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.

/* global variables for the MPFFS reader code */

#include "types.h"
#include "struct.h"

struct inode *mpffs_active_index;
int mpffs_root_ino;
int mpffs_init_done;