On 2020-Oct-04, at 1:22 AM, Josh Dersch via cctalk wrote: I have run across TI ICs from that era with odd/unknown series numbering, in particular the SN3900 and SN4500 DTL ICs. Notably: - by pinout they match up with standard DTL series ICs, - I have only found these in equipment from one manufacturer: calculators built by Canon. I received a solitary page of datasheet for some of them (by way of a Canon service center many years ago), but I have never seen them mentioned in TI databooks from the era, even in those sections where they list e.g. "other products from TI" and proceed to list little known series and part numbers. So an obvious guess is these were house numbering systems of standard parts done for the purchaser/equipment manufacturer but with TI's format scheme rather than a format specified by the manufacturer. Another guess would be standard parts tested and selected for purchaser-specified parameters, although that seems a little excessive for these cases. The 54/7400 series originated with TI in 65, I'm not aware of them producing any other TTL series, other than perhaps second-sourcing some other manufacturer's. I guess that's another possibility - another manuf's TTL series, labeled differently. Odd that this Motorola CPU is filled with ICs manufactured by TI. It's conceivable, although it seems less probable, that they're DTL rather than TTL. On the whole, best guess would seem to be 7400-series inside. -- On another issue, did you trace the +/-15V lines to the core address/inhibit drivers? Could some of the remaining wires from the PS be other core supplies - 15V was a little low compared to most core systems I've seen.