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Is it possible to view a RS 485 diffential signal using a Rigol DS1102E 100MHz Digital Oscilloscope, Dual Analog Channels, 1 GSa/s Sampling, USB Storage?

I can see a signal that looks more like amplitude modulation, but not a square wave. How can I view serial data?

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    \$\begingroup\$ Are you connecting the ground clip of the o-scope probe to the ground reference of the circuit? \$\endgroup\$ – Dan Laks Nov 18 '14 at 1:30
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    \$\begingroup\$ why not attach the seen signal? \$\endgroup\$ – Erik Friesen Nov 18 '14 at 1:35
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    \$\begingroup\$ You can certainly "see" the signal, either by using one probe on one side, or using both probes in 'subtracting' mode for differential. It may be not look like squares if there is a long wire, poor line adaptation... If it simple asynchronous serial bitstream (8bits, 1 start...), you can decode it simply watching the waveform. \$\endgroup\$ – TEMLIB Nov 18 '14 at 1:35
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    \$\begingroup\$ Check this :electronics.stackexchange.com/questions/76514/… \$\endgroup\$ – diverger Nov 18 '14 at 1:35
  • \$\begingroup\$ If you have a 2-channel scope, connect channel 1 to A and channel 2 to B, if the scope has an add/subtract mode you can make a good RS485 trace from that, or just view the two channels alongside each other. \$\endgroup\$ – John U Dec 18 '14 at 16:21
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Depending on the nature of the data being transferred, you might need to set the input coupling to DC for good results. Also make sure you're not using the digital filter, or have it set to LPF at a cutoff frequency at least 3x your bit rate. Use of AC coupling or the wrong digital filter settings could cause the kind of display you see. As TEMLIB wrote, you should use the "subtract mode" between probe 1 and probe 2 for the two sides of the differential pair. Press the "MATH" button to select that.

I don't know whether the DS1000E series is isolated from the line power ground; traditionally there were problems with using the ground of a scope probe on a signal that wasn't at the power line ground potential, resulting in undesired and possibly damaging current flow through that ground connection. If the DS1000E is isolated from the power line ground, and you're not simultaneously probing other signals, you could use a single probe, without subtract mode, with the ground clip to one side of the RS-485 pair and the probe tip to the other. Some technicians "float the scope" by cutting the ground pin from the power cord, but doing that is DEFINITELY NOT RECOMMENDED. If you truly need an isolated scope, power it from a true isolation transformer.

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  • \$\begingroup\$ It is not isolated. I was considering using a 2 pin receptacle adapter to isolated the ground. I didn't have much luck using the two probes in subtract mode. On a older analog scope I was able to use the math, but wasn't able to sync and capture wave form. \$\endgroup\$ – BKnight Nov 20 '14 at 13:05
  • \$\begingroup\$ What did you see with subtract mode? That really should work, so it's probably worth some time to track down the issue. If you can post screen shots with the two probes separately, then in subtract mode, perhaps that will be helpful in figuring it out. \$\endgroup\$ – Eric Smith Nov 21 '14 at 22:29
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    \$\begingroup\$ Most of the time it is going to be more useful to use both probes in distinct mode or even one alone. Unless you have severe distortion, either one contains the information. And if you are trying to understand distortion, seeing the relationship of the two signals plotted on top of each other is often more useful than the difference. \$\endgroup\$ – Chris Stratton Jan 17 '15 at 16:10

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