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Re: [cosmacelf] help with 4K board

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  • Richard
    Cancel previous tip, After a Re Read of the message I discovered that It is not Bit 7 that is the problem. But Bit 1. 7A = 01111010 78 = 01111000 Bits=76543210
    Message 1 of 10 , Jun 27, 2007
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      Cancel previous tip,
      After a Re Read of the message I discovered that It is not Bit 7 that is the
      problem.
      But Bit 1.
      7A = 01111010
      78 = 01111000
      Bits=76543210

      I would start by removing, A11,19,27,35 and A8 on the 4k board.
      and checking the results, if the bit is still low, you have a short on
      this the D1(58) line.
      if the result is 7F 3F then add the chips back one at a time to see which is
      locking it low, Starting with A8

      Richard
    • glendroger
      OK... I ve gone over this with a magnifying glass, tracing everything. I m desparately avoiding desoldering the chips to avoid the mess. I just noticed
      Message 2 of 10 , Jul 7, 2007
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        OK... I've gone over this with a magnifying glass, tracing everything.
        I'm desparately avoiding desoldering the chips to avoid the mess. I
        just noticed something new...

        Press FF and you get FD... for the first 8 times. Then you get FF...
        for the next 8 times!!! And so on.

        So
        00 to 07 - bit D01 is forced low
        08 to 0F - bit D01 is fine
        10 to 17 - bit D01 is forced low
        18 to 1F - bit D01 is fine

        When I type in C0 FO 00 and try to run the monitor with the 4K board
        in it won't run so I can't check to see what is happening upwards in
        memory.

        Does that make sense to anyone?

        Thanks!

        Glen

        --- In cosmacelf@yahoogroups.com, "Richard" <stateamuse@...> wrote:
        >
        > Cancel previous tip,
        > After a Re Read of the message I discovered that It is not Bit 7
        that is the
        > problem.
        > But Bit 1.
        > 7A = 01111010
        > 78 = 01111000
        > Bits=76543210
        >
        > I would start by removing, A11,19,27,35 and A8 on the 4k board.
        > and checking the results, if the bit is still low, you have a short on
        > this the D1(58) line.
        > if the result is 7F 3F then add the chips back one at a time to see
        which is
        > locking it low, Starting with A8
        >
        > Richard
        >
      • randy129
        ... I m not familiar with that particular board, but it sounds like A0 is shorted to D1. Randy
        Message 3 of 10 , Jul 8, 2007
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          --- "glendroger" <groger@...> wrote:

          > just noticed something new...
          >
          > Press FF and you get FD... for the first 8 times. Then you get FF...
          > for the next 8 times!!! And so on.

          I'm not familiar with that particular board, but it
          sounds like A0 is shorted to D1.

          Randy
        • randy129
          ... oops, I mean A3 shorted to D1. Randy
          Message 4 of 10 , Jul 8, 2007
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            > I'm not familiar with that particular board, but it
            > sounds like A0 is shorted to D1.

            oops, I mean A3 shorted to D1.

            Randy
          • Steve Valin
            Glen, I have seen this on both of my unexpanded ELF II boards. The culprit in my case was a flakey 2101 SRAM. It had a weak driver on one of it s I/O pins. I
            Message 5 of 10 , Jul 8, 2007
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              Glen,

              I have seen this on both of my unexpanded ELF II
              boards. The culprit in my case was a flakey 2101
              SRAM. It had a weak driver on one of it's I/O pins.
              I had to use a scope to see what was going on.
              Basically, when you load a value into the SRAM using
              the input switch, it will write the value to the SRAM,
              then turn around and read it back out, which is
              latched to the displays. I replaced the flakey 2101
              parts and the problem went away. You could have a bad
              SRAM. I would try swapping your SRAM around to see if
              the problem follows the SRAM or stays the same.

              -steve

              --- glendroger <groger@...> wrote:

              > OK... I've gone over this with a magnifying glass,
              > tracing everything.
              > I'm desparately avoiding desoldering the chips to
              > avoid the mess. I
              > just noticed something new...
              >
              > Press FF and you get FD... for the first 8 times.
              > Then you get FF...
              > for the next 8 times!!! And so on.
              >
              > So
              > 00 to 07 - bit D01 is forced low
              > 08 to 0F - bit D01 is fine
              > 10 to 17 - bit D01 is forced low
              > 18 to 1F - bit D01 is fine
              >
              > When I type in C0 FO 00 and try to run the monitor
              > with the 4K board
              > in it won't run so I can't check to see what is
              > happening upwards in
              > memory.
              >
              > Does that make sense to anyone?
              >
              > Thanks!
              >
              > Glen
              >
              > --- In cosmacelf@yahoogroups.com, "Richard"
              > <stateamuse@...> wrote:
              > >
              > > Cancel previous tip,
              > > After a Re Read of the message I discovered that
              > It is not Bit 7
              > that is the
              > > problem.
              > > But Bit 1.
              > > 7A = 01111010
              > > 78 = 01111000
              > > Bits=76543210
              > >
              > > I would start by removing, A11,19,27,35 and A8 on
              > the 4k board.
              > > and checking the results, if the bit is still low,
              > you have a short on
              > > this the D1(58) line.
              > > if the result is 7F 3F then add the chips back one
              > at a time to see
              > which is
              > > locking it low, Starting with A8
              > >
              > > Richard
              > >
              >
              >
              >



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