UNIBUS extension card/cable sets
ard at p850ug1.demon.co.uk
Fri Jun 26 12:59:30 CDT 2015
> 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?
> > 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! :-)
> > 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
More information about the cctalk