CD interconnect

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The CD interconnect (sometimes given as C/D interconnection, or some variant thereof) is the name given to the 'bus' which is present on the C and D connectors of a Q/CD backplane.

It is not actually a full bus (i.e. a given pin is not connected to 'all' instances of that pin, in every slot in the backplane); rather, slots are connected together in pairs, and these connections are used to connect together board pairs, without the need of a so-called 'over the back' connector cable.

In general (see below for details), the bottom (2, or solder-side) pins on one slot are connected to the top (1, or component-side) pins on the next slot. It runs down the right-hand side of the backplane, when facing the side of the backplane where the boards plug in.

Use by PMI

The CD interconnect is used by Private Memory Interconnect cards, such as the KDJ11-B CPU, and PMI memories, to carry PMI signals.

NOTE WELL: For reasons which seem utterly incomprehensible, many boards designed for Q/CD slots (such as PMI memory cards) do not avoid the QBUS pins on the CD connectors of a Q/Q backplane which contain 'hazardous' (to TTL circuitry) voltages. (The exact details of why are as yet unclear.) [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.


The connection details are that for backplane slot 'N', in connector C, pins Cx2 are connected to the Cx1 pins in slot 'N+1', for x = B, D-S, and U-V. Similarly, in connector D, Dx2 is any slot is connected to Dx1 in the next slot, again for pins B, D-S, and U-V.

In addition, in slot 'N', in connector C, pin CA1 is connected to pin CC1 in slot 'N+1', and likewise in connector D, for DA1 to DC1. Finally, in slot 'N', pin CT2 is connected to pin DT2 in slot 'N+1'.

Finally, power (+5V) is available on pins CA2 and DA2), and ground on CC2, CT1, DC2 and DT1.


The DEC H9273-A backplane (used in the BA11-N mounting box) and H9276 backplane (used in the BA11-S mounting box) are both full Q/CD backplanes (i.e. every slot is a Q/CD slot); the former is Q18, and the latter Q22, although the former is easy to modify to Q22.

The DEC H9278-A backplane (used in the BA23-A mounting box) is a hybrid; the first three slots are Q/CD, and the rest are Q/Q.