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Some more comments:
RS232 communication as normally used, with or without parity, is an open loop system (no feedback).
In such an open loop system you must have an extremely low error rate, and that requires a very high "signal-to-noise" ratio. To send a 10 million byte file to a CNC without error you must have zero errors in 100 million bits. And really it means the error rate must be much less than 1 bit per 100 million.
XMODEM or similar programs send a block of data, for example 128 bytes, with included check data. At the destination the received block of data is tested for errors. If one or more errors exist the sender is asked to resend the block. This will be repeated some number of times until the system gives up. The lower the "signal-to-noise" ratio the greater the number of retransmissions. This is a feedback system that can provide error detection and correction up to some point.
This will take some time in the CNC CPU, but not much.
The feedback system can operate at a much lower "signal-to-noise" ratio than the open loop system and provide error free operation.
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