Reverse-engineering the 1977 Unisonic 21 calculator/game (part 3)

The three signals from the oscillator section, A, B, and C, feed into a clock phase generator section. The three signals are converted into four. Here's the logical diagram:

So how was I able to get the timing of each phase? It turns out that pin 1, not connected on the PCB, is the B_XNOR_C signal. I measured that in relation to the A signal on pin 2, and so all clock phase periods are known.

Annotated clock phase generator section.

Annotated clock phase generator section.

Polygonized. Yellow is metal, black is a contact to diffusion, red is diffusion powered by the high voltage (logical 1, VSS), blue is diffusion powered by the low voltage (logical 0, VDD), green is other diffusion, and pink is oxide. Areas that look gray are actually blue plus yellow.

Polygonized. Yellow is metal, black is a contact to diffusion, red is diffusion powered by the high voltage (logical 1, VSS), blue is diffusion powered by the low voltage (logical 0, VDD), green is other diffusion, and pink is oxide. Areas that look gray are actually blue plus yellow.

Four gates in 25 transistors. See the [PDF] and KiCad schematics at the [Unisonic21 GitHub project].

Now there are two sections on the progress map!