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14686Re: [cosmacelf] RE: Membership Card Serial Terminal Connection?

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  • Mark G Thomas
    Jan 15, 2014

      On Tue, Jan 14, 2014 at 07:01:35PM -0800, wheagy@... wrote:
      > Lee/Mark,
      > A question about the ROM-stack hand-drawn schematic. In Lee's webpage
      > writeup it says "Install Q1 [on the PC board as usual] to address the
      > RAM at 32k-64k." I though the usual was for 0-32K (connections 1, 2,
      > and3). Is this a difference between how the stack method would work
      > using a second Membership Card as opposed to stacking the EPROM on top
      > of the RAM? I want to be sure the way the hand-drawn schematic shows
      > the transistors is correct?

      Good catch. I believe the original quote from Lee was supposing I wanted
      to address the RAM at 32k-64k and the ROM at 0k-23k, opposite the way I
      ended up doing it.

      Since the elf2k ROM expected to be at 32k-64k, I installed Q1 on the
      PCB in it's default position (pad 1,2,3), then added the second transistor
      to decode the alternately wired ROM, as I roughly drew in the schematic
      on Herb's site.

      > It turns out my EPROM is actually a 28C256. Pins 1 and 27 are swapped
      > compared to the 27C256. I did the swap and tried running the EPROM.
      > It starts counting down and stops the POST at 0x97, which is "EPROM
      > checksum wrong". I've double checked all the connections and am
      > thinking the circuit is correct except for possibly as described above.

      Herb posted some good suggestions about isolating the problem.

      Please double check P2 and P3 are set for their 32k RAM positions (default),
      and the transistor and ROM wiring is correct. It sounds to me like you are
      real close to success.


      Mark G. Thomas (Mark@...)
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