On-Board Diagnostics (OBD), as the name suggests, refers to the in-vehicle system that detects and reports the status of vehicle health. With the help of an OBD port (fitted in a vehicle) and OBD connector, the repair technician/after-sales service provider gets access to critical vehicle parameters like vehicle speed, engine data, emission data and more in the form of Diagnostic Trouble Codes (DTC)
In order to facilitate a standardized method of diagnosis and correction of vehicle emission related malfunctions across the geographies, International Standards Organization (ISO) introduced ISO 15031 standard for automotive industry.
ISO 15031 is a standard for Communication between vehicle (OBD port/connecter, defined by SAE J1962) and external equipment (Diagnostic Tester) for emissions-related diagnostics
OBD II stack is a set of software specifications or services implemented based on ISO 15031 standard. This OBD II software stack is best suited for passenger car and light commercial vehicle applications
OBD II stack can be implemented over five signaling protocols or physical layers, as shown in the diagram:
Vehicle communication can be broadly classified into two:
In-vehicle networking: This refers to exchange of data between various automotive electronic control units (ECU) which are part of the vehicle network.
Automotive Protocols like CAN (Controller Area Network), LIN (Local Interconnect Network), FlexRay, MOST (Media Oriented Systems Transport) are defined for in-vehicle network communication.
Usually this is a broadcast network and one can add listen-only nodes easily.
Vehicle Diagnostics/External communication: Vehicle Diagnostics communication is used for diagnosis and repair of vehicle sub-systems or ECU.
This type of communication is useful for monitoring and reporting of system malfunction monitoring and vehicle parameters. An external tool may be used to re-configure and re-flash the ECU to rectify the malfunction.
Vehicle Diagnostics is a query-response communication. The external tool asks for a particular information (diagnostic request) and the ECU (the control unit to which the query is directed) responds with the answer (diagnostic response)
For diagnostic communication, the physical medium (also known as the signaling protocol) is same as the medium used for in-vehicle communication – CAN, LIN, Flexray, MOST.
Additionally, there are also dedicated protocols like K-Line, J1850 (VPW, PWM) and more that work on serial link. Over these physical layers, diagnostics applications are implemented as per the protocol standards like KWP2000 (ISO 14230), UDS (ISO 14229).
In passenger cars protocols like KWP2000 (ISO 14230) and UDS (ISO 14229) are used as signaling protocols/physical layers, but what data is sent and what conversion factors are used is proprietary to the vehicle OEM (Original Equipment Manufacturer).
Thus, it is not easy to get access to the vehicle data unless one has been granted access by the automotive OEM.
OBD II qualifies as a protocol for vehicle diagnostics communication (query-response communication).
OBD II is a part of emission legislation. Hence it follows the CARB (California Air Resources Board) initiated protocol mandates. OBD II protocol monitors parameters in emission relevant ECUs as per the guidelines of CARB.
OBD II also follows CARB guidelines for data storage and data accessibility to be provided to the external scan tools.
Diagnostic protocols like UDS and KWP2000 support re-configuration and re-flashing of ECU (writing to EEPROM and Flash), but OBD II supports only read access.
The external scan tool requirements (defined in ISO 15031) mandate that the following modes should be supported:
Using Mode $01 and $09, several vehicle parameters become accessible to any external scan tool and the data format is also standardized.
OBD protocol has witnessed several revisions (versions update) over the years. Hence OBD protocol still supports various legacy physical layers.
Most of the modern vehicle models implement OBD II over CAN.
Legacy protocols are J1850 (2 variants – PWM, VPW), K-Line and ISO9141-2. Once communication is established in one of the medium, OBD supports query-response communication.
Following are the hardware and software modules for OBD implementation:
|OBD services||NA||ISO 15031|
|OBD over CAN||CAN support in microcontroller
|ISO15765 (CAN TP layer)|
|OBD over J1850 (VPW/PWM)||PWM support in microcontroller
VPW/PWM driver based on J1850 standard
|OBD over K-Line||UART support in microcontroller
K-Line driver (based on ISO14230)
|OBD over ISO9141-2||UART support in microcontroller
Auto protocol detection is possible to identify which protocol is supported by the vehicle
Despite the limitations, OBD is the only option to support vehicle diagnostics applications for a wide range of passenger cars/after-market products already on road.
OBD II is very critical for integrating connected car and telematics solutions for millions of vehicles already on road.
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