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30046Re: IMSAI 8080

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  • s100doctor
    Apr 1, 2013
      --- In midatlanticretro@yahoogroups.com, "DanielB" <danlb_2000@...> wrote:
      >

      >
      > Finally made some more progress on the IMSAI. The problem with the Examine switch ended up being a bad 74107 JK Flip Flop on the front panel. I also replaced some of the caps on the one-shots on the front panel. So the examine and deposit seem to be working ok now, but I am still having some flaky behavior, but I am making progress.
      >
      The one-shots are ALWAYS off on the IMSAI and MITS front panels. the caps and resistors and the monostable chips all "drift". There's also any number of "glitch" conditions which cause retriggering, that only a fast (100MHz) oscope can see. Check the Web for discussions about these front-panel one-shots, and check to be sure yours are working within reasonable timing range. replace R's C's and chips as needed.

      The other flaky source, is RAM. RAM and RAM boards "age" with time, and degrade and get soft bits. Try to get that IMSAI to run some aggressive memory test programs, or run them on boards on another system, preferably with a faster processor. If your IMSAI is using a Z80, be aware that Z80 instruction fetch cycles are FASTER than memory reads. 8080'd don't do that. thus you can have a board that passes Z80 read/write tests but fails on running programs! the "worm" program moves itself through memory and displays its memory address, until it fails and of course you 'see' the failure address.

      Herb Johnson
      IMSAI rassler
      retrotechnology.com
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