UNIBUS extension card/cable sets

tony duell ard at p850ug1.demon.co.uk
Fri Jun 26 12:59:30 CDT 2015

[BC11A cable]

> You're right, the alternates are grounded (ohmmeter shows it). I cannot see
> how they did it; I think there must be a comb-shaped trace along the top of
> the card, where it's hidden once the Flexprint cable is soldered down. The
> intermediate ground conductor on one trace, on one end, _is_ connected to
> ground, so the rest could pick it up via a comb-shaped trace.

>     > Well, Unibus is terminated into 180 Ohms and 390 Ohms, isn't it?
> Yes.
>    > The thevenin equivalent is thus around 123 Ohms.
> DEC spec for UNIBUS is 120 +/- 18 ohms.

Although 100 ohms is just out side that spec, I'll bet the mismatch wouldn't cause problems
on most systems.

>    > Most ribbon cables have a characteristic impedance when used with
>    > alternate wires grounded of around 100 Ohms (I seem to remember that is
>    > certainly right for the twist-n-flat ones).

> What's the number for the regular flat? (I have a ton of the latter, but none

Pretty close to 100 ohms I think. You might be able to find a specification somewhere. Alas
I don't have a suitable TDR, or I'd link up a reel of cable and try terminating the end and see
what value gave minimum reflections.

> of the twisted kind. And speaking of the twisted kind, I've always wondered
> what kind of machine they used to produce it - the mind reels!)


> By definition, regular flat must work 'OK', because DEC created these cards,
> and specified the use of ordinary BC05L-xx cables, so whatever its number
> is, it must be acceptable! :-)

It does....

>    > That's a small mismatch, but I don't think it is going to cause big
>    > problems.
> BTW, is my understanding that the issue is the _junction_ of the two
> different impedences, and not so much the length of the section with a
> different impedance, correct? (The sound-based mental model I'm using is two
> different diameters of pipe - going from a larger cross-section to a smaller
> could produce echos - aka reflections - from the junction, but after that, it
> should be OK.)

Generally yes. I think you'll get a reflection at one end of the mismatched cable
and a similar, but inverted, one at the other end. The length of the mismatched
bit will presuambly affect the time between the reflections but not how big they


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