YAAC does not have any TNC modem capability built into it; it expects you to provide some external implementation (in hardware or software). Given you're using a Raspberry Pi with UDRC-II, your best option would be to use DireWolf as your software TNC. As such, you wouldn't even need to use YAAC as a digipeater or I-gate, as DireWolf has that capability all by itself (and it would be more efficient at it too, because DireWolf isn't trying to update all the graphical screens YAAC has), although you could choose to not enable DireWolf's digipeating and I-gating capability and use YAAC to do so for the logging.
Even if you use DireWolf as a stand-alone digipeater and/or I-gate, you could still run YAAC and connect it to the KISS or AGWPE ports on the DireWolf process so you could watch what DireWolf was processing.
Andrew, KA2DDO
author of YAAC
