Define a macro that emulates a serial port:

add unsigned long last_time;       /* create local symbol */
define int ser_port(offset,base)   /* macro definition */
    int offset;                /* offset of device register */
    unsigned short *base;      /* base of port */
{
    unsigned short value;
    if (offset == 0)
    {                           /* control register */
        if (last_time && ((@cycle - last_time) < 20))
        {
            error (0, "ser_port: access less than
                allowed time: %d", @cycle - last_time);
            return (0);
        }
        last_time = @cycle;
        value = base[offset];  /* cache written value */
        base[offset] = 0;      /* reset */
        if (value == 0x20)
        {                      /* want to read */
            ...
        }
        ...
    }
    ...
    $SETREG @PC = #line_num$; /* reset PC to skip the patched lines */
}
.

Start a debugging session and set a breakpoint on the source code to stop execution immediately before it accesses the serial port, for example at line 20 of module_name.c, and attach your macro to this breakpoint:

BREAKINSTRUCTION \MODULE_NAME\#20 ;ser_port(0,&ser_port)

Continue debugging using the newly-inserted serial port emulation.