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I got two devices communication serially with RS485. I got a serial analyzer with which I am able to listen to the whole communication but I am unable to determine which bytes come from which device. Is it possible that I can somehow split up the line while still letting the devices communicate with each other, so I can determine the direction of the communication?

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  • \$\begingroup\$ Is it a half duplex line? Like 2 wires only? I that case it won't be easy without knowing the protocol. \$\endgroup\$ – Eugene Sh. Apr 5 '17 at 15:44
  • \$\begingroup\$ Yes, half duplex, 2 wires: Data and GND \$\endgroup\$ – Liess Jemai Apr 5 '17 at 15:45
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    \$\begingroup\$ If it is RS485 there should be 2 wires for data as it is differential signal. And a ground (well, a third one. But sometimes people mistakenly omit it ) \$\endgroup\$ – Eugene Sh. Apr 5 '17 at 15:49
  • \$\begingroup\$ You need access to R/W control or some clever Peak amplitude detector to detect idle and so see which is closer by amplitude or choose correct protocol and decode address for destination. fte.com/images/STStats.gif \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 5 '17 at 16:14
  • \$\begingroup\$ DCE/DTE address decode is setup in Configuration menu for 1 or 2 bytes which starts every block communication. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 5 '17 at 16:44
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Use current probe. There is a 120R resistor on each side, but only one side drives at a time. So with current probe you can know the direction. Typically the current would be around 25mA, visible enough.

Actually its best to use current and voltage probe together. So when voltage on a wire is positive relatively to the other wire, current poits to receiver. Otherwise it points to transmitter.

By the way. If current probe is not available, just cut the wire, insert a 10R resistor and measure voltage on it.

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For 2-wire circuits, Serialtest expects to see the transmit lines of your circuit connected to the corresponding pins on the RS-422/485 ComProbe. Tap off of the Transmit + line (TX+) of your circuit, and connect that to pin 1 (TX+) on the RS-422/485 ComProbe, then tap off of the Transmit - line (TX-) and connect that to pin 2 (TX-) on the RS-422/485 ComProbe

enter image description here

The user can also choose to monitor either DTE only, DCE only or both DTE and DCE ends of the Asynchronous communication in setup "CONFIGURATION" along with baud rate = 2x Network bit rate , and other Async parameters.

Destination address is first, 1 or 2 bytes in setup for DCE and DTE.

enter image description here

This may not match your equipment, but logically it should be similar.

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  • \$\begingroup\$ What software is that? Is it free? \$\endgroup\$ – Marko Buršič Apr 5 '17 at 17:43
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    \$\begingroup\$ Yes but only the Demo is free .... Title is in photo.....I just installed it for this question but denied the printer install driver (optional) then uninstalled with REVO uninstaller after question. Looks pretty good. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 5 '17 at 17:46
  • \$\begingroup\$ Whoever marked this -1 gave no idea why so I consider it trolling. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 7 '17 at 4:28

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