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OBD2 [On-Board Diagnostics II] defines a communications protocol and a standard connector to acquire data from passenger cars. It was required by U.S. EPA on all gasoline powered cars and light duty trucks manufactured for the U.S. after 1996 to help monitor/inspect vehicle emissions. OBDII will light a lamp called a MIL (malfunction indicator lamp), also known as the "check engine" light on the dash. A scan-tool may also be used to probe the OBDII connector OBDII data as defined by the SAE J1979 standard. The OBD-II standard allows for multiple electrical interfaces, which complicates the hardware used to interface with the vehicle.
The J1850 VPW single wire protocol, used by GM may be found on an OBDII bus, if so the connector will have contacts in pins 2, 4, 5, and 16, with no contact in pin 10. The J1850 PWM two wire protocol, used by Ford may be found on an OBDII bus, if so the connector will have contacts in pins 2, 4, 5, and 10, with no contact in pin 16. The ISO 9141-2 single wire protocol, used by Chrysler may be found on an OBDII bus, if so the connector will have contacts in pins 4, 5, 7, 15 and 16. The protocol and command set is fixed by SAE J1979, so they are the same for all three protocols, only the electrical layers are changed. The CAN Bus may also be found on the OBDII bus.
The J1850 bus is used for diagnostics and data sharing applications in vehicles. The J1850 bus takes two forms; A 41.6Kbps Pulse Width Modulated (PWM) two wire differential approach, or a 10.4Kbps Variable Pulse Width (VPW) single wire approach. The single wire approach may have a bus length up to 35 meters (with 32 nodes). 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. Developed in 1994, J1850 may be phased out for new designs. The ISO 9141-2 single-wire asynchronous interface operates at 10.4kbps
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