Why dont u rather use a max232. No voltage problems or extra power
supplies required. It generates all the volatges needed to convert RS232
to TTL levels at the pic side. I've used it and it's working just fine.

On the voltage level debate. There is an extensive appnote on RS422/485
that fully describes the principles of RS232 with protocals, voltage
levels etc.

http://www.bb-elec.com/techlibr.html

Best regards

Jattie van der Linde