Answer Posted / carl_ellis

192 views of this thread up to this point, yet no one's
answered by Jan 2014.

I have read a reference to motor speed and direction
controller that use or used a bipolar +/- 20mA as an input
demand signal, but that's the only use I've ever heard of.

Bipolar +/- current signals are rare in the process
industries. Even the TTY teletypewriters of yesteryear
used a unipolar current loop, not a bipolar current.

That's not to say that it can't be done, and many industrial
analog inputs have bipolar +/- DC voltage functionality,
like -50 to +50mV, -1.0V to +1.0V, or -12 to +12V dc, and
Phoenix Contact has an instrumentation signal isolator that
has bipolar current output at +/- 20mA, but I haven't a clue
what one would use the bipolar function for.

Of course, pumping a bipolar current through a precision
dropping resistor would provide a bipolar +/- dc voltage
signal.

Carl Ellis
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