you need some software on the PC side that monitors the connection and can activate the DNC automatically.
I think RYM - CNC Monitoring & Communication Solutions have the software required.
The main man to ask is Dan Fritz.... rym is his company
but if you have a memory card slot just use that. it's far easier and quicker.
Fanuc 31i/32i can read from very large compact flash card via PCMCIA card adapter. The machine may also have USB connectivity so you can use common USB thumb drive.
There's really no need for old technology like DNC on latest Fanuc controls.