Ken Shirriff Reverse Engineers an Unusual Logic Board from the 1960s' Univac 1004
Originally mistaken as an IBM board, this diode-transistor logic (DTL) board implements a full adder, inverter, and more.
Noted reverse engineer and computer historian Ken Shirriff has turned his attention to a board designed for the Univac 1004 in the 1960s — identifying the unusual component as an AND-OR-INVERT logic board with some unknown features.
"The IBM 1401 team at the Computer History Museum accumulates a lot of mystery components from donations and other sources," Shirriff explains of the origins of his latest board. "While going through a box, we came across [this] unusual circuit board.
"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. However, this board is larger, has double-sided wiring, the connector is different, and the labeling is different."
The board, which has an edge connector to one side, was identified by Robert Garner as a plug-in for the Univac 1004 plugboard-based "Card Processor" — capable of running programs of up to 31 steps in length, "coded" through the use of a tangle of wires on a plugboard.
"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," Shirriff writes. "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 transistor."
The result of careful analysis: The identification of the board as an AND-OR-INVERT logic board, implemented in diode-transistor logic (DTL). "That is, various inputs are ANDed together, the AND results are then ORed together, and finally the result is inverted," Shirriff explains.
"After tracing out the board's logic," he continues, "I recognized that it implemented a full adder. 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. Without knowing more about the circuitry, I can't determine the role of these circuits."
Shirrif's full write-up, complete with schematics, is available on his website.