Serial + Async + RTU violates modbus protocol, results in bugs

[k2nsloos]

Hi,

I recently tried to communicate with an SC2004MBS over RS485 using the pymodmus async library.
However I noticed that several commands got dropped by the slave, whenever they we're issued in a small time frame.

Upon reading the Modbus specification (http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf) I found two issues:

1. At most one transaction can be active at any given time.
2. The minimal idle time between messages is at-least the time equivalent of 3.5 characters

Whenever a request is build using ModbusClientProtocol class, it is passed to the following function:

def execute(self, request):
        ''' Starts the producer to send the next request to
        consumer.write(Frame(request))
        '''
        request.transaction_id = self.transaction.getNextTID()
        packet = self.framer.buildPacket(request)
        self.transport.write(packet)
        return self._buildResponse(request.transaction_id)

This function build the packet and immediately writes it to the transport. Possibly violating issue 1 and 2.

Because if request B is written to the transport before request A is fully sent, then they'll be fused together resulting most likely in an invalid message (CRC). The specification dictates that slaves must drop these kind of packets silently.

Furthermore the 3.5 characters of silence is not enforced before sending any reply, which is also violates the specification.

[dhoomakethu]

fixed with pymodbus v1.3.0rc1