Again RS-485 full duples is called RS-422. As I stated it's no sweat to handle 256kb with a little assembler (approx 20 lines). Re RS-485/422 have a look at the application notes at National Semiconductor, they will save you bundles of time. Many facts about RS-422/485 are not well known and reading these notes makes it problem free. I tried the first time to go on my own (was before the www) and it took me weeks to get it to go until finally some information popped up.

Hav en god dag

Erik