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
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 |
| 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. '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.
External links
- Microcomputer Products Handbook (19850 - backplanes are covered in Part X, 'Backplanes, Enclosures, and Expansion Cables'
- Q-bus Expansion Concepts (uNote #029)
- Douglas Electronics still sells extender cards for DEC backplanes
