Telocator Alphanumeric Protocol (thing)

aka TAP. The protocol used to communicate with alphanumeric paging services. It's usual means of operation is a 300bps dialup modem.

A typical session goes something like:

Server                    Client
ID=                       ESC PG1 CR
114 Welcome CR ACK CR
ESC [ p CR                STX Pager ID CR
                          Pager Message CR ETX Checksum CR
211 Message sent CR ACK CR
                          ESC EOT CR
115 Goodbye CR ESC EOT CR

Checksum algorithm:
Sum all characters making up message (everything between and including STX/ETX) then transmit the checksum as 3 bytes:
Checksumdigit = (((Checksum>>8)%16)+'0');
transmit(Checksumdigit);
Checksumdigit = (((Checksum>>4)%16)+'0');
transmit(Checksumdigit);
Checksumdigit = (((lChecksum)%16)+'0');
transmit(Checksumdigit);