I was playing with old software to connect to PLATO, and it appears that modern PLATO systems like Cyber1.org and irata.online wrap their connection in a Telnet-like protocol (0xFF -> 0xFF 0xFF and 0x0D -> 0x0D 0x00).

DOSBox's modem emulator supports a Telnet mode, with ATNET1 or (in DOSBox-X at least) AT+NET1

However, this mode doesn't convert 0xAA 0xAA into 0xAA unless the other end indicates that it's sending binary.

Is this correct behavior and the PLATO Telnet-like stuff is wrong, or should DOSBox always turn 0xFF 0xFF into 0xFF?