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.

/* this header file defines the data structure for baud rate machinations */

struct baudrate {
	char	*name;
	speed_t	termios_code;
	int	bootrom_code;
};