All SME devices implement a MODBUS RTU compliant RTU slave. The default configuration of the MODBUS RTU slave is 38400 and ODD parity. Connection is by RS485-2W where the interface is not electrical isolated.


Wiring is RS485-2W. The device does not have internal biasing resistors and termination resistors and proper setup of the RS485 network has to be ensured. 


Termination: Termination should be only at the end and beginning of the bus to avoid reflections. Requirements on termination differ not only on the baud-rate used but also on the slew-rate of device internal drivers. As a general rule, if there are devices on the RS485 bus that allow fast transmission speeds, and cable lengths exceed a few meters, then termination is required. 

Biasing: Biasing of the RS485 BUS is required. If biasing is omitted, communication may work unreliably or not at all. See figure below for Pull-Up/Pull-Down resistors. 

Connections: Connection stubs on the RS485 bus should be kept to the minimum.

Common: RS485 does allow common mode voltages of +12V / -7V. It is therefore recommended to use a common wire to ensure common mode voltage is within safe limits for all receivers. In case of problems it might be necessary to introduce RS485 isolators into the network. The common terminal for the SME device is the 0V terminal (GND) for the power supply.



The SME device allows the configuration of the Modbus address using the RCP software. Please refer to the RCP manual for instructions.

Factory default settings of Modbus parameters:

address=0 or 1



The system is capable of supporting: 

  • parity settings of odd, even and none
  • baudrates of 9600, 19200 and 38400 


In case after connection your PLC/device does not read the registers. 

1. Swap the A & B wires for RS485 connection

2. Check you have the correct resistors

If it still does not connect. Contact rheonics support by raising a ticket on the support portal.