UNIBUS extension card/cable sets

tony duell ard at p850ug1.demon.co.uk
Thu Jun 25 12:41:17 CDT 2015


[One row on each Berg is 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.

I am not at all surprised they are grounded, it's the only logical place for them 
all to end up. 

> 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). I will check.

> 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 never really understood the Qbus rules either. The ones I saw make very little 
electrical sense (unlike Unibus, which is designed as a transmission line). I don't
think I've ever taken Qbus outside one backplane, as a result just about anything
will work. On Unibus too, you can generally get away with no terminator at the
far end if you are in one backplane, or even one mouting box. Can be useful
for testing...

> 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?

Well, Unibus is terminated into 180 Ohms and 390 Ohms, isn't it? The thevenin 
equivalent is thus around 123 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). That's a
small mismatch, but I don't think it is going to cause big problems.


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