UNIBUS extension card/cable sets

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Jun 25 12:02:30 CDT 2015

    > From: tony duell

    > There are 3 40 pin Berg headers, one row of each appears to be ground

Ah, hadn't noticed that! But then again, I hadn't looked at them closely
yet! :-) Yes, they do connect to ground - all the UNIBUS ground pins are
ganged together, and connected to the A-row Berg pins on all 3 connectors.

So every other wire on the 40-conductor flat cables should be ground - that's
even better than the classic BC11A, where almost every other line is, from
what I can see, simply left floating (which is better than nothing, but not
as good as grounding them, is my understanding).

    >> I was wondering if maybe the M9015 was an M9014 with termination
    >> resistors, or something (the way the QBUS versions come with and with
    >> termination)

    > I would be very surprised. Unibus is normally terminated at the ends
    > and not in the middle.

Right, but the very similar QBUS does have terminations (of a sort - the
rules for when you need terminations on QBUS extensions are so complex that I
don't really grok them yet) 'in the middle', so...

I just couldn't find out _anything_ about M9015's, so I was just guessing in
the dark. Real data gratefully received.

    > From: John Wilson

    > I was kind of assuming that there's some impedance-matching (etc.)
    > problem with using ribbon cables for more than one hop.

Hmm. Well, I dunno; that may be beyond my (minimal :-) level of analog
expertise. I would have assumed that it's the _change_ from one impedance
level to another that's the issue (you can get a reflection off the
junction), so whether one's using long or short cables between a pair of
M9014's, it shouldn't be _that_ big a deal (modulo propagation delays, which
_are_ an issue with length). Perhaps someone else can opine?

But I hope we can do fairly long runs with the 40-conductor (aka BC05L-xx),
that could save us when we run out of BC11A's, if that strange Flexprint flat
white cable the BC11A uses is no longer available.


PS: From what I can see so far (done half the pins), the M9014 and M9042
do have an identical pinout on the Berg connectors.

More information about the cctalk mailing list