OBD-II Protocol -- SAE J1850 VPW PWM

http://www.auto-diagnostics.info/j1850

j1850

The SAE J1850 bus bus is used for diagnostics and data sharing applications in vehicles. The J1850 bus takes two forms:

41.6Kbps Pulse Width Modulated (PWM) two wire differential approach (Ford vehicles)
10.4Kbps Variable Pulse Width (VPW) single wire approach (GM vehicles)

The single wire approach may have a bus length up to 35 meters (with 32 nodes). Developed in 1994, J1850 may be phased out for new designs. The J1850 Interface is a class B protocol.

A high resides between 4.25 volts and 20 volts, a low is any thing below 3.5 volts. High and low values are sent as bit symbols (not single bits). Symbols times are 64uS and 128uS for the single wire approach. The bus uses a weak pull-down, the driver needs to pull the bus high, high signals are considered dominant. A passive logic 1 is sent as a 128uS low level, an active logic 1 is sent as a 64uS high. A passive logic 0 is sent as a 64uS low level, an active logic 0 is sent as a 128uS high. The J1850 protocol uses CSMA/CR arbitration.