UNIBUS extension card/cable sets - test-drb@ccmp.vtda.org

26 Jun 2015

...
  From: tony duell 
...
 > 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 am surprised. DEC didn't waste copper like that.
It's been a long
 time since I worked on a BC11A, but I thought alternate wires were
 grounded. Maybe a track right along the edge of the PCB where the cable
 comes off (so you can't see it). 
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.

...
 > 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). 
...
  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
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.)

	Noel