What is the extra data transmitted between Sealevel Ethernet Serial Servers and the SeaLINK driver?

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The extra data is non-serial Telnet command messages such as COM port settings and modem control signals. SeaLINK devices communicate using the RFC-2217 Telnet Com Port Control protocol so they also send control sequences and certain bytes in a Telnet compliant manner.

Every Telnet control sequence begins with the “Interrupt As Command” (IAC) byte, which has the decimal value 255. Serial data is sent with the IAC value (decimal 255) repeated as a two-byte (escaped) sequence. The receiving device interprets that two-byte sequence as a single (unescaped) serial data byte with decimal value 255 instead of a Telnet control sequence.