QBUS backplanes

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):

1-2
4-3
5-6
8-7
9-10

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

Several manufacturers produced QBus compatible backplanes:

DEC Backplanes

The following table lists the backplanes produced by DEC:

ID	Type	# Slots	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; removable termination packs
54-17507	mixed	12+1	Q22	BA123	120 ohms	slots 1-4 are Q/CD; slots 5-12 are Q/Q; slot 13 provides only power and the DCOK Signal
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. '# slots' is the number of slots; for Q/Q quad backplanes, the number of dual-width board which can be plugged in is twice this.

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.

MDB Backplanes

MDB Systems produced many third-party QBus options, including backplanes and complete chassis systems:

ID	Type	Width	Address Width	Assembly	Chassis	Termination	Comments
40328	Q/Q	8	Q18	MLSI-BPA84	MLSI-BA11-200	none	Wire wrap pins, can be wrapped to Q22

'Type' indicates slot height and wiring, a value with a slash indicates quad height slots. 'Width' is the number of slots; if the backplane is quad-height, twice as many dual-height modules will fit.

As with DEC backplanes, MDB backplanes that are Q/Q will damage QBus cards with CD interconnects.

Further reading

• Micro Backplane Mechanical Mounting Guidelines, MicroNote #026

External links

• Microcomputer Products Handbook (19850 - backplanes are covered in Part X, 'Backplanes, Enclosures, and Expansion Cables'
• Q-bus Expansion Concepts (MicroNote #029)
• Douglas Electronics still sells extender cards for DEC backplanes
  • 6-DE-11-C dual extender
  • 6-DE-8-C quad extender