Unibus / Qbus

[shadoooo]

Hello,
we are discussing on separate thread about doing an universal interface for
PDP11.
I'm taking all the relevant documentation about Unibus and Qbus busses,
aiming to check the possibility of doing a board compatible, with some
adjustments, with both worlds.
I started to read the 1979 specifications, however it's not all clear to
me, specially about Unibus.

What I understood:
- Qbus is complete on A and B connectors, so a dual card could be done.
Some backplanes have a true serpentine, while some other has C and D with
other signals, but those are of particular usage with dual-board interfaces.
Basically both dual and quad boards can be done, with the latter using A
and B and simply propagating grant on C and D, supposedly connected in
standard serpentine.

Unibus: the specifications are describing A and B, but backplanes are
complicate than that, and can have Unibus, Modified Unibus, Extended
Unibus, SPC...
What for?
If all the signals are in AB, why they are connected again in CDEF?
There's some complete documentation about the different backplane types,
and the standard approach for an Unibus board?

Thanks
Andrea