48 minutes ago, picosecond said:
Yes, TTL and CMOS have different specifications for low and high voltage levels. Let's assume a minimum VCC of 4.5V for this example.
For the F521, a "1" at the output means a voltage greater than 2.5V. This is the VOH(min) parameter in the datasheet.
For the AC11032, a "1" at the input is guaranteed to be recognized only if the voltage is 3.15V or greater. This is the VIH(min) parameter in the datasheet.
So a perfectly valid "1" for the F521 could be wrongly interpreted as a "0" by the AC11032. Obviously that would be bad. If you ever wondered why there are both AC and ACT logic families this is the reason. ACT input voltage thresholds are shifted downwards to be compatible with TTL output voltages.
This article has a more complete explanation: https://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/
As an aside, in your last schematic you could connect RW to that unused 'F521 input and save some glue logic.
I think I was under the assumption that eventually the signal levels would reach 0 or 5v and that'd be totally okay given the amount of time, of course this would be predicated on stable power, right? I'll definitely look into alternatives, but finding low propagation delay logic gates has been somewhat challenging.
As for running RW through the grounded input on the low address range F521, it's definitely a good idea, and would work, but I'm using the CLKWR signal out of that AND gate for write enable on the ram chips anyways. I don't think it's required, but I do have the time and wanted to try to make sure all address and select logic is taking place on low clock, with writes only occurring during high clock. I got lazy and didn't give the same consideration for reads, but meh, not really necessary, at least as far as I can tell. Trying to learn, the best I can, from my understanding of the mistakes Adrian pointed out on the 2nd proto board with timing.
As an aside, I just updated my PCB layout and seen all the logic IC's and their associated rats nests drop into my, so far, beautifully laid out board and traces. I admit, I leaned back in my chair and started wondering why in the hell I'm trying to do this all through hole lol. I may just scrap my progress and switch to SMD. If I can figure out how to program the ROM while it's soldered onto the board I may switch to SMD and try to shoot for some faster RAM/ROM chips and maybe shoot for 12mhz.... Oh I think I could use a clock divider and leverage another flipflop in the IO area to change the speed on the fly. I think that'd be kind of fun.