Elm Electronics – Circuits for the Hobbyist
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ELM327
3 of 5
ELM327QSA
Ordering Information
These integrated circuits are 28 pin devices, available in either the 300 mil plastic DIP format or in the 300 mil SOIC
surface mount type of package. To order, add the appropriate suffix to the part number:
300 mil 28 pin Plastic DIP..............................ELM327P
300 mil 28 pin SOIC....................................ELM327SM
J1850 Bus- (pin 14)
This active high output is used to drive the J1850
Bus - Line to an active (dominant) level for J1850
PWM applications. If unused, the output can be left
open-circuited.
RTS (pin 15)
This active low “Request To Send”
input can be
used to interrupt processing in order to send a new
command. Normally high, the line is brought low for
attention, and should remain so until the Busy line
(pin 16) indicates that the ELM327 is no longer busy.
This input has Schmitt trigger waveshaping.
Busy (pin 16)
This active high output shows the current state of the
ELM327. If it is at a low level, the processor is ready
to receive ASCII commands and characters, but if it
is at a high level, commands are being processed.
RS232Tx (pin 17)
This is the RS232 data transmit output. The signal
level is compatible with most interface ICs (output is
normally high), and there is sufficient current drive to
allow interfacing using only a PNP transistor, if
desired.
RS232Rx (pin 18)
This is the RS232 receive data input. The signal
level is compatible with most interface ICs (the level
is normally high), but can be used with other
interfaces as well, since the input has Schmitt trigger
waveshaping.
VDD (pin 20)
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.
ISO K (pin 21) and ISO L (pin 22)
These are the active high output signals which are
used to drive the ISO 9141 and ISO 14230 buses to
an active (dominant) level. Many new vehicles do not
require the L Line - if yours does not, you can simply
leave pin 22 open-circuited.
CAN Tx (pin 23) and CAN Rx (pin 24)
These are the two CAN interface signals that must
be connected to a CAN transeiver IC for proper
operation. If you are connecting to an existing CAN
system, the integrity of that sysem might be
jeopardized if a proper interface is not used. See the
Example Applications section for more information.
RS232 Rx LED (pin 25), RS232 Tx LED (pin 26),
OBD Rx LED (pin 27) and OBD Tx LED (pin 28)
These four output pins are normally high, and are
driven to low levels when the ELM327 is transmitting
or receiving data. Current capability is suitable for
directly driving most LEDs through current limiting
resistors, or interfacing to other logic for status
reporting. If unused, these pins should be left open-
circuited.
Pin Descriptions (continued)
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