Answer / carl_ellis

The 'test' connections on a 2 wire, loop powered pressure transmitter have a diode behind them.

If a milliampmeter is connected across the test terminals (in parallel with the diode), the impedance of the meter is less than the diode's, so current will flow through the milliampmeter. There is not enough voltage drop to forward bias the diode and the diode will not conduct with a low impedance meter circuit connected across it. The diode turns off and the current flows through the meter.

Test terminals are provided for loop troubleshooting so that loop current can be measured without breaking or opening the loop circuit so that the milliampmeter can be put in series in the loop circuit.

Carl Ellis
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