OBD2 Communication standard

Description of the standard

While the port is standardised, several communication protocols remain possible depending on the manufacturers. They are represented below by the 5 columns ISO 9141-2, ISO 14230, SAE J1850, ISO 15765 and SAE J1979. It is the job of the software in the ELM interfaces to decode these various communication standards. Our software, converts the frames from the ELM interfaces into interpretable data based on the SAE J1979 standard.


Summary of EOBD standards

The different protocols

The protocols based on lines K and L

The protocols described below all use the same medium (same electrical connection). However, differences in the data sent mean that they are not compatible with each other.

ISO9141-2

This protocol is used mainly by European manufacturers.
Communication speed: 10.4 kbps
It uses the following pins on the connector:

  • pin 7: for line K
  • pin 15: for line L which may be optional

ISO14230 (KWP2000 or KW2000)

ISO14230 is the successor to ISO9141, and repeats the main characteristics. This protocol is used mainly by European manufacturers. Within this protocol, there are 2 "sub" protocols that differ mainly in their level of initialisation: slow init or 5 baud init and fast init.

Slow init (or 5 baud init)

Communication speed: from 1.2 to 10.4 kbps
It uses the following pins on the connector:

  • pin 7: for line K
  • pin 15: for line L which may be optional

Fast init

Communication speed: from 10.4 kbps only
It uses the following pins on the connector:

  • pin 7: for line K
  • pin 15: for line L which may be optional

KWP OBD frame oscillogram

Example of an engine speed request frame followed by its response in ISO1420
Yellow = K line, Blue = L line

KW1281, KW71 and KW82

These protocols (specified by SAE J2818) were mainly used by German car makers prior to the application of the EOBD standard.

Protocols based on SAE J1850

WM (SAE J1850)

This protocol is mainly used by Ford. But it doesn't necessarily apply to FORDs sold in Europe that use the ISO protocol.
Communication speed: from 41.6 kbps only
It uses the following pins on the connector:

  • pin 2: BUS +
  • pin 10: BUS -
SAE J1850 PWM OBD frame oscillogram

Example of an engine speed request frame
Yellow = Bus +, Blue = Bus -

VPW (SAE J1850)

This protocol is mainly used by General Motors.
Communication speed: from 10.4 to 41.6 kbps
It uses the following pins on the connector:

  • pin 2: BUS +
SAE J1850 VPWM OBD frame oscillogram

Example of an engine speed request frame
Yellow = Bus +

Protocols based on CAN

CAN (ISO 15765)

This is the protocol that will ultimately be used by all vehicles. It offers the best rapidity and flexibility.
Communication speed: from 125 to 500 kbps
It uses the following pins on the connector:

  • pin 6: CAN High
  • pin 14: CAN Low
CAN OBD frame oscillogram

Example of an engine speed request frame in 500 kbps CAN bus
Yellow = CAN high, Blue = CAN low, emission rate = 230µs

Note: the voltage levels are slightly different for "low speed" frames (125 kbps).

CAN (SAE J1939)

This protocol is mainly used for heavy duty vehicles (trucks, agricultural machinery, site machinery).
Communication speed: from 125 to 500 kbps
It uses the following pins on the connector:

  • pin 6: CAN High
  • pin 14: CAN Low
Heavy Duty Tractor diagnostic connector

Most often, the diagnostic connector
is not the same on this type of vehicle, and looks like the one above