Ken Shirriff Reverse Engineers a Board From the Apollo Saturn V Rocket's Launch Vehicle Computer

Noted reverse engineer Ken Shirriff has taken on a somewhat more unusual project: To determine the operation of an original circuit board taken from the Saturn V rocket used in the Apollo moon missions.

"In the Apollo Moon missions, the Saturn V rocket was guided by an advanced onboard computer system built by IBM," Shirriff explains. "This system was built from hybrid modules, similar to integrated circuits but containing individual components. I reverse engineered a circuit board from this system and determined its function: Inside the computer's I/O unit, the board selected different data sources for the computer."

Shirriff isn't the first to have taken a look at this specific design: "This board was first studied by Fran Blanch in The Apollo Saturn V LVDC Project," he explains. "Then EEVblog made a video about it. Now it's my turn to analyse the board."

While the board under investigation appears to use device that look like integrated circuits, appearances can be deceiving: "ULD [Unit Logic Device] modules contained multiple components," Shirriff writes. "They used simple silicon dies, each implementing just one transistor or two diodes. These dies, along with thick-film printed resistors, were mounted on a .3-inch-square ceramic wafer. These modules were a variant of the SLT (Solid Logic Technology) modules used in IBM's popular S/360 series of computers."

Knowing the core technology is one thing, but with no documentation it's not clear what the circuit board was actually designed to do. "To determine what the board did, I tediously beeped out the connections between chips with a multimeter to create wiring diagrams," Shirriff explains. "(Shortly after I finished, LVDA manuals with schematics turned up making my reverse engineering effort unnecessary.) The board forms a 7-input multiplexer, selecting one of 7 input lines and storing the value in a latch. With 1960s technology, this simple function required a whole board of chips.

"Of the 26 chips on the board, 18 of them were analogue chips that buffered and processed the input signals. The inputs were 28-volt signals, while the logic requires 6-volt signals. Each input (except #7) passes through a "Discrete Interface Circuit" that converts the input to a logic signal."

Shirriff's full write-up, including reverse engineered schematics and a look at the board's specific role in the Launch Vehicle Data Adapter (LVDA), can be found on the Righto.com website.