Hi,
Okay, thanks. I asked as it's related to a real-world situation, and this article:
Continuity: So Easy to Check, Except When It's Not discusses the same issue, for example in a house with single wires crossing the house (or even all are the same colour, now I think of it) that a person needs to know which is which.
In my case, I'd have liked to turn off the AC main circuit breaker to disconnect an acquaintance's house from the AC mains and - after checking to see no voltage is present with a DMM - continuity test a couple of cables - one of which is probably buried a few cm deep under concrete and is maybe about 15m long - to see:
a) if a specific one starts and ends where I think it does, and
b) if there is a short between the two or three wires in the buried cable as someone may have split more than the outer sheath when they accidentally hit it with a chisel (no-one was hurt or anything).
I know where the phase and neutral finish, in a circuit breaker in a house, presumably the same circuit breaker that tripped when the cable was hit, but after a visual inspection I have no idea which of a few same-colored cables in a spaghetti-like junction box is the other end of that probably damaged cable. There's a fault and a light, a plug and something or other else don't work (since then I would assume).
Anyway, if it just can't work due to wfeldman's explanation, then that's that. Really, I already knew, it's just annoying me that it isn't/doesn't appear to be possible and it would be ever so useful occasionally. Especially a bidirectional one (say an h-bridge push-pull or a sinewave) that with continuity e.g. lit an LED at each end, save walking a lot when guessing which cable is which in an unpowered state or having a long ground cable/adapting COM cable to make it as long as needed.
One is a rather low ohmic driver, where the other is a high ohmic receiver.
Interesting, I hadn't actually thought about that aspect, thanks. I guess the output and input impedance could be matched (?).
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Hi,
electrical potential, or voltage, is measured relative to a specified reference, as you know
the 5V on the left is 5V because you measure it relative to the reference on the left
the 5V on the right is 5V because you measure it relative to the reference on the right
taking the 5V from the left and connecting it to the right is meaningless unless you also bring the reference with it
Thank you very much. It's been annoying me all afternoon, I understand it but couldn't put the concept into words.
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Hi,
They are certainly different but that doesn't really matter anyway. The critical points are:
1. the ground potential difference must not reverse the polarity at the receiver end or it will damage the IC.
2. the ground potential must be less than that needed to change the logic level perceived by the receiver end.
3. in the case of a break or short in the cable, it must cause a change in logic level at the receive end.
The last point is important because a break may leave the CMOS input floating and that could create a false report.
I think I would tackle this by sending a low frequency AC down the cable so it can be safely DC isolated at both ends.
That's great, thanks. So it could be done with care, then, super. You make some interesting points I hadn't considered, either. The (infrequent) joy of electronics: turning an idea that seems impossible into a functional reality.