ELM322
OBD (VPW) to RS232 Interpreter
Description
Since the 1996 model year, North American
automobiles have been required to provide an OBD,
or On Board Diagnostics, port for the connection of
test equipment. Data is transferred serially between
the vehicle and the external equipment using this
connection, in a manner specified by the Society of
Automotive Engineers (SAE) standards. In addition
to operating at different voltage levels, these ports
also use a data format that is not compatible with the
standard used for personal computers.
The ELM322 is an 8 pin integrated circuit that is
able to change the data rate and reformat the OBD
signals into easily recognized ASCII characters. This
allows virtually any personal computer to
communicate with an OBD equipped vehicle using
only a standard serial port and a terminal program.
By also enhancing it with an interface program,
hobbyists can create their own custom scan tool.
This integrated circuit was designed to provide a
cost-effective way for experimenters to work with an
OBD system, so a few features such as RS232
handshaking, variable baud rates, etc., have not
been implemented. In addition, this device is only
able to communicate using the 10.4KHz J1850 VPW
protocol that is commonly used in General Motors
and some Daimler Chrysler vehicles.
Features
• Low power CMOS design
• High current drive outputs - up to 25 mA
• Crystal controlled for accuracy
• Fully configurable using AT commands
• Standard ASCII character output
• High speed RS232 communications
• 10.4 KHz J1850 VPW protocol
Connection Diagram
PDIP and SOIC
(top view)
V
DD
XT1
XT2
OBDIn
1
2
3
4
8
7
6
5
V
SS
OBDOut
Tx
Rx
Applications
• Diagnostic trouble code readers
• Automotive scan tools
Block Diagram
XT1
2
3
3.58 MHz
XT2
Timing and
Control
Tx
Rx
6
5
RS232
Interface
Interpreter
OBD
Interface
4
OBDIn
OBDOut
7
ELM322DSE
Elm Electronics – Circuits for the Hobbyist
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