Ken Shirriff Tackles Reverse Engineering a Diode-Transistor Logic Apollo Computer Buffer Unit

Noted reverse engineer Ken Shirriff has taken a crack at another piece of classic hardware associated with the Apollo space program: a transistorized shift register box, originally built in 1965.

Last year Shirriff, who makes a hobby of figuring out how vintage and esoteric hardware works, turned his attention to a circuit board of unknown purpose taken from the Apollo Saturn V rocket's Launch Vehicle Computer. His latest piece of space history, though, is less mysterious: A 16-bit shift register dubbed the Computer Buffer Unit.

"[It] interfaced test equipment to the Apollo Guidance Computer," Shirriff explains of the grey box's purpose. "While a shift register is a trivial circuit nowadays, back then it took a box full of transistors that weighed about 5 pounds."

As well as diving into the history of the guidance computer and the minicomputer-driven Acceptance Checkout Equipment-Spacecraft (ACE-S/C) control room built specifically for testing it, Shirriff has dived into Computer Buffer Unit's innards — revealing six boards wired into a vertical backplane.

"The circuit boards can be opened up like a book to provide access to the inner boards," Shirriff notes. "The boards are not soldered directly to the backplane, but are connected by short, flexible wires, allowing them to swing apart. To prevent short circuits between the boards, they are separated by white sheets of (probably) silicone."

"The circuitry is constructed in a very unusual way that I haven't seen before. Instead of mounting components directly on the circuit boards, components are mounted on small boards, each forming a module with a logic gate or two. These smaller modules are then soldered on pins above the main circuit boards, forming two-layer boards. Essentially they built pseudo-integrated-circuits on small boards, and then constructed the circuitry from these modules."

These "pseudo-ICs," Shirriff explains, are an example of diode-transistor logic, or DTL — a 60s solution to the high cost of the transistors required for the better-known transistor-transistor logic (TTL) approach. Each module has a purpose: Input inverters, match gates, latch gates, secondary latches, a load driver, a shift driver, and a clock driver. All put together, and you have a bulky but entirely functional shift register.

"This box also illustrates the rapid pace of integrated circuit technology since the 1960s," says Shirriff. "The first commercial MOS integrated circuit was a 20-bit shift register introduced in 1964 and by 1970, Intel was producing a 512-bit shift register. In 1971, Western Digital was selling a UART chip, putting a complete parallel-to-serial and serial-to-parallel communication system onto a chip. Thus, it took 6 years to shrink the complex shift-register box down to a single chip (more or less)."

The full write-up is now available on Shirriff's blog.