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.

/*
 * Commands which don't belong anywhere else
 */

#include <sys/types.h>
#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <stdlib.h>
#include "pktmux.h"
#include "etm.h"
#include "tmffs1.h"

void
cmd_check_ffs1(argc, argv)
	char **argv;
{
	u_char cmdpkt[5];

	cmdpkt[1] = ETM_FFS1;
	cmdpkt[2] = FPI_TMFFS_VERSION;
	cmdpkt[3] = FPI_END;
	send_etm_cmd(cmdpkt, 3);
}