This is covered in the FAQs but to re-iterate:

Q-bus is a 2-connector bus.
Most DEC backplanes have four connectors in a row.
Some cards want to use their pair of connectors.
All Q-bus slots between the CPU and the last card must
be full.
The connectors are identified as A/B/C/D on the backplane.

So given all of these requirements, DEC made two kinds of "slots" in their 
backplanes, one had Q-Bus on the first two connectors (A/B) and nothing on 
the second two connectors (C/D). Those slots are called Q/C/D slots since 
they have Q-bus and then two more connectors. The other kind of slot has 
Q-bus on A/B and on C/D. These slots are called Q/Q slots (two Q-bus 
"places").

In the BA123 chassis (like you have) the card cage has four Q/CD slots and 
8 Q/Q slots (the last slot is "special for the RQDX connector card.) The 
"path" of the Qbus is a serpentine S as follows:
	The top have of the first four slots are Q bus slots 0 - 3, then the
	top half of slot 5 is Q-bus slot 4, the bottom half is Q-bus slot 5,
	the bottom half of slot 6 is Q-bus slot 6, the top half slot 7
	etc.
	
	+---+---+---+---+---+---+---+
        |   |   |   |   |   |   |   |
        | 8 | 7 | 4 | 3 | 2 | 1 | 0 |
        |   |   |   |   |   |   |   |
	+---+---+---+---+---+---+---+
        |   |   |   | C | C | C | C |
  etc   | 9 | 6 | 5 | / | / | / | / |
        |   |   |   | D | D | D | D |
	+---+---+---+---+---+---+---+

Make sure there are no gaps from slot 0 to slot 'n' (where n is the last 
board you have plugged in). Note that the BA23 chassis only has 3 Q/CD 
slots and the BA213 chassis is *all* Q/CD slots.


--Chuck