I am usually a fan of the DS1232 (ADM1232, MAX1232) unfortunately since the built in watchdog timer cannot be disabled during development process I could not use it in this design
The 1233 is the same w/o the watchdog.

I am a fan of internal watchdogs, in my opinion it is much more likely taht a runaway program will toggle a port pin, that it is that two specific consequtive writes to a SFR will happen.

Erik