Instead asynchronous framing uses “control-octet transparency”, also called “byte stuffing” or “octet stuffing”. The frame boundary octet is 01111110, (0x7E in hexadecimal notation). A “control escape octet”, has the value 0x7D (bit sequence ‘10111110’, as RS-232 transmits least-significant bit first). If either of these two octets appears in the transmitted data, an escape octet is sent, followed by the original data octet with bit 5 inverted. For example, the byte 0x7E would be transmitted as 0x7D 0x5E (“10111110 01111010”). Other reserved octet values (such as XON or XOFF) can be escaped in the same way if necessary.
The “abort sequence” 0x7D 0x7E ends a packet with an incomplete byte-stuff sequence, forcing the receiver to detect an error. This can be used to abort packet transmission with no chance the partial packet will be interpreted as valid by the receiver.