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I came across a mention of a "PSI bus" in reference to late-model HP 3000 machines. I thought this was simply a typo for "PCI bus", but a google search suggests this really was a thing.

Looking over what little documentation I can find that mentions it, I can't even tell what it was. Is this a physical specification? I see that it hangs off the HPPB/NIO bus internally, so it would seem the interface side is HPPB.

For those who know of this, was it used on any other system? It appears to be appear only on HP-PA systems, not the "classic" line?

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    PSI might also stand for "programmable serial interface" e.g. something more custom than the standard serial protocols.
    – Brian H
    Commented Nov 6, 2021 at 14:46
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    Seems to a bus off the HP Precision Bus. ref: "System organization" and also diagrams under sections "Workstation and serial connections" and "Buses" from 3kranger.com/HP3000/mpeix/doc3k/B59608250.11943/14.htm
    – Brian
    Commented Nov 7, 2021 at 0:11
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    I think @Brian hit it - it appears this is not a bus so much as a smart serial card with protocol support for Bisync and similar. Commented Nov 8, 2021 at 12:34

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I'm not sure where you read about a PSI bus (1) but the PSI was actually a programmable serial interface card that could plug into the HP-PB Precision Bus and allow communication with a wide variety of serial devices, using protocols such as SDLC and X.25.

Documentation for it can be found here and, if you read through that, it's really more of a physical interface (on both the HP and comms equipment side) that can be configured to talk to many different devices.


(1) The link given in this answer does talk about the "Hewlett-Packard Precision Bus Programmable Serial Interface (HP-PB PSI) card" so it may have been something like that where the "bus" was mistakenly associated with the PSI card rather than the Precision Bus.

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