Talk:CPU/Memory Interconnect

CMI clarification

I think we need to be more careful the CMI material. It turns out that is only one large backplane in the 750, into which plug:

• the KA750 cards (which have internal busses, such as the MBus and WBus, running between them)
• /750 CMI option cards (RH750, etc)
• memory array boards (in a separate section of the backplane, with different connectors from the other two types of boards)

The CMI appears on the CPU backplane, and goes to some slots/cards, but most of the 'main' slots (specifically, slots 1-6 and 10) are 'special'; i.e. they are customized to a particular board, which has to go in that slot - and that slot can't hold anything else. (An example is the MIC board's slot, 10; the CMI runs to it, but the internal memory bus to the memory array boards - which does not seem to have a formal name - also goes to this slot.) Similarly, according to the KA750 TD, pg. 15, the MBus and WBus both run to the DPM, MIC and FPA cards/slots; but the UBI card/slot has only the WBus, but not the MBus. The CMI runs to some cards/slots - MIC and UBI - but not others: not the DPM, nor the FPA.

So it would be wrong to describe the FPA as a 'CBI' card. It's not; it's part of the KA750, and plugs into the /750 backplane which carries the CMI to other slots, but that's all.

I think we shouldn't put much weight on whether boards plug into the 'main' slots (1-10) on the /750 backplane, because that section holds a mix of special-purpose slots, and 'CMI option' slots (7-9). Although cards that the CMI goes to (MIC, UBI, RDM, WCS) are in some sense 'CMI' cards... because they have dedicated, customized, slots, I would rather reserve the term 'CMI card' for the cards that can go in the CMI option slots: RH780, etc.

Does this make sense? Jnc (talk) 19:10, 2 June 2022 (CEST)

Yes, you are right! I read some manuals again caring how DEC named its components.

There is neither a "Memory Backplane" nor a "CMI Backplane", but only ONE "CPU Backplane" with 10 "Extended hex slots" for CPU cards including 3 "CMI option slots" (7, 8, and 9), and 9 "Hex slots" for memory boards. The 9-slot UNIBUS Backplane DD11 is a separate unit, of course.

So there is a CMI bus, and there are three CMI option slots in the CPU backplane, but there is no CMI backplane.

May I leave the corresponding corrections to you? Vaxorcist (talk) 13:18, 3 June 2022 (CEST)

Yes, I would be happy to do them; in fact, I have been thinking since last evening how best to arrange the overall 750 description. (And I uploaded that wonderful block diagram, File:VAX750block.jpg, which I believe should be central to most articles about the 750.) Let me go re-do the main 750 article (as the first step). Jnc (talk) 15:45, 3 June 2022 (CEST)

Wonderful diagram indeed - go ahead! Vaxorcist (talk) 18:31, 3 June 2022 (CEST)

OK, all done. In addition to trying to clean up the main /750 article, I also tried to rationally allocate content between it, and the KA750, MS750 and CMI pages. E.g. I left the 'CMI configuration' table in the CMI page, as it could logically go either there or the main /750 page, but leaving it in the CMI page meant the /750 page was shorter.

Thank you for putting up with my 'help' on the /750! :-) Jnc (talk) 23:50, 3 June 2022 (CEST)