OBD Primer
There is a lot of information about On Board Diagnostics (OBD) systems for current vehicles we will cover it with the view of a scanner implementation. At the time of this writing there are already manufacturers using engine management software that are labeled as OBDIII but here we focus on the standards concerning the connectivity to a scanner.
History
In 1988 California required that all automotive makers provided a system that would identify trouble codes this was OBD generation I or OBD1 and required a Check Engine light to signal the driver of a problem. OBD1 was then in effect for 1988-1995 model years and they were mainly Emission codes. OBD generation II or OBD2 was a requirement for ALL cars 1996 and newer in the US and was a superset of the OBD1 with additional codes for diagnosing emission and regular maintenance items and required that the computer monitors and performs diagnostics tests on vehicle emission control systems on a regular basis while the vehicle is driven, also the Check Engine Light officially became a Malfunction Indicator Light(MIL)
Flavors of Current OBD
In the US vehicles 1996 and newer have been required by Federal regulations to have the OBD2 system in place, in the EU 2001 and newer vehicles are required to have the EOBD which is pretty much equivalent to OBD2 and most of the P0 codes are the same. EOBD (ISO15031) states that the same protocols can be used as OBDII with the exception of CAN ( controller area network ISO 15765 ). This is not used in the american OBDII system
The OBD connector (J1962)
The OBD implementation provides a standard 16-pin connector(J1962) where the user can plug the OBD2 scanner to read/reset the Diagnostics Trouble Codes(DTC) and even though the protocol of the information is a standard simple packet based one there are three main different transmission schema of this protocol, there is the ISO which is used by most European, Asian and Chrysler makes, then GM and FORD use a VPW and PWM respectively. The J1962 accommodates all of the 3 transmission protocols as follows:
4,5,7,15 and 16 it is ISO-9141-2 or KWP-2000
2,4,5,10 and 16 it is SAE J1850 PWM
2,4,5,and 16 but not 10 it is SAE J1850 VPW
The chart below has some useful information about these pins.
The OBD Message Format in simple terms
The OBD transfers the information like most other protocols with a Message format(OBD) which consists of a header, a body and a CRC byte as a checksum per frame of info, a picture is worth a thousand words the figure below shows how the scanner queries for a Current Engine coolant Temp. and how the OBD responds
This information is transmitted in the three different flavours stated above but in essence the information is encoded the same way. Additionally there are some extra rules for the protocol like a minimum response time, once a connection is established some pulses need to be interchanged to keep the connection alive.
Implementations
Once the transmission encoding and the packing of the information is defined implementing it is straightforward. There has been some very simple implementations where all is needed is connecting the signal pins for each of the transmission protocols to a regular serial port and through software the decoding was accomplished. Then there are proprietary implementations whereby a black box does all the decoding and the information displayed on a display panel. Also there are chipsets that help on building such interfaces e.g. the ELM 323 for ISO standard performs all the encoding/decoding/syncing it from the OBD port to a regular serial port there are quite a few users following this method and shareware programs are widely available for such systems.
Controller Area Network(CAN) and OBD
First established by Bosch as an industrial control network was soon chosen by car companies for its robustness. e.g. Mercedes has used CAN to enable communications between internal modules as early as 1992. As we have seen before the OBDii allowed for different ways of encoding(PWM,VPW,ISO) creating the respective complications. CAN allows for a single encoding and communcations and has already been integrated into the OBDii spec by the ISO committee. The California CARB has already embraced the CAN protocol and manufacturers can start implementing in 2003 and must be compliant by 2008. There is already pin definitions(CAN-H, CAN-L, GND) in the connector to carry out CAN.
Sources of information
The Society of Automotive Engineers(SAE) publishes full description of their standards as Jxxx documents and the ISO do the same on their documentation. i.e. The ISO9141 is the standard used by European, Asian and Chrysler makes, the J1962 is the SAE standard that defines the OBD connector and so on.
