QBUS backplanes

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QBUS backplanes come mainly in two physical types, dual-width and quad-width. In the quad-width backplanes, each slot held four 'connectors', DEC's term for a group of edge connector pins, denominated 'A'-'D'.

The QBUS itself is fully carried in a dual slot, and the quads are further sub-divided into two types, the so-called Q/Q and Q/CD. In quad Q/Q units, 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):


etc (as shown here).

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. [NOTE: The exact failure mode here is still not understood; the PMI spec was examined, but no clash of pin assignments was found. The warning is accurate, though: MicroNote 28 says "MSV11-J MODULES CAN[NOT] BE PLACED IN A Q/Q BACKPLANE SLOT. IF THIS IS ATTEMPTED PERMANENT DAMAGE WILL BE DONE TO THE BOARDS".] So, plugging such a card into a Q/Q backplane will generally destroy the card.

Backplane types

The following table lists the backplanes produced by DEC:

ID Type Height Address Width Chassis Termination Comments
H9270-A Q/Q 4 Q18 BA11-M none
H9270-Q Q/Q 4 Q18/Q22   none
H9273 Q/CD 9 Q18 BA11-N none
H9275 Q/Q 9 Q22   120 ohms
H9276 Q/CD 9 Q22 BA11-S none
H9278 mixed 8 Q22 BA23 none slots 1-3 are Q/CD; slots 4-8 are Q/Q
H9281-Q Q 4/8/12 Q18/Q22   none/120 ohms
DDV11-B hex Q/Q 9 Q18 H909-C none E-F connectors un-wired
DDV11-C hex Q/Q 4 Q18   none E-F connectors wired as CD interconnect

'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.

The DDV11-C details are somewhat speculative; in the VS11, it is used with a VSV11, and the VSV11's Video Bus is carried over the E-F connectors. Puzzlingly, the VS11 documentation shows that a DDV11-D may also be used - but it claims to have the E-F connectors un-wired.

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