Sub VFO on Digital Modes

PROBLEM: I borrowed a TNC and am playing a bit with RTTY, etc. Every thing is fine copying stuff, sending stuff, etc UNTIL
I try to use the SUB RX and then there is nothing. Either the SUB output doesn't go to the back panel or I'm not pushing the right
button. I get intimidated by digital modes anyway since I am such a poor typist and maybe I am just overwhelmed...or at least whelmed. Tom Horton K5IID

SOLUTION: .OZ6TL straightened me out... there is a switch inside the radio that must be switched if you want to use the Sub Rx audio for ASFK or RTTY output. It is spelled out in real words on page 53!

 
But Don, W5FKX states that he simply splits off the audio from the audio out on the back of the and sends it  both to the RTTY modem and to the remote speaker. That sounds very logical to me Thanks Don!!