If you want modbus RTU like binary data transfer, then you can use parity error to indicate command code like start of message or end of message.
For example let 8 bit code of start of message is 0xAA and let 8 bit code of end of message is 0xBB, then send 0xAA or 0xBB with parity error. At the receiver side check for parity error. If parity error occured, then do not reject the byte, check for 0xAA or 0xBB. If it is 0xAA or 0xBB then take necessary action. If the error byte is neither of them then flag it as a real error.