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  • Hello.
    When I run the next code I can see that the master.getValue(el), send the message to the server to close the socket.
    In the attached whireshark picture you can see it. In the whireshark picture the is the controller (modbus tcp server) and the is my computer.
    In this moment I have problems reading values every 500ms from one controller because every time that I read the master.getValue close the socket.

    Do you know How can we close the socket only at the end of the readings?


     NumericLocator el = new NumericLocator(255, RegisterRange.HOLDING_REGISTER, 0, DataType.TWO_BYTE_INT_UNSIGNED);
            NumericLocator fjk = new NumericLocator(255, RegisterRange.HOLDING_REGISTER, 1, DataType.TWO_BYTE_INT_UNSIGNED);
            for (int i = 0; i < 1113; i++) {
                try {
                    System.out.println("el: " + master.getValue(el));
                    System.out.println("fjk: " + master.getValue(fjk));
                catch (Exception e) {

    Attachment: download link

  • The problem is solved.
    When we create the master we can see a paramer keepalive in the constructor.

    ModbusMaster master = modbusFactory.createTcpMaster(ipParameters, keepalive);

    I was keepalive=false. If keepalive = false then every time that the client (master) read of the server (slave) the socket is open------close.

    If keepalive = true then the master close the socket at the end of all the readings. But I do not know how the keepalive works exactly, is it parametrizable...

    Do you know How the keepalive works exactly? And Where Can I find Modbus4j documentation?

    Best regards.