code: public void open() throws ModbusException {
  
          // Start the listener if it isn' already running
  
          if (!isRunning) {
              try {
                  listenerThread = new Thread(listener);
                  listenerThread.start();
                  isRunning = true;
              }
              catch (Exception x) {
                  closeListener();
                  throw new ModbusException(x.getMessage());
              }
          }
      }
docs: Opens the listener to service requests
  
  @throws ModbusException If we cannot listen