26 Jun
2015
26 Jun
'15
10:59 a.m.
[BC11A cable]
Although 100 ohms is just out side that spec, I'll bet the mismatch wouldn't cause
problems
on most systems.
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
are.
-tony
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. > 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!)
Eeek!
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.)