For a service like FTP, to disconnect would be QUIT/r/n. If a command string is not specified, the connection is closed by sending a FIN packet and then an RST packet.
The /r (carriage return) and /n (line feed) are the conventions for sending these control characters to terminate a string. You can use:
/r = 0x0a/n = 0x0d/t = 0x09 or /xnn where nn is any hexadecimal value from 00 to FFThe disconnect string is:
Send=QUIT/r/n