code: final public void writeSingleCoil(int serverAddress, int startAddress, boolean flag) throws
              ModbusProtocolException, ModbusNumberException, ModbusIOException {
          processRequest(ModbusRequestBuilder.getInstance().buildWriteSingleCoil(serverAddress, startAddress, flag));
      }
docs: This function code is used to write a single output to either ON or OFF in a remote device.
  The requested ON/OFF state is specified by a constant in the request data field. A value of
  TRUE requests the output to be ON. A value of FALSE requests it to be OFF.
  The Request PDU specifies the address of the coil to be forced. Coils are addressed starting
  at zero. Therefore coil numbered 1 is addressed as 0.
  
  @param serverAddress a slave address
  @param startAddress  the address of the coil to be forced
  @param flag          the request data field
  @throws ModbusProtocolException if modbus-exception is received
  @throws ModbusNumberException   if response is invalid
  @throws ModbusIOException       if remote slave is unavailable