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32133Re: The Demo setup for MARCH's Altair- current status

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  • Mike
    Aug 18, 2013
      Easiest way to check out ROM, is to put a short routine in RAM that reads a location in ROM (like the first) and then jumps back to read again. Basically two instructions. You can then scope out the signals (best to use an extender card, if you have one).

      If it seems to be working, you can expand the routine a bit to compare the read value to a constant and repeat. If it occasionally fails, you can trigger a scope or logic analyzer by reading a different (trigger) location when the failure occurs. This amounts to about 6 instructions.

      regards,
      Mike W.

      --- In midatlanticretro@yahoogroups.com, corey986 <no_reply@...> wrote:
      >
      > Ok so some of you know I'm slowly working my way though the home brew computer room at MARCH making sure we have something to show for each machine.
      >
      > I've been working on what to have the altair do and show. Evan wants it to be pretty early 1975 which is a great idea to show people what could be done with this giant box before anyone got crafty enough to hook then up to teletypes. So I've been limiting myself to 256bytes and front panel only.
      >
      > Now the Altair had a PT 1702a ROM card which would help in having these demos easily available, but it had some damage to the power circuit that Systems Glitch is going to fix, so until then I was going to use a 32k bytesaver that MARCH owns. I was going to limit myself to 2k (the max of the PT card). I already created a rom image that runs Steve D's fool on the hill and daisy in a giant loop, kill the rotating bit and a small program that runs up and down the address lights continually. I'll add more later.
      >
      > The problem I am having is this darn 32k bytesaver. First I did have to repair the finger on the 12v line. It had partially delaminated from the pcb and was folded over itself. While it still made contact it could potentially damage the s100 connector.
      >
      > Next I tested each IC in the leaper tester and replaced any questionable ones.
      >
      > I set the address switches and checked for shorts and put the card in my sol-20 since it had a crowbar circuit just in case. Well no luck !!!!
      >
      > Ok then I took the card out and ESR metered the caps. Two 10mf 20v ones could not keep a steady reading. Usually only big caps have that problem (not really a problem for big ones) but for little ones I think this may be an issue so I changed them. still nothing...
      >
      > Next up I may have to put it in my altair which had a front panel so I can step though accessing the EPROM and use my scope to check.
      >
      > Worst case I'll give up if by mid week I can't solve this as I don't want to spend hours on it, the backup plan is to lend MARCH my 8k bytesaver with some bent up pins on the 2716 so it thinks its a 2708. I'll just have to be careful with it as its a perfect example that a paid a ton for becuase it has all box, docs and everything, my other bytesaver isn't built (still in the parts baggies)
      >
      > Anyway... Just keeping everyone in the loop.
      >
      > Cheers,
      > Corey
      >
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