An idle time of more than 27 bit times (3 chars) on the wire
signifies the end of an incoming packet, so the callback (a) would be
necessary even when there's nothing waiting to transmit. It also
needs to give the current length of the input buffer, or stick a
marker in the buffer somehow - at the same time as I receive this
callback, I know that some machine somewhere else on the RS485 bus
will immediately start transmitting, before I have a chance to clear
the input buffer.
There's no end of packet character? How.... awful.
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