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.

  • 2
    \$\begingroup\$ Why are you measuring AC? What do you expect to see? \$\endgroup\$ – pipe Jul 17 '17 at 10:59

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:

  1. Most multimeters have bandwidths for RMS measurements up to 10kHz. Differential signals are most often used way beyond that bandwidth.
  2. Multimeters assume sinusoidal waveforms. If you were measuring a pure rectangle you could work with a conversion factor, but this will fail for transient signals like digital streams.
  3. Generally you have very poor control over the aspect of the signal you are measuring, especially when there are "mute" intervals between frames. You will most likely compare two different signals, even when using two multimeters at the same time to measure N and P waveforms, since they are not synchronized.

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.

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  • \$\begingroup\$ Thank you so much for the explanation. This helped a lot! \$\endgroup\$ – Rohits112 Jul 17 '17 at 14:28
  • \$\begingroup\$ Why an oscilloscope for "differential signals" and what has it to do with AC? \$\endgroup\$ – Curd Aug 9 '17 at 16:44
  • \$\begingroup\$ @Curd: Well, what else would you use to measure two waveforms and calculate the difference? Of course you could also use a logic analyzer etc., but that is IMHO even more specialized equipment than an oscilloscope. There's a lot to be said about differential probes, but I think the question was more general. I don't get where you see AC in my answer, but well, if a signal changes with time, it sure has some AC (alternating current) components. And the oscilloscope is the most common measurement equipment for signals (voltages changing with time). \$\endgroup\$ – cx05 Aug 10 '17 at 13:51

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