Qbus Backplanes - test-drb@ccmp.vtda.org

5 Jun 2003

On Jun 5,  7:28, Ethan Dicks wrote:
...
  --- vance(a)neurotica.com wrote:
 > Hello people.  Is there any way to tell if a backplane is QQQQ or QQCD by
...
  > looking at it, and without having to pull the
backplane out of the
 > chassis? 
...
  In some older chassis, you can see the backplane
wiring, but not on
 boxes like the BA23 or BA123.

   What if I can find the model number of the
chassis? 
 That's useful... The model numbers that look like BA-23, BA-11N, BA-11M,
...
  etc., are the ones you want.

 Also, remember in newer boxes, it's three slots of Q22-CD, with the
 rest Q22-Q22. 
That's true for a BA23 (3 slots plus 5), but in a BA123 it's four Q-CD,
eight Q-Q, and the thirteenth slot is not bussed at all.

...
  Older boxes can be wired for either 18-bit or 22-bit
addressing,
 which may or may not involve Q-CD slots.  I _think_ the BA-11N is a
 18-bit Q-CD box... that's what I remember my 11/23 is in (but I
 added the extra wires some time ago to make it Q22). 
Yes, a normal BA11-N is Q-CD.  So is a BA11-S.

If you can read the H92xx number off the backplane (it's often visible
without too much effort, for example it's on the top of a BA11-M/N/S
backplane) you can look that up to see if it's straight (Q-CD),
serpentine (Q-Q) or mixed.

The following is not exhaustive, but includes the common ones:

PDP-11/03   BA11-M  H9270     18-bit  4 x Q-Q
PDP-11/23   BA11-N  H9273     18-bit  9 x Q-CD
                    H9275     22-bit  9 x Q-Q
PDP-11/23+  BA11-S  H9276     22-bit  9 x Q-CD
microPDP-11 BA23    H9278     22-bit  3 x Q-CD, 5 x Q-Q
                    H9281     18-bit  various sizes, only two slots
(AB)
microPDP-11 BA123   54-17507
                 or 70-22019  22-bit  4 x Q-CD, 8 x Q-Q, 1 x non-bus
                    DDV11     18-bit  9 x Q-Q, slots EF are separate

Q-Q backplanes are usually referred to as serpentine (or sometimes as
zig-zag), Q-CD as straight.  Serpentine slot order goes like this:

    slot 1  ---->  slot 2
                     |
                     V
    slot 4  <----  slot 3
      |
      V
    slot 5  ---->  slot 6

                    etc

Some of the backplanes have optional jumpers to enable/disable LTC
interrupt, or for termination.  Many carry the SRUN_L signal only on
row 1. Beware of such differences when changing backplanes or linking
two together.  The DDV11 is unusual in being hex-height instead of
quad-height; the extra slots are nor bussed, except for +5,V, 0V, and
-12V, but may carry odd voltages, depending on which OEM or
experimenter last used it!

There's a rumour that the first 3/4 slots in a BA23/123 are special and
you can't use them for anything except memory or processor; that's
rubbish, they're normal Q22-CD slots and you can put anything that's
Q-CD compatible in them, so long as it's after the processor.  What
*is* special is the range of PMI memory boards; if you put them in
*front* of a KDJ11-B they use the CD interconnect for the Private
Memory Interface
but if you put them *after* the CPU they use the Q-Bus like all other
memory boards.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York