There is none. The MFJ-1375 uses the DTR (data terminal ready) and/or CTS (clear to send) pins to key the transmitter.
This link (http://www.tldp.org/HOWTO/Serial-HOWTO-4.html#flow_control
) offers an explanation of what the signals are. Google for examples of how to set them up on a PC (I used to be able to do it, but that was on an Atari 850 interface box).
Good luck with your project.
N3KQX - echolink #268847