PDP 11/40 Unibus termination query...

Tom Uban uban at ubanproductions.com
Thu May 21 08:06:48 CDT 2009

> I'll check the NPG jumpers -- here's a stupid question -- how do I find
> where they're supposed to be?  I've looked all over for a simple diagram
> showing which pins are CA1 and CB1, but I'm not finding them...

The backplane information is clearly described in the Peripherals Handbook.
If you don't have a copy and you intend to work with these machines, then
you need to get yourself a copy.

The 'C' in CXX is the slot. In a hex backplane, there are six slots, A - F.
Each slot has 18 sets of two pins, each set being identified by a letter:
ABCDEFHJKLMNPRSTUV. You will note that any letter which looks remotely like
a digit is skipped. The second character 'A' in your example (CA1) indicates
the first set of pins. Each slot has two sides designated as '1' and '2' and
they are denoted by the third character '1' in your example (CA1).

  A1 .  .  A2
  B1  .  . B2
  C1 .  .  C2
  D1  .  . D2
  E1 .  .  E2
  F1  .  . F2
  H1 .  .  H2
  J1  .  . J2
  K1 .  .  K2
  L1  .  . L2
  M1 .  .  M2
  N1  .  . N2
  P1 .  .  P2
  R1  .  . R2
  S1 .  .  S2
  T1  .  . T2
  U1 .  .  U2
  V1  .  . V2

This notation is read as viewed from the wire wrapped (pin) side of the
backplane, left to right, and top to bottom. So for your example, for a given
card, CA1 is the 3rd slot from the top, the 1st set of pins in that slot,
and the left hand pin of the set of two.

Hope this helps.


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