I do not believe it will drip feed from an external port. Port A is a RS232 port. It is used for uploading and downloading. Files are typically transfered to the hard drive, flash card or whatever mass storage device you have and you DNC from that device. You could also drip feed from the floppy too.
Check your operating manual. You only need to drip feed if your program exceeds 256KB which is the buffer size of the BMDC board.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)