Modbus RTU slave

To use Modbus RTU slave, we need to decide which COM port receives data, and connect this COM port to an RTU slave.

NOTE:  Before you use RTU slave, make sure you have already assigned a COM port to RTX64. For further details about how to assign a COM port, see Modbus RTU.

  1. In the Project pane, add a new program with FBD. You can decide its name and task, or use the settings we provided.

    2. Language: FBD

    Name: mb_rtu_slave

    Task: Fast

    NOTE:  For more information about how to add a new program, see PLC programming.

  2. In Library Tree, drag the following function blocks (FBs) to the source editor and give them meaningful names. COM_Connect is the COM port that the RTU slave uses to receive data. Mb_RTU_Server is an RTU slave. After you add the FBs to the source editor, they will also be added to local variables.

  3. Add the following variables, which control the Enable state of COM_Connect and Mb_RTU_Server.

    4.  

    		 Variable 1 Variable 2

    Attributes

    Name: connect_target

    Type: BOOL

    Address: Auto

    Name: enable_slave

    Type: BOOL

    Address: Auto
    Usage Enable or disable COM_Connect. Enable or disable Mb_RTU_Server.

  4. Add the following inputs and outputs to the FBs as the screen shot shows.

  5. Compile and run the PLC program. To toggle the BOOL variables, drag them to the Watch panel and set their values. Remember, you need to use connect_target and enable_slave to turn on the Enable state of the FBs so they can start working. To check the Enable state, add the two FBs to the Watch panel, expand rtu_target and rtu_slave and check their states. If the following states are TRUE, it means they are enabled.

    6. If you want to use the RTU master and slave examples in this guide to test Modbus data transmission, for Modbus RTU slave, make sure Index in the slave's COM_Connect is different from that in the master's, and make sure the DeviceId in the Modbus master's command (e.g. Mb_ReadCoils_10) and Modbus slave's Mb_RTU_Server are the same.

    NOTE:  For more information about compiling and running, see Compile and run your PLC program.

    NOTE:  For more information about toggling a boolean, see Step 8 in Start and stop the KINGSTAR Subsystem through variables.