NUL character in an ASCII-Based protocol over UART
You specify your packet format as {STX,DATA,ETX}.

The entire DATA content of the packet is contained within the DATA field, between the STX and ETX.

If you decide to send NUL-terminated strings in your packets then the NUL is part of the string - part of your DATA field.

So you would send:  STXHELLO\0ETX