when designing the computer was the IBM 604 Customer Engineering manual. Here is an extract from that manual talking about semiconductor diodes.

Inverting Amplifier

The inverting amplifier is the cornerstone of the entire build. It is based very heavily on the basic inverting amplifier laid out by IBM in the IBM 604 Customer Engineering manual. However, the vacuum tube computer is running at considerably less voltage (+24V and -12V), which required the design to be slightly tweaked.

OR and NOR Gate

A basic OR gate can be built using just diodes. Connect the cathodes together, and the anodes are used for the input signals. By combining a basic diode OR gate with the fundamental inverting amplifier shown above, a very simple, low voltage NOR gate can be built. NOR gates are also universal logic gates, meaning any other logic gate can be created using a combination of NOR gates. This is a technique that is employed very heavily in the vacuum tube computer design.

NAND and AND Gate

A diode resistor logic AND gate can be built by connecting the anodes of the diodes together, and using the cathodes as the input. It should be noted that each input needs a pull-down resistor to negative 12V, and the anodes need a pull-up resistor to +24V. The output is then pulled from the anodes. However, the pull-up resistor must be very large for the AND gate to generate acceptable signals, which unfortunately makes the output incredibly high impedance (meaning it supplies extremely small amounts of current). As such, a diode resistor AND gate should be paired with an inverting amplifier to not only restore the signal integrity but also provide a lower impedance output. This design also creates a NAND gate, which like the NOR gate mentioned above, is also a universal logic gate.

Buffer

The output from our standard inverting amplifier is moderately high impedance in and of itself, which can present problems when one output must connect to many different inputs. This is an issue known as "fanout." In instances where we need a very low impedance output signal that can supply very large numbers of inputs without losing signal integrity, a cathode follower can be used. The output is pulled from the cathode, which ultimately means that the signal is not inverted. However, with such a small plate resistor, the output impedance is only limited by the internal resistance of the tube, which is many orders of magnitude smaller than the 33k plate resistor used on the inverting amplifier.