[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!) 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.) 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
