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ELM322 Datasheet Preview

ELM322 Datasheet

OBD (VPW) to RS232 Interpreter

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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
www.DataSheeatb4leU.tcoocmhange 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.
Applications
• Diagnostic trouble code readers
• Automotive scan tools
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)
VDD 1
XT1 2
XT2 3
OBDIn 4
8 VSS
7 OBDOut
6 Tx
5 Rx
Block Diagram
3.58 MHz
XT1 2
3 XT2
Timing and
Control
Tx 6
Rx 5
RS232
Interface
Interpreter
OBD
Interface
4 OBDIn
7 OBDOut
ELM322DSE
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
1 of 18




ELM Electronics

ELM322 Datasheet Preview

ELM322 Datasheet

OBD (VPW) to RS232 Interpreter

No Preview Available !

ELM322
Pin Descriptions
VDD (pin 1)
This pin is the positive supply pin, and should
always be the most positive point in the circuit.
Internal circuitry connected to this pin is used to
provide power on reset of the microprocessor, so
an external reset signal is not required. Refer to
the Electrical Characteristics section for further
information.
XT1 (pin 2) and XT2 (pin 3)
A 3.579545 MHz NTSC television colourburst
crystal is connected between these two pins.
Crystal loading capacitors (typically 27pF) will
www.DataSheet4aUlso.cnoomrmally be connected between each of the
pins and the circuit common (Vss).
OBDIn (pin 4)
The OBD data is input to this pin, with a low logic
level representing the active state (and a high,
the passive). No Schmitt trigger input is
provided, so the OBD signal should be buffered
to minimize transition times for the internal
CMOS circuitry. The external level shifting
circuitry is usually sufficient to accomplish this -
refer to the Example Applications section for a
typical circuit.
Rx (pin 5)
The computer’s RS232 transmit signal can be
directly connected to this pin from the RS232
line as long as a current limiting resistor
(typically about 47K) is installed in series.
(Internal protection diodes will pass the input
currents safely to the supply connections,
protecting the ELM322.) Internal signal inversion
and Schmitt trigger waveshaping provide the
necessary signal conditioning.
Tx (pin 6)
The RS232 data output pin. The signal level is
compatible with most interface ICs, and there is
sufficient current drive to allow interfacing using
only a single PNP transistor, if desired.
OBDOut (pin 7)
This is the active low output signal which is used
to drive the OBD bus to its active (high) state.
Typically this is accomplished by switching a PNP
type transistor on - see the Example Applications
section for more details.
VSS (pin 8)
Circuit common is connected to this pin. This is
the most negative point in the circuit.
Ordering Information
These integrated circuits are available in either the 300 mil plastic DIP format, or in the 208 mil SOIC surface
mount type of package. To order, add the appropriate suffix to the part number:
300 mil Plastic DIP............................... ELM322P
208 mil SOIC..................................... ELM322SM
All rights reserved. Copyright 2001, 2002, 2003, 2004 Elm Electronics.
Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be
given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information
described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these
products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or
systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve
reliability, function, or design.
ELM322DSE
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
2 of 18


Part Number ELM322
Description OBD (VPW) to RS232 Interpreter
Maker ELM Electronics
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ELM322 Datasheet PDF






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