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Old December 7th, 2017, 05:36 PM   #3
danw
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Join Date: Oct 2004
Location: midwest, USA
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Is it likely that Modbus RTU over RS-485 is the only signal on the RJ-45? Probably. If that's the case, then it is a known fact that swapping the driver lines will not damage the driver lines. Comm won't work if the lines are swapped, but there's no damage. So you can break out those lines and give it a try.

You might be able to confirm the signal GND by testing for continuity to the frame ground. It might or might not be the same point.

If those pins 4 and 5 do not modulate when Modbus is polling, then use a voltmeter to check for which pins modulate/oscillate when Modbus is polling. It is very likely too be 2 wire half duplex and only 2 wires out of the 8.
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