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Modbus RTU RS485 Fault finding with Transmit and Receive LEDs

Issue

Using the RD and TD LEDs to help commission and Fault finding a Modbus RTU Continuum Xdriver

Using the RX and TX LEDs to help commission and Fault finding a Modbus RTU AS, AS-P, AS-B

Product Line

  • Building Operation Automation Server Premium
  • Continuum bCX4040 (BACnet)

Environment

  • Continuum
  • Automation server

Cause

How to use the TD RD(TX RX) LEDs to help commission or check an installation

Resolution

Continuum
The Comm Port LEDs on a net controller can be helpful when first setting up or fault finding an Xdriver (ModbusRTU)
Every time the net controller transmits the TD LED will flash and every time another device on the bus transmits (such that the net controller can receive it) the RD LED will flash.
Thus if Xdriver seems not to be working correctly this is often the first thing to check
A simple check with Modbus RTU would be to stop any automatic polling on the network, then manually refresh a modbus point and watch for a single Transmit (TD) followed by a Receive (RD).

SmartStruxure
The Comm Port LEDs on an AS controller can be helpful when first setting up or fault finding ModbusRTU
Every time the AS transmits the TX LED will flash and every time another device on the bus transmits (such that the AS can receive it) the RX LED will flash.
A simple check with Modbus RTU is to create only a  single device (or inhibit polling to all but one device) and watch for a single Transmit (TX) followed by a Receive (RX).

Modbus Simulation
Another useful technique when commissioning or troubleshooting with Modbus is to simulation software on a PC (with RS232/485 convertor for RTU if required).
A Slave device or the Master node can be simulated
eg ModSim, ModScan, ModbusSlave, ModbusPoll, Mod_RSsim

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