Option to strip the TX echo on half-duplex sensors

[clausbroch]

IEC 62056-21 optical probes (such as the Smart Gateways USB-C probe) are half-duplex and echo back transmitted bytes on the RX line. PyKMP does not account for this — it reads until the first stop byte (0x0D), which matches the end of the echoed request, and never sees the actual response.

A fix is to read and discard the echo bytes (matching the length of the sent request) before reading the response.

Edit the PyKMP client source (client.py).
Find the send_request method (around line 140) and locate these two lines:

        self.write(request_physical_bytes)
        response_physical_bytes = self.read()

Add a line between them to consume the echo:

        self.write(request_physical_bytes)
        # Strip TX echo from optical probes that echo back sent bytes.
        self.read(num_bytes=len(request_physical_bytes))
        response_physical_bytes = self.read()

Save the file and test:

pykmp-tool -vv get-serial
The "Received bytes" line should now start with 40 (response from meter) instead of 80 (echo of request).

This should of course not be implemented as the default, but instead as an option, e.g. --strip-tx-echo.
I would like to hear opinions about this approach?

[jktjkt]

My optical head won't echo the transmission back, so I have not seen this problem. However, if you need this, I suggest you leverage the fact that the KMP protocol uses a different start-of-frame marker for the reader-to-meter direction, and another one for the meter-to-reader communication. I.e., you can start reading the packets normally, and then silently discard frames which begin with constants.ByteCode.START_TO_METER. That should work even for optical heads which do not echo back the original transmission, and therefore it won't need any explicit flag to enable.