Modbus4J Keepalive: Configuring Persistent TCP Sockets

Repeated master.getValue() calls were opening and closing the Modbus TCP socket for each read because the TCP master was created with keepalive=false. Create the master with keepalive=true to retain the connection across the reading sequence.

Identify the socket-close condition

The controller at 192.168.2.101 operates as the Modbus TCP server, while the computer at 192.168.2.69 runs the client master. A packet capture showed the connection closing after each getValue() call, disrupting an intended 500 ms polling cycle.

Configure the Modbus4J TCP master

Pass true as the keepalive argument when creating the TCP master. Keep the same master instance for all reads in the sequence.

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

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) {
        e.printStackTrace();
    }
}

Apply and verify the correction

1. Create the master once with keepalive=true before entering the read loop.
2. Use that master instance for both holding-register reads throughout the loop.
3. Capture traffic between 192.168.2.69 and 192.168.2.101. Confirm that the socket no longer closes between consecutive getValue() calls and that the application still receives both register values.

Recognize the documented limits

The available evidence establishes the Boolean constructor setting and its observed effect, but it does not define an idle timeout, retry policy, parameterized keepalive interval, or the exact API call used to terminate the master after polling. Do not assume that additional keepalive timing parameters exist without checking the Modbus4J version-specific API or official documentation available with the library distribution.

FAQ