6502.org

I've been studying the schematics of the old machines to learn about how dynamic memory refresh is implemented.

The PET 2001 schematics are available onine, and I'm looking at the main board (part 320349). (The schematic PDF is https://www.zimmers.net/anonftp/pub/cbm ... 320349.pdf)

What drives the #INIT line? It's used all over Sheet #6 to clear counters and flip flops and the clock phase shift register. Clears a flip flop on the display board, Sheet #7. And enables the character ROM and another flip-flop on sheet #8.

Nothing seems to drive it. The #RESET line is developed on Sheet #1 wit ha 555 timer, but #RESET has nothing to do with #INIT.

How is #INIT driven on this machine? Is it just always high? It's pulled high by R12 on Sheet #6, but that's about it.

It's also connected to a test point. It's possible that it's always high under normal operation, but can be pulled low during testing to put the system into a known state. That would require the circuit it's attached to (which looks like video generation) not minding that various flip-flops are in random states at power-up. Which it might not.

I was as confused as you when trying to understand the PET video circuit.
I came to the conclusion that it is just a line pulled up by a resistor (see bottom left of sheet 6 of your linked schematics)
It was probably used to re-init the video circuit during testing via the test point.

André

Edit: here's the video on this PET's graphics circuit if you are interested: https://youtu.be/pwb5GvmoAFE