Sample RtsModbusSerialClient from https://github.com/bashwork/pymodbus/issues/33

gistfile1.txt

class RtsModbusSerialClient(ModbusSerialClient):

    def _send(self, request):
        ''' Sends data on the underlying socket

        :param request: The encoded request to send
        :return: The number of bytes written
        '''
        if not self.socket:
            raise ConnectionException(self.__str__())
        if request:
            self.socket.setRTS(True)
            result = self.socket.write(request)
            self.socket.setRTS(False)
            return result
        return 0

    def _recv(self, size):
        ''' Reads data from the underlying descriptor

        :param size: The number of bytes to read
        :return: The bytes read
        '''
        if not self.socket:
            raise ConnectionException(self.__str__())
        self.socket.setRTS(False)
        result = self.socket.read(size)
        self.socket.setRTS(True)
        return result