> Your point about an open line at high impedence makes sense.
>=20
> However, as I understand it, current loop originated with teletypes. I wa=
s
> under the impression that the whole thing was electromechanical and the
> circuit was open when the 'switch' was open. Did they have a resistor in
> parallel with the switching element, or something like that?
>=20
> ~ Bob Ammerman
> RAm Systems

That is my understanding as well, and matches with how long distance telegr=
aph lines used to operate, and all microprocessor development systems that =
I have come across worked the same way as well.

The only time I have seen 4mA and 20mA used is analogue instrumentation loo=
ps where the signal varies 4-20mA.


--=20
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--=20
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