Difference between revisions of "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 [[DEC card form factor#Edge connector contact identification|C and D connectors]] of a [[QBUS#Backplanes|Q/CD backplane]].
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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 [[DEC edge connector contact identification|C and D connectors]] of a [[QBUS#Backplanes|Q/CD backplane]].
  
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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.
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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 ([[DEC card form factor#Edge connector contact identification|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.
 
In general (see below for details), the bottom ([[DEC card form factor#Edge connector contact identification|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.
  
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'''''NOTE WELL:''''' For reasons which seem utterly incomprehensible, many boards designed for Q/CD slots (such as [[Private Memory Interconnect|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.
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==Use by PMI==
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The CD interconnect is used by [[Private Memory Interconnect]] cards, such as the [[KDJ11-B CPU]], and [[PMI memories]], to carry PMI [[signal]]s.
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'''''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 [[QBUS#Backplanes|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: [https://www.ibiblio.org/pub/academic/computer-science/history/pdp-11/hardware/micronotes/numerical/micronote28.txt 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.
  
 
==Details==
 
==Details==
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==Backplanes==
 
==Backplanes==
  
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The DEC H9273-A backplane (used in the BA11-N cabinet) and H9276 backplane (used in the BA11-S cabinet) are both full Q/CD backplanes (i.e. every slot is a Q/CD slot); the former is [[QBUS#Variable address size|Q18]], and the latter Q22, although the former is easy to [[Upgrading QBUS backplanes|modify to Q22]].
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The DEC [[H9273 backplane|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 [[QBUS#Variable address size|Q18]], and the latter Q22, although the former is easy to [[Upgrading QBUS backplanes|modify to Q22]].
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The DEC H9278-A backplane (used in the BA23-A cabinet) is a hybrid; the first three slots are Q/CD, and the rest are Q/Q.
 
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==Use by PMI==
 
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The CD interconnect is used by [[Private Memory Interconnect]] cards, such as the [[KDJ11-B]] processor card, and the [[MSV11-J]] memory card.
 
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The connection is somewhat idiosyncratic, because it is desirable to be able to plug in a variable number of memory cards - i.e. create a true 'bus' by connecting together more than two slots. If the CPU card sent out the PMI bus on the bottom (2, or solder-side) pins, then the memory card would have to take the PMI bus in on its top (1, component-side) pins, and repeat the bus through on its bottom pins, for use by a notional 'next memory card'. However, if the next card is ''not'' a PMI memory card, this will result in the PMI bus being sent to a card which does not use it - possibly with harmful effects.
 
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So, instead, PMI CPU cards emit the PMI bus on their '''''top''''' CD connector pins, and the memory cards take the bus in on their '''''bottom''''' pins, and repeat it through to their '''''top''''' pins. Thus, PMI memory cards must be placed in a Q/CD backplane '''''above''''' the CPU card.
 
  
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An exception to this rule is the backplane in the [{PDP-11/84]], in which the CD connectors of the QBUS section of the backplane are wired to form a true bus; in this machine, the PMI memory cards are placed ''below'' the processor card.
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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.
  
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{{PDP-11}
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==Further reading==
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* ''C-D Interconnect Scheme'', [[MicroNote]] #037
  
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[[Category: Bus Architectures]]
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[[Category: DEC Buses]]

Latest revision as of 14:11, 25 July 2024

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.

Details

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.

Backplanes

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.

Further reading

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