Ken Shirriff Reverse Engineers a Mystery Comparator Chip — Built with an Unknown Purpose in Mind

Noted reverse engineer Ken Shirriff has turned his attention to what was initially an unknown integrated circuit in attractive metal-and-ceramic packaging, later identified as a vintage comparator — though built for an as-yet unknown purpose.

"I recently saw an interesting die photo of an unknown chip on Twitter, so I did some analysis of it," Shirriff explains by way of introduction. "Much of the die is unused, especially the large metal area in the middle. Because this chip's circuitry is relatively simple, it only uses a fraction of the available space."

"Modern chips are usually in a black epoxy package, but this chip has a white ceramic package. The tiny silicon die is visible in the middle, with bond wires connecting the die to the lead frame, the metal connections to the chip's gold-plated pins."

The ceramic packaging provides a rough era for the part, but even with a high-resolution die shot and the relative simplicity of the circuitry inside identifying the chip's purpose is a challenge. "In this chip, the circuit is used as a comparator, a circuit that compares the two inputs and generates a logic output that indicates which input is higher," Shirriff writes. "The side with the current will get pulled low, while the other side is pulled high by the resistor. Thus, the output can be treated as a logic signal.

"So what is this chip? Maybe it's simply four comparators, but they could have a specific purpose. The chip could be a converter for four differential input signals, e.g. DCS (Differential Current Switch) logic. Another chip in the family seems to be Emitter-Coupled Logic, so this chip could be four ECL inverters (but it doesn't make sense to have four pins for the reference voltage). It's a bit puzzling that two comparators have inverted and non-inverted outputs, while two have single outputs."

The part is from around 1970, Shirriff estimates, and is marked "OQ104" with a "P" which may indicate it's a Philips chip — but neither that markings nor a reverse engineering of the die image reveal its true purpose. "For now," Shirriff concludes, "the exact identity of this chip is a mystery."

The full write-up is available on Shirriff's blog, where anyone with more information about the part is encouraged to leave a comment in order to help solve the mystery.