Looking over a list of various VAX processors, I noticed a reference to a "CI" bus. Poking about in Google I can only find very passing mentions of this, and no details. I suspect this had some other un-acronym name that might work better?

1 Answer 1


At a certain point in the 1980s, 'I' things were all the rage in DEC.

BI - Backplane Interconnect (the bus in the 8xxx series VAXen - see Wikipedia)

CI - Computer Interconnect (showed up as star coupler for VAXclusters, also to HSC intelligent storage controllers)

NI - Network Interconnect (ethernet to you).

SI - Storage Interconnect - controller-to-disk.

The point was to define a hierarchy of standardized interconnect structure, with different characteristics of speed, operating area, number of nodes supported, etc.)

Figure 1-3 in this manual shows various interconnects, including the CI.

The DEC Technical Journal, number 5, from 1987, has some CI details.

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    That "I" nomenclature was mainly used in architectural diagrams (like that Figure 1-3) - it didn't so much mean it was the name of an actual technology (like "MASSbus"). right?
    – davidbak
    Commented Sep 24, 2020 at 15:39
  • There was very definitely one definition of the "CI" protocol, just like "NI" meant "Ethernet" (10 Mb/S at the time) and not just any old network interconnect, and "BI" was one specific backplane technology. From the point of view of a programmer in DEC, CI was the term for a specific technology. It showed up in product names too: e.g., the CI780 module that plugged into a VAX 11/780 backplane to allow it to be connected up to a star coupler.
    – dave
    Commented Sep 24, 2020 at 22:45
  • If you look at the Software Product Description for VAX clusters, you'll see "OpenVMS Cluster software supports any combination of the following interconnects: • CI (computer interconnect) (Alpha and VAX) …." so it's definitely an actual thing.
    – dave
    Commented Sep 24, 2020 at 22:53
  • "just like "NI" meant "Ethernet" - gebus, thanks DEC. Commented Sep 28, 2020 at 13:40
  • I think you have a sequencing error. First you decide what busses you need, architecturally speaking - so we needed a local area network interconnect (to replace the point to point DDCMP links that DECnet was previously built on). So that's your "NI". Then you make a case that the best candidate for NI is Ethernet (though maybe a 10Mb/s Ethernet rather than the existing 4Mb/s Ethernet, I don't know the timing here).
    – dave
    Commented Sep 28, 2020 at 15:29

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