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12285Workin' on the IMSAI...

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  • Mike Loewen
    Apr 20, 2009
      I've finally had a few more hours to work on the IMSAI, and things are
      looking up. After slowly bringing up the electrolytic capacitors in the
      power supply to their rated voltage (in stages, monitoring the current), I
      left them sit there for several hours. After bleeding off the charge, I
      put everything back together and powered up the machine with only the
      front panel plugged in. No smoke!

      Next, I plugged in the Cromemco ZPU card, the Fulcrum Omniram 64KB
      static RAM card and the Compupro active terminator card. I tried the
      Deposit and Examine switches, but was getting inconsistent results. After
      some online reading, I decided to disable the Power On Jump (POJ) on the
      ZPU card. Now, the Deposit and Examine switches were working as expected.
      I toggled in a short program, found here:


      All it does is move an LED back and forth in the Programmed Outputs
      register on the front panel, Cylon-style. You can change the speed by
      turning on the high address switches. It worked:


      The next step was to insert the Morrow Disk Jockey 2D/B floppy
      controller and connect the dual 8-inch drives and a serial terminal. The
      2D/B has its own POJ logic, which sends the CPU to F800H on reset. No joy
      on booting from diskette, though. From the front panel, I can read the
      jump table at F800H and can follow the program flow. The red LED on the
      2D/B comes on indicating the controller is ready, but the drive select LED
      doesn't pulse as described in the manual. At this point, I ran out of
      time. I'll pick it up later and try to figure out if the drive is

      Mike Loewen mloewen@...
      Old Technology http://sturgeon.css.psu.edu/~mloewen/Oldtech/
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