I loaded YAAC-157 onto my Raspberry Pi running Buster to replace YAAC-152 and without changing anything, it nicely connected with the TNC-Pi9k6 including receiving and transmitting.? Therefore, I would guess that there is something in Raspberry Pi OS Stretch that interferes with it supporting YAAC to connect with the serial port /dev/ttyS0.? Note that in Buster, the serial port shows as /dev/ttyAMA0 but the user notes I found from WVCARC who makes the TNC-Pi9k6, and from John Hansen who had made it before he retired, all working with John Wiseman who designed the card, said that this change had occurred with Stretch.
If there is anything I can to to help with this, including testing, please let me know.
