> > Not exactly, there is 18 bit Q-bus, 22 bit Q-bus, and Q/CD Q-bus (also 22
> > bit) Quad wide cards may or may not work in a Q/CD slot, Dual wide cards
> > will work in the AB slot of Q/CD or a Q slot if they are the same number of
> > bits as the backplane (18/22). Its all written up in the MicroNOTES on the
> > PDP-11 FAQ site.
> 
>   Well, aside from q18 vs. q22.  Q/CD vs. Q/Q is more of a physical
> issue, unless you're dealing with a multi-board set like the RLV11,
> which (if I recall correctly) actually uses the CD interconnect for
> its own purposes.
> 
>   The notion that I had intended to convey was that the vast majority
> of Qbus boards are compatible with both MicroVAX and Qbus PDP-11
> machines.

By and large this is true but there exceptions all over the place that 
despite their oddness do fit the rather flexible spec.  the oddest was a
custom hex wide with AB/AB/CD bussing, would have loved to get that one 
but the box was huge rack mount and the cards were vertical.

Allison