If you are using MODBUS the defined "minimum" delay between the end of one message and the start of the next message is 3.5 characters at the defined bit rate, as long as you exceed this minimum you are able to determine when a message has finished and transmit a new message. could you not merely sit in a for loop until the time has expired? (yes it is wasteful but would save on a timer) an interrupt could bring it out of the loop early if something else came in the port.

regards


Marshall Brown