ELM323
Pin Descriptions
VDD (pin 1)
RS232Tx (pin 6)
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.
This is the RS232 transmit or data output pin. The
signal level is compatible with most interface ICs,
and there is sufficient current drive to allow
interfacing using only a PNP transistor, if desired.
LED Drive Outputs (pins 7, 8, 9, and 10)
These four pins are driven to low levels when the
ELM323 is transmitting or receiving RS232 or OBD
data. Otherwise, they are at a high level. Current
capability is suitable for directly driving most LEDs
through current limiting resistors. If unused, these
pins should be left open-circuited.
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 also normally
be connected between each of these pins and the
circuit common (Vss).
OBDIn (pin11)
LFmode (pin 4)
The OBD data is input to this pin, with a high logic
level representing the active state of the OBD K line.
No Schmitt trigger input is provided, so the OBD
signal should be buffered to minimize transition
times for the internal CMOS circuitry.
This input is used to select the default linefeed mode
after a power-up or system reset. If it is at a high
level, then by default messages sent by the ELM323
will be terminated with both a carriage return and a
linefeed character. If it is at a low level, lines will be
terminated by a carriage return only. This behavior
can always be modified by issuing AT L0 or AT L1
commands (see the section on AT Commands).
OBDL (pin 12) and OBDK (pin 13)
These are the active high output signals which are
used to drive the OBD bus, using external NPN
transistors. Data transfer normally occurs only by the
K line, but the standards require that the L line be
implemented as well in order to properly initialize the
bus. See the Example Applications section for more
details.
RS232Rx (pin5)
A 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 47KW) is
installed in series. (Internal protection diodes will
pass the resistor current safely to the supply
connections, protecting the ELM323.) Internal signal
inversion and Schmitt trigger waveshaping provide
the necessary signal conditioning.
VSS (pin 14)
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 150 mil SOIC surface mount
type of package. To order, add the appropriate suffix to the part number:
300 mil Plastic DIP...................................ELM323P
150 mil SOIC........................................ ELM323SM
All rights reserved. Copyright 2001, 2003 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.
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