code: void checkValidity() throws ModbusException {
          if (request != null && response != null) {
              if (request.getUnitID() != response.getUnitID()) {
                  throw new ModbusIOException("Unit ID mismatch - Request [%s] Response [%s]", request.getHexMessage(), response.getHexMessage());
              }
              if (request.getFunctionCode() != response.getFunctionCode()) {
                  throw new ModbusIOException("Function code mismatch - Request [%s] Response [%s]", request.getHexMessage(), response.getHexMessage());
              }
          }
      }
docs: Checks the validity of the transaction, by
  checking if the values of the response correspond
  to the values of the request.
  
  @throws ModbusException if the transaction is not valid.