End of response to an AT command

Question

How to be sure what is end of an AT command send to GSM module?

I need some character or string that represent end of AT command for every case due to finding when

Answer 1

Referring the original Hayes command set Wikipedia is stating:

Carriage return character, is the command line and result code terminator character, which value, in decimal ASCII between 0 and 255, is specified within parameter S3. The default value is 13.

So it should be possible to set any character to indicate the end of the response (and as well the end of any command sent to the modem a well).

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Anyhow s are not only returned as last character by many modem's responses.

So it might be an interesting experiment whether writing a different character value to S3 would just change the last character sent at the end of the modem's response, or if this would change any sent during the modem's response to be the value stored in S3.