The manual for the radio tell you for running RTTY, you go into RTTY mode, to run packet, you go into packet mode..a bit contrary to the signal link operation. It works fine in the packet mode on the radio, running RTTY but just seems odd to me they (Tigertronics) don't use the RTTY jack for that purpose. I suspect being a sound card interface and not a traditionaly TNC, that is why.
Typically the dedicated RTTY connection on a radio is for true FSK operations. Using a soundcard device like a Signal Link, the output is AFSK and probably will come out of another port like a digital mode interface used for packet and such. At least it has been my experience with most rigs I have interface from Yaesu and Kenwood.