Serial Port
Most older ThinkPads have a Male DB-9 RS232 Serial Port, capable of running up to 115,200bps
Current ThinkPads no longer have a physical serial port, but still have the logic integrated, all that is required is to attach the ThinkPad to a supported Dock or Port Replicator with a serial port.
ThinkPads produced from 1994 onwards have a NS16550A compatible uart.