I highly recommend either http://www.scantool.net/scan-tools/ics/stn1110.html or http://www.scantool.net/scan-tools/m...roobd-200.html depending on the maturity of your design, and how much design work you want to have to do yourself. That would eliminate a lot of unneccesary coding that has already been done. With either option, you can have your PIC talk directly to the STN chip, and it would handle all the protocol stuff. You just have to give it standard ELM style commands to retrieve data. Much easier.