AT command interface works! The cause of the breakage was the same Nucleus API issue with NU_Create_Timer() which we encountered at the very beginning of this project with Riviera timers: the code in uartfax.c from TCS211 was passing 0 as the initial dummy value for the timer duration, and our FreeNucleus version doesn't like it. The fix is the same: pass 1 as the initial dummy value instead.

During some of my debug sessions (wild goose chases after difficult bugs)
I hacked the bloody hell out of some Nucleus modules, trying to instrument
them to catch the bugs I was after.  This directory is a stash-away place
for such ugly hacks.