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I have a pair of connectors that mate with each other. One is labeled BUS, the other TAG. What are they for?

bus-tag connectors

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  • Not enough information. What does this have to do with retro computing? What is the context? Are you trying to identify equipment, or do we know what systems they come from? Because there are a lot of hardware widgets out there.
    – user12
    Commented Apr 21, 2016 at 4:34
  • I was hoping the picture would be sufficient. The Amdahl sticker suggests mainframe computer hardware from 20+ years back. I was wondering what they were and what they were used for.
    – fadden
    Commented Apr 21, 2016 at 5:17
  • So, a google search for that part number gets relevant hits. Did you try that?
    – user12
    Commented Apr 21, 2016 at 12:38
  • A google search for "amdahl 312508-203" returns two hits, both product lists that only add the word "terminator" to the set of available knowledge. Dropping the "-203" expands the number of results, but it's still just part lists. Was there a result that you found particularly enlightening?
    – fadden
    Commented Apr 21, 2016 at 15:35
  • I got 4 hits, all suggesting these are bus terminators. Given these are Amdahl, they are going to be almost identical to IBM 360 equipment.
    – user12
    Commented Apr 21, 2016 at 15:46

2 Answers 2

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Yes, these are bus and tag terminators for mainframe channels that used to run at 3MB/sec speeds. Used as late as 1998 in P/390 development systems. The IBM and Amdahl terminators are equivalent.

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I guess (is that allowed as an answer?) that these are bus terminating resistors. High speed signals are usually passed along impedance matched transmission lines, the termination resistor mostly removes any reflections from an unused port.

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  • Any idea why there would be two different types of terminators? I would guess some sort of asymmetric I/O setup, but it seems odd that the connectors can be physically attached to each other... I don't know if that's deliberate or just an artifact of the design (e.g. a desire to reduce manufacturing costs by using the same cases for both). Peeking in the sides, they both have rows of resistors, but the resistors in BUS are different from those in TAG.
    – fadden
    Commented Apr 21, 2016 at 15:40
  • 1
    These will be pretty much identical to DCC bus terminators for IBM 360 and similar HP products. See en.wikipedia.org/wiki/IBM_System/360
    – user12
    Commented Apr 21, 2016 at 15:46
  • Aha! en.wikipedia.org/wiki/IBM_System/… in particular.
    – fadden
    Commented Apr 21, 2016 at 15:49
  • 1
    @fadden -- see en.wikipedia.org/wiki/Bus_and_Tag for the two types Commented May 24, 2021 at 10:29

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