This page serves as a summary of known libraries and rough status of maturity/development.
SAE J1850 PWM, Status: very early in development
PWM @ 41.6 kB/sec, standard of the Ford Motor Company
No known compete Arduino libraries for this. There are some partial libraries that could be combined into one complete library.
SAE J1850 VPW, Status: very early in development
variable pulse width — 10.4/41.6 kB/sec, standard of General Motors
ISO 9141-2, Status: very early in development
This protocol has an asynchronous serial data rate of 10.4 kBaud. It is somewhat similar to RS-232; however, the signal levels are different, and communications happens on a single, bidirectional line without additional handshake signals. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.
ISO 14230 KWP2000, Status: very early in development
(Keyword Protocol 2000)
Physical layer identical to ISO 9141-2
Data rate 1.2 to 10.4 kBaud
Message may contain up to 255 bytes in the data field
ISO 15765 CAN, Status: libraries well developed
250 kBit/s or 500 kBit/s.
As of 2008 all vehicles sold in the US are required to implement CAN as one of their signaling protocols.