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.

/*
 * Handling of TMFFS2 responses from ETM
 */

#include <sys/types.h>
#include <stdio.h>

extern u_char rvi_msg[];
extern int rvi_msg_len;

void
handle_ffs2_response()
{
	if (rvi_msg[3])
		print_etm_pkt_raw("FFS2 error");
	else if (rvi_msg_len == 5)
		async_msg_output("FFS2 command successful");
	else
		print_etm_pkt_raw("FFS2 response");
}