Want to see MODBUS (RS-485) Traffic

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deep.kap

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I want to see the traffic(data communicating) between Modbus master and slave devices on PC (Serial Terminal or any Modbus software).

When I tried to connect Master to PC, I can able to see data communicating between them on Modbus Slave software. And I also tried to connect Slave to PC, can able to see data on Modbus Master software. But when i connect Master and Slave together, they are not communicating. i also tried to connect directly Tx-Rx of Master to Tx-Rx of Slave (Bypassing MAX485), then they are communicating. But when MAX485 comes between them they are not communicating.

I'm using:
MAX485
MODBUS 3-wire protocol
RS485 to RS232 converter

Thanks

 

RS-485 MODBUS connection can be perfectly eavesdropped with RS-485 adapter and PC, much easier than RS-232 MODBUS which needs two RX ports to monitor master and slave simultaneously.

Just take care that the RS-485 adapter stays passive, TX-enable should be never activated.

You didn't tell about the RS-485 interfaces in your MODBUS setup, and how transmitters are enabled. A "popular" fault is however lack of pull-up and pull-down resistors that set the RS-485 idle level.
 

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