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re: [midatlanticretro] PDP-11/20

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  • B. Degnan
    OK/mental note taken. ... slightly ... could ... Deposit ... returned ... zero ... consecutive ... wasn t ... removed.
    Message 1 of 3 , May 27, 2013
      OK/mental note taken.

      -------- Original Message --------
      > From: "David Gesswein" <djg@...>
      > Sent: Monday, May 27, 2013 11:40 AM
      > To: midatlanticretro@yahoogroups.com
      > Subject: [midatlanticretro] PDP-11/20
      >
      > Posting this so next person who looks at the 11/20 will know its state.
      >
      > I did a little testing on it on the last day. I checked the voltages
      > on the paddle boards and 5V was slightly less than 4.8 and -15 was
      slightly
      > over -15. I didn't find the right document or section looking at what I
      could
      > find with my laptop to know what the other voltages on the little power
      > paddle boards should be.
      >
      > The load address switch lit up the address the switches were set to.
      Deposit
      > set the data lights to the switches set. Reading the address back
      returned
      > all zeros. I didn't know what address the core was at. I tried address
      zero
      > and addresses like 0x8000, 0xf000. Some of the upper addresses strange
      > things happened when I read such as the address jumping to non
      consecutive
      > addresses or run light coming on. The low addresses didn't do this.
      >
      > Looking in through the side the first couple of cards were being pressed
      > by the foam. The foam was stiff but the amount they were pressed in
      wasn't
      > that far. The later cards didn't seem to be pressed at all. I didn't see
      > anything obvious indicating the foam caused the problem but it was
      removed.
      >
      >
      > ------------------------------------
      >
      > Yahoo! Groups Links
      >
      >
      >
    • s100doctor
      ... I looked over David s shoulder at the work he reported and got similar impressions. I was concerned that the foam - actually more like rubber anti-static
      Message 2 of 3 , May 28, 2013
        --- In midatlanticretro@yahoogroups.com, David Gesswein <djg@...> wrote:

        > Looking in through the side the first couple of cards were being pressed
        > by the foam. The foam was stiff but the amount they were pressed in wasn't
        > that far. The later cards didn't seem to be pressed at all. I didn't see
        > anything obvious indicating the foam caused the problem but it was removed.
        >

        I looked over David's shoulder at the work he reported and got similar impressions. I was concerned that the "foam" - actually more like rubber anti-static matting which is heavier - was so unyielding, that it pushed some of the one-width cards sideways out of position or contact. The two-width and four-width cards can't be pushed "sideways" in that fashion. I believe I suggested removing that material; I saw it was removed in any case.

        The 11/20 is challenging in that the front panel is upside-down relative to the cards. Thus if the 11/20 is mounted for use, the cards hang "under" the backplane. It may be that *soft* foam is needed (not necessarily antistatic) to hold the cards in place. More DEC-11 experienced people can address this.

        Herb Johnson
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