In quad Q/Q backplanes, both sides of each quad slot are fully wired for QBUS, and so a single slot can hold two separate dual-width QBUS devices. The device locations are usually arranged for grant priority in so-called 'serpentine' order, i.e. one with the devices in the following kind of order (facing the backplane from the board side):
In a quad Q/CD backplane, the CD connectors form a private bus, sometimes called the CD interconnect, used to connect together board pairs. (The CD connectors run down the right-hand side, when facing the side of the backplane where the boards plug in, with the CPU at the top.)
It is usually possible to upgrade 18-bit backplanes to 22-bit; see Upgrading QBUS backplanes.
NOTE WELL: For reasons which seem utterly incomprehensible, many boards designed for Q/CD slots (such as PMI cards) do not avoid the QBUS pins on the CD connectors which contain 'hazardous' (to TTL circuitry) voltages. Plugging such a card into a Q/Q backplane will generally destroy the card.
The following table lists the backplanes produced by DEC:
|H9278||mixed||8||Q22||BA23||none||slots 1-3 are Q/CD; slots 4-8 are Q/Q|
|DDV11-B||hex Q/Q||9||Q18||H909-C||none||E-F connectors un-wired|
'Type' indicates whether it is dual-width ('Q') or quad. 'Height' is the number of slots; the number of dual-width board which can be plugged in is twice this, for Q/Q quad backplanes.