Modbus TCP master

To use Modbus TCP master, we need to decide which TCP port sends data, and use this port to send a Modbus command to a TCP slave.

NOTE:  Before you use TCP master, make sure you have already assigned a network interface card (NIC) to RTX64. For further details about how to assign an NIC, see Modbus TCP.

  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_tcp_master

    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. TCP_Connect is the TCP port that the TCP master uses to send data. Mb_ReadCoils_10 is a command that reads a slave's Coil. You can use other commands if you want. After you add the FBs to the source editor, they will also be added to local variables.

  3. Add the following variables. Two of them control the Enable and Execute state of TCP_Connect and Mb_ReadCoils_10, respectively. One of them stores the IP address of a TCP slave, and another displays the value the master reads.

    4.   Variable 1 Variable 2 Variable 3 Variable 4
    Attributes

    Name: target_ip

    Type: BYTE

    Address: Auto

    Array: 0..3

    Init value: 192, 168, 125, 10

    Name: connect_target

    Type: BOOL

    Address: Auto

    Name: read_coil

    Type: BOOL

    Address: Auto

    Name: coil_state

    Type: BOOL

    Address: Auto

    Array: 0..9
    Usage Store the IP address of a TCP slave. Enable or disable TCP_Connect. Enable or disable Mb_ReadCoils_10. Display the value read.

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

    5. NOTE:  To know the range of Coils, Discrete Inputs, Holding Registers, and Input Registers, in Library Tree, expand ModbusSlaveData, double click an item.

  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 turn on the Enable and Execute states of the FBs so they can start working.

    6. If you want to use the TCP master example in this guide to test Modbus TCP data transmission, do the following:

    1. Connect your computer to an I/O module using an Ethernet cable.
    2. Assign an NIC to RTX64 for Modbus.
    3. Add connect_target, read_coil, tcp_target, tcp_read_coil, and coil_state to the Watch panel. Use connect_target and read_coil to enable the two FBs and check their Enable and Execute states. They will be TRUE if the FBs are enabled successfully.
    4. After both FBs start to run, coil_state gets the value from the I/O module it reads. In this guide, we read the address 1282. When you read your I/O module's data, you should check the address range of your I/O module and enter the address you want to read.

    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.