I have been doing some research on APRS recently and acquired a used Paccomm Tiny-2 MK-II TNC, a TNC-2 clone.
Above, a top view of the PCB.
There is no shortage of bad signals to test the tolerance of the TNC, with two local overdriven digis, and travellers through the area.
The TNC is reasonably tolerant of the poor signals, not as good as the MFJ1270B, but better than a AM7910 based TNC, better than the more modern TNC-X, OT-USB, and OT-T3.
It curiously uses a male DE9 connector, the opposite to convention for DCE, so a F-F gender changer is needed with standard cables.
The TNC is wired for a 27C512 EEPROM with a bank selection switch on the back panel so it is possible to conveniently switch between to 32kB applications.
I have tested the TNC with Paccomm v5.0, TF2.7b, SMACK v1.3 and UIDIGI 1.9B3, and it has worked properly with all of them.
The TNC contains a reset circuit that allows the processor to run when voltage is low, but it inhibits the RAM chip. It is a rather strange arrangement, but it seems to be effective in that memory corruption observed with some other TNCs does not seem to occur (Duffy 1999).
Open squelch DCD
Paccomm manufactured an accessory to add Open squelch DCD (OSDCD) to the TNC. The TNC did not come with the OSDCD option so I developed the Open squelch DCD for Paccomm Tiny2 Mk2.
The OSDCD works better than the DCD of the more modern TNC-X, OT-USB, or OT-T3.
The MFJ-1270B running UIDIGI1.9B3 could be used to good effect as a digi, WIDE or fill-in. With OSDCD accessory the receiver can be run open squelch, and is relatively immune to false indication on non-data signals.
My recommendation for use with UIDIGI is to burn an EPROM with a conventional TNC monitor that supports the CAL command for audio drive setup in the lower half of the EPROM, and UIDIGI in the upper half.
UIDIGI does have a TEST command which will send an isochronous test signal, but it is not as flexible as the CAL command in TNC-2 monitors.
The TNC was purchased used and it had a bunch of mods which needed to be undone, a flat battery, and still had the reverse polarity cap on the MAX232. The cap had gone leaky and required replacement.
Duffy, O. 1999. Flash TNC. http://owenduffy.net/module/FlashTnc./FlashTnc.htm