I have now RPI running as Modbus TCP server/slave with OpenPlc, with AdvancedHMI,  if I add modbus slave to be connected with USB-RS485 to the RPI, do my RPI need to be configured as Modbus Master? or the slave device could run as an IO expansion?

Great software! Thank you.

Thanks, Rox
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All the configuration you need to make is in the Slave Devices page on OpenPLC. The slave device will run as an IO expansion to OpenPLC.
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