Upgrading QBUS backplanes

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Upgrading a QBUS backplane from 18-bit to 22-bit is a fairly simple process. Basically, one has to wire pins BC1, BD1, BE1 and BF1 (BDAL18-BDAL21, respectively) on all QBUS slots together into a bus. So one wires BC1 on slot 1 to BC1 on slot 2, slot 3, etc, etc.

A couple of notes: First, the QBUS is because if the backplane is a so-called Q/CD backplane, in which the C and D connectors are a CD interconnect private bus used to connect together board pairs, clearly one doesn't run the extra BDAL lines to the C/D connectors, bur rather only on the A-B connectors wired for QBUS (which run down the left-hand side, when facing the side of the backplane where the boards plug in).

Second, for optimal analog behaviour, the 'out' slot on the backplane should be the last slot one wires to, so that there are no branches in the transmission lines 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):


etc., one might naively think one has to run the extra bus lines back and forth to match. However, only the bus 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 (albeit 10 QBUS slots) backplane above, one could/would wire:


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 quad slots, e.g.:


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:


with a vertical run on the left side, stopping short of the last slot; then a diagonal up to 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 that is not currently known.

All the obvious caveats apply: make sure not to get confused by the mirror pin and slot numbers on the front and back sides (the wiring will be on the back, whereas the diagrams above are on the front), etc.

See also

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