> From: Jacob Ritorto
> Would you happen to have notes or references about how to do it?
It's not too hard; basically, one has to wire pins BC1, BD1, BE1 and BF1
(BDAL 18-BDAL21, respectively) on all _QBUS_ slots together into a bus. So
wire BC1 on slot 1 to BC1 on slot 2, slot 3, etc, etc.
A couple of notes: First, I said '_QBUS_' because if you have a Q/CD
backplane, clearly one doesn't run the extra BDAL lines to the CD slots, only
the QBUS slots (which run down the left-hand side, when facing the backplane).
Second, for optimal analog behaviour, the 'out' slot on the backplane should
be the last slot you wire to, so that there are no branches in the
transmission line for BDAL18-BDAL21 (which can produce reflections - aka
noise - on the transmission lines). How to do this efficiently (in terms of
the wiring) can be a bit tricky, depending on the backplane configuration.
E.g. if one has the standard 'serpentine' backplane, i.e. one with the slots
in the following kind of order (facing the backplane from the board side):
1-2
4-3
5-6
8-7
9-10
etc., one might naively think one has to run the extra bus lines back and
forth to match. However, only the _grant_ lines have to follow this pattern
(and they are already there); the added lines don't have to follow the same
pattern, as long as there are no branches.
So, for the example 5-slot backplane above, one could/would wire:
1-4-5-8-9--2-3-6-7-10
i.e. a single vertical run on the left hand side, a single diagonal from 9
back to 2 (shown with "--"), and then another vertical run on the right hand
side. Much simpler than wiring back and forth in slot order; there are no
branches; and the last slot is the 'out' slot.
For backplane with an _even_ number of layers, e.g.:
1-2
4-3
5-6
8-7
it's a little more complicated: a single vertical run on each side
cannot be connected in such a way as to have the 'out' slot (8) be the
last slot. One has to do something a little more complex:
1-4-5--2-3-6-7--8
with a vertical run on the left side, stopping short of the last slot; then
a vertical run on the right side, then a lateral back across on the last
layer.
Obviously one _could_ run the wires back and forth, in slot order, but that
will take a lot more wire, which at the very least is more work (especially
on backplanes which don't have full wire-wrap pins, just the little stubby
pins that have to have the wires soldered to); whether it also increases the
delay down those transmission lines enough to be noticeable is something I
don't know the answer to.
All the obvious caveats apply: make sure not to get confused by the mirror
pin and slot numbers on the front and back sides (you'll be wiring on the
back, whereas the diagrams above are on the front), etc.
Noel
