On-board Diagnostics Resources
the datasheet links have not been revised recently. They can all be
found on the manufacturer's web sites. The broken links will be
repaired soon. Sorry for the inconvenience.
Information on OBDII and
other in-vehicle networks
There are many types different
in-vehicle networks. Light vehicles and heavy vehicles use different
styles of networks. Different manufacturers have different network
types. A number of vehicle network standards have been introduced
which are being adopted by the vehicle manufacturers. However, the great
thing about standards is that there are enough of them that everybody can
choose their own.
The state of California
introduced some uniformity to in-vehicle networks when they defined their
OBDII requirements (On-Board Diagnostics). They defined a data
protocol and a standard vehicle connector which enables a device (called a
scantool) to query the vehicle's computer and ask it if the vehicle is
polluting. The idea is that the vehicle monitors itself and it only need
be queried once in a while. There is now a OBD3 proposal which will have
the cars reporting pollution violations themselves via radio. Europe
is also formulating their own version of OBDII called EOBD.
While the OBDII standard
defined a single data protocol and physical connector, it allowed three
different electrical interfaces to be used; J1850 PWM, J1850 VPW, and
ISO-9141-2. More electrical interfaces are being approved for OBDII
purposes such as KWP2000 and CAN. The result is that there are a
number of different flavours of OBDII. The following definitions
will help explain the differences between the OBDII vehicle network
flavours and non-OBDII networks.
Here is a summary of some of
the vehicle networks standards...
J1850 VPW (Variable Pulse
Width) – this is the 10.4kbps protocol used mainly on GM vehicles.
J1850 PWM (Pulse Width
Modulated) – this is the 41.6kbps protocol used mainly on Ford
vehicles. Dual wire.
ISO9141-2 – This is the
10.4kbps asynchronous protocol used mainly on Chrysler, European, and
Asian vehicles. The protocol messages are the J1850 messages but
the physical interface is different. The protocol bit format
is the same as an asynchronous PC serial port but the unique bit rate
and voltages require a converter to be used for PC to vehicle
communications. A slow 5bps handshake sequence is require to
initialize the ISO interface to start communications. Most
vehicles will terminate the communications session if a command is not
received for 5 seconds. Single wire.
ISO/DIS14230-4 (also called
Keyword Protocol 2000 or KWP 2000) – This is a newer version of the
ISO9141 protocol. It has a provision for a fast initialization
which may be used instead of the slow 5bps handshake sequence.
CAN (Controller Area Network)
- CAN is becoming the most widely used bus for in-vehicle
communications. It is a high speed/high performance protocol.
LIN (Local Interconnect
Network) - LIN is a low cost, low speed, in-vehicle network
intended to complement the higher performance CAN network. Vehicle
systems with modest network requirements are placed on a LIN sub-network
to reduce costs. The LIN sub-networks connect to the vehicle's CAN
network via gateways. LIN uses a single-wire enhanced ISO9141
implementation (non-OBDII protocol).
Ford's UBP (UART Based
Protocol) - Proprietary, non-OBDII diagnostic protocol.
Chrysler's CCD - Proprietary,
non-OBDII diagnostic protocol.
GM's 8192 UART (ALDL Assembly
Line Diagnostic Link) - Proprietary, non-OBDII diagnostic protocol.
OBDII Related Standards
J1962 - SAE (Society of
Automotive Engineers) standard defining the physical connector used for
the OBDII interface.
J1850 - SAE standard
for the Class B Communications Network Interface
(this standard defines the actual J1850 signaling and timings)
J1939, ISO 11898
J1978 - SAE standard
for OBD II scan tools
J1979 - SAE standard
for diagnostic test modes
J2012 - SAE standard
for EPA emission test report format.
J2178-1 - SAE standard
for Class B Communications Network Message: Detailed Header Formats and
Physical Address Assignments
J2178-2 - SAE standard
for Class B Communications Network Message: Data Parameter Definitions
J2178-3 - SAE standard
for Class B Communications Network Message: Frame IDs for Single Byte
Forms of Headers
J2178-4 - SAE standard
for Class B Communications Network Message: Message Definitions for
Three Byte Headers
OBD2 Vehicle Network
The best source of
information on the J1850 protocol itself is the book of SAE standards
called HS-3000, “SAE On-Board Diagnostics for Light and Medium Duty
Vehicles - Standards Manual 1999”. It is available from B
& B electronics bookstore. The book is also available
directly from the SAE
The HS-3000 manual was
written by a committee and it isn’t an easy read. You will find
the signal levels and bit timings for the protocols. You will find
the message formats for many of the messages although you will be
flipping pages since the information is distributed through a number of
different standards documents. You will not find vehicle or
manufacturer specific message format information.
The IC manufacturers
datasheets provide a good summary of the standards documents and they
are much easier to read. The explanation of the J1850 VPW format
was not very clear in the SAE standards. Intersil and Motorola
make some J1850 VPW parts and their datasheets have good explanations of
VPW. The Oki datasheets have a good explanation of the J1850 PWM
VPW (GM style) Part Datasheets and App Notes
“J1850 Byte Level Interface Circuit” datasheet.
“J1850 Bus Transceiver For Multiplex Wiring Systems”
“Protecting The J1850 Bus For Module Loss Of Ground Using The
“J1850 8-Bit 68HC05 Microcontroller”
"VPW J1850 Multiplexing and Motorola’s Byte Data Link
Controller (BDLC) Module"
“J1850 Multiplex Bus Comms Using the MC68HC705C8 & SC371016
J1850 Serial Link Transceiver
Search the Motorola site
www.mot-sps.com for J1850 for more information on J1850 enabled
J1850 PWM (Ford style)
Part Datasheets and App Notes
OKI Semiconductor MSM6636B
- SAE J1850 PWM Automotive Bus Controller, parallel multiplexed bus
OKI Semiconductor MSM6636
- SAE J1850 PWM Automotive Bus Controller, serial interface
OKI Semiconductor Users'
OKI Semiconductor Users'
(Chrysler, European, Asian, International) Part Datasheets
“MC33199DW ISO9141 serial link driver (has K& L lines)”
ISO9141 serial link interface (K line only) PDF file.
(K line) and Si9243A
(K and L lines) bus transceivers. PDF Files.
Driver ICs for ... ISO9141 PDF File (3 pages)
White Papers and Articles
Technologies, Inc., "In-Vehicle
Networking Briefing Slides" (93 KB).
AVT, Inc., "In-Vehicle
Networking Briefing Slides with Presenter's Notes" (105 KB).
These articles summarize the
EPA legislation that requires cars to have on-board diagnostic
capabilities. The communications network that accesses the diagnostics
is discussed in detail.
Evaluation Engineering Computer
Networks Take to the Road: Part 1
Evaluation Engineering Computer
Networks Take to the Road, Part 2
magazine. Critique of On
Board Diagnostics by Ray T. Bohacz.
Intel Corp., D. John
the J1850 Protocol"
Engineering Inc. makes low cost ($72+ US) OBD interfaces for use
with your own software.
AutoTap - OBDII Diagnostic
Scanner from B&B Electronics ($290+ US with software, various
Schematics & PCB Layout open source ISO-9141 to RS-232 converter
- On-Board Diagnostic System home page hosted by B&B
Electronics Autotap Cable Pinout PDF file (1 page).
Engineering sells SAE J1962 connector to DB9F cables and SAE J1962
Pioneer Packard is the
distributor for Delphi Packard which manufacturers OBD2 connectors.
Their part numbers for the SAE J1962 male connector are: 12110252 shell,
12047581 pins, 12110254 locking clip.
Products Inc : OBDII Diagnostic Connector industrial strength
connector for manufacturers.
LIN (Local Interconnect
Application Notes and White Papers
LIN Press Conference Press
Release 6 Mar 2000 PDF file (34 pages, 2.9MB).
LIN Protocol Implementation Using PICmicro PDF file.
LIN Protocol Implementation Using PICmicro Source code.
Heavy Vehicle Network
J1708 Application Notes and White Papers
National Semiconductor AN-915
Automotive Physical Layer SAE J1708 and the DS36277 PDF file.
White Papers, and Books
A Simple Way to
Prevent Blindzone Accidents PDF file.
Pease's homepage for his book How
to Drive Into ACCIDENTS ...and How NOT To.