Adjusting sync modbus server to accept custom ModusRequestHandler

[tilman19]

This allows to have a custom send method triggering a hardware Driver Enable (DE) using the RTS pin e.g. on Raspberry Pis for the server (modbus slave) as well. This solution is based on #33 (comment) (Thanks for this one!)

Something similar is already available for the async-io server, but there it is hard to get the correct timing for the RTS pin.

class MyModbusSingleRequestHandler(CustomSingleRequestHandler):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def send(self, message):
        if message.should_respond:
            pdu = self.framer.buildPacket(message)
            tx_time = (len(pdu) + 1) * (10.0 / self.request.baudrate)
            self.request.setRTS(False)
            time.sleep(0.001)
            status = self.request.send(pdu)
            time.sleep(tx_time)
            self.request.setRTS(True)
            logging.debug('send: [%s]- %s' % (message, b2a_hex(pdu)))
            return status

server = StartSerialServer(context, framer=ModbusAsciiFramer, identity=identity,
                           port='/dev/ttyAMA1', timeout=.005, baudrate=9600, handler=MyModbusSingleRequestHandler)

[sonarqubecloud]

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