Hi Kathy!
I have made optoisolated RS485 interface using photocouplers (PC849 from SHARP). But if you need very fast working interface, you need some "extra solutions" or better photocoupler. This solution works fine up to 30..35kHz. I can send you "PDF how-to" file directly to your email if you want, because I cannot make an attachment here.

Good luck,
Jacek