When you communicate with a PC you are now working with RS232 which requires a MAX232 or other circuitry to convert to TTL (the elm devices uses that circuitry for serial in/out). However, with the elm device you are talking TTL and can connect directly to a pic. BTW you can eliminate all the serial circuitry between the elm and pic (you will still need to flip Rx in software or add circuitry to do it via hardware).