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Thread: looking for some information on some old IBM 3865-2 Modems

  1. #1
    Join Date
    Mar 2020
    Florida, United States

    Question looking for some information on some old IBM 3865-2 Modems

    I posted this originally somewhere else, but I was logged out at the time and it may have submitted without a body when I tried again so here we go:

    So I recently purchased two IBM 3865-2 modems from the early-mid 80s that were designed for older mainframe systems (I have no idea specifically which mainframes they were designed for or work with, but I beleive I recall reading references to them being used with System/390 and 3090 family mainframes)

    Unfortunately information on these seem to be very scarce. I've scoured Google search results and Google's sunset archives and the majority of hits seem to be other people selling these things. Outside of that I've been able to find a few books / papers / patents that brefily mention them, but dont really seem to give me any information about their operation and some sales ads that have some interesting buzzwords and feature names, as well as a handful of old IBM pages that basically list the modem as compatible with some other old hardware, but are not very useful either.

    I'm mostly curious to figure out how they operate. Something like a operation manual or somehow on documenting their experience with these things. I have absolutely no experience and ofc dont hace an old IBM mainframe sitting in my apartment to help me learn. The maintaince manual comes with a good but of useful information regarding testing and repair. I know it's a microprocessor based modem and after taking an admittedly very cursory look at the manual many of the tests seem to operate by sending byte sequences to the modem, so I imagine normal operation must work bu sending some series of commands down to the the modem, but I have no way of knowing what that was or is there was some sort of standard command set for IBM's mainframe modems. Additionally the connector to whatever you might connect it to, be it some terminal or communication controller looks like a 25-pin serial connector, but I'm not sure if RS-232 was common in mainframes or not. At first I wondered if I might be able to find the rom and strip it from one of them for analysis, but many of the IC's on this seems to be proprietary and I'm not sure how far I could get if anywhere at all.

    Does anyone here perhaps have any experience with these modems or mainframes in general that may be able to help me clear up some of the confusion I've listed above (existence of a command set, what systems they were orginally designed for use with, etc.)? Even something like a operation manual or document would be great and I would gladly purchase it.

  2. #2
    Join Date
    Jan 2007
    Pacific Northwest, USA
    Blog Entries


    Yes, the 3865 is RS-232 compatible, and that includes the usual reverse- and sub-channel.

    But it's a 9600 bps synchronous modem, used for dedicated (leased) lines. Not your usual consumer modem. Think Bell 209.


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