One simple yet critical circuit is the clock generator.

It must be controlled with the start/step/stop button : it's a SPDT switch with 3 positions, including a middle (no contact) and a temporary position.

So the clock generator must work both in monostable and multivibrator modes...

I already have a 24V electromechanical pulse counter as well as a safe-like rotary knob that generates pulses to help with longer sequences and reduce wear on the SPDT button.

The pulse counter can run at maybe 8 pulses per second, which is 3 times slower than the expected clock frequency. The pulse rate must be limited in this case... a predivider might be necessary.

Initially I wanted to use the mains' frequency (50Hz over here) to "clock" the whole circuit. A dedicated transformer uses a single diode to rectify the 50Hz sine and create 10ms-long half-sine pulses, with 10ms gaps. But 50Hz might be too fast... and it wouldn't be synchronised with external signals (race conditions become possible).

A divide-by-2 circuit is not too hard to make with a few relays. A divide-by-4 predivider is possible too. But apparently this part is more sophisticated than I would think, but I'm not surprised because it was a problem already with the #SPDT16and #YGREC16...

The rotary encoder sends 100 pulses per rotation, and I can use the A or B output at will. It works with 4V/50mA approximately (some spikes/overshoot appears on the output at 5V). I'll try to see if I can gang that output to the pulse counter, through a transistor for example...

Further thinking makes me to ditch the EM pulse counter altogether. It's too slow and will only make things uselessly too complicated.

A much better approach is to use the 5 prototype digits to build the cycle counter, since the predivider is not that hard to build (and can reuse the internal relays) and a 5×4 bits binary ripple counter could be great (ditch decimal !). At least, the counter could work at full speed !

Now, the cycle control unit has :

• One RESET/RUN toggle switch (to clear the state or let the program run)
• 4 push buttons : STOP, STEP, SEQ, START
  * STOP just clears a latch that is set by START
  * SEQ momentarily shorts the latch's output
  * STEP sends only one pulse to nudge the oscillator for one cycle (monostable)
  * STEP and SEQ only work when the latch is in STOP state
• One rotary button (acts like multiple STEP pulses)
• Pulses are counted and displayed with 5 hex digits => another reset button is required.

it's getting crowded now... so I must split it: the pulse counter is relegated to a separate module, as an "event counter" that can be triggered by other modules, such as the breakpoints.