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?
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.
1That "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?– davidbakSep 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. 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. Sep 24, 2020 at 22:53
"just like "NI" meant "Ethernet" - gebus, thanks DEC. 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). Sep 28, 2020 at 15:29