(Topic ID: 152320)

Williams System 7 reset controller

By falco

8 years ago



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  • 2 Pinsiders participating
  • Latest reply 8 years ago by falco
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#1 8 years ago

Hi all - just a question to clarify my understanding of how the reset controller on a System 7 works. It takes two power supply rails, +12 unreg (via Q9) and +5 reg (via Q8). Both of those have to be good in order for reset to be high. But... why? +12 isn't used for anything else on the board, as far as I can see. And +5 reg is derived directly from +12 unreg in the PSU. So - why does reset depend on them both, rather than just +5?

Confused!

#3 8 years ago

Right, I see. I understand the problem they were trying to get around, but basing reset on +12 unreg seems an odd way to do it, rather than being more careful around +5v monitoring. Hard to guess what was in peoples' minds with ancient designs! At least I know now that my assumptions are correct. I have a machine here that won't boot because reset won't go high, but it goes OK on the bench. But, my bench supply is about +14 on unreg +12, whereas in the machine it's very close to 12v. Maybe C10 on the PSU has gone a bit weak and there's too much ripple to keep reset happy. Could be that's exactly the situation that the reset circuit was designed to avoid. Will need to have a closer look...

#5 8 years ago

Yep, sure, I realize that. It just seems wiser to monitor the actual supply that's critical, rather than the noisy, unregulated source. In this case, it looks to me like unreg +12 could actually drop to maybe a stable 7V without having any effect on regulated +5. It may not be ideal, but taking the MPU down because of that seems like the wrong thing to do - the MPU only cares about regulated +5. If I were designing it I'd do it such that it didn't care how the +5 came about, so long as it doesn't drop below the threshold required for stable running. Of course, that's what a lot of later machines did, with quite a similar regulation scheme in the PSU. This particular machine will be an interesting test case.

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