Though differential signals are measured through oscilloscope or differential probe, i tried measuring the signals through a multimeter out of curiosity and my observation was that when a voltage was measured across Rs-485 'A' terminal and Ground, the multimeter gave high reading >100V.
Could not find the reason why the reading showed such a high voltage
. It would be great if anyone could help me with it.
Best Answer
As you said in the question yourself, differential signals are usually measured using an oscilloscope. Using a multimeter may only give you reasonable results for DC-Offsets (or DC common mode signals).
Using a multimeter to measure the "changing" part of the waveform will generally fail due to several reasons:
If you still want to go ahead you should at least perform an analog differential-to-single-ended conversion (using a difference amplifier), so that you are at least not comparing different sections of the N and P signals. If the multimeter inputs are floating, you can also measure directly across the P and N lines. Similarly, you could use an OpAmp adder to measure the common mode part of your differential signal.
To sum up: Unless you specify exactly the multimeter you are using and exactly what waveform you are measuring, the results will be meaningless. Especially when the multimeter is in some kind of auto mode.