ELM327L
Pin Descriptions (continued)
IgnMon / RTS (pin 15)
discussion on page 86 (‘Modifications for Low Power
Standby Operation’) provides more detail on how to
use this output.
This input pin can serve one of two functions,
depending on how the Power Control options
(PP 0E) are set.
If bit 7 of PP 0E is a ‘0’, pin 16 will function as a
‘Busy’ output, showing when the ELM327 is actively
processing a command (the output will be at a high
level), or when it is idle, ready to receive commands
(the output will be low).
If both bit 7 and bit 2 of PP 0E are ‘1’s, this pin will
act as an Ignition Monitor. This will result in a switch
to the Low Power mode of operation, should the
IgnMon signal go to a low level, as would happen if
the vehicle’s ignition were turned off. An internal
‘debounce’ timer is used to ensure that the ELM327
does not shut down for noise at the input.
By default, bit 7 of PP 0E is ‘1’, so pin 16 provides
the Power Control function.
RS232Tx (pin 17)
When the voltage at pin 15 is again restored to a
high level, and a time of 1 or 5 seconds (as set by
PP 0E bit 1) passes, the ELM327 will perform a
‘Warm Start’ and return to normal operation. A low to
high transition at pin 15 will in fact restore normal
operation, regardless of the setting of PP 0E bit 2, or
whether pin 15 was the initial cause for the low
power mode. This feature allows a system to control
how and when it switches to low power standby
operation, but still have automatic wakeup by the
ignition voltage, or even by a pushbutton.
This is the RS232 data transmit output. The signal
level is compatible with most interface ICs (the
output is high when idle), 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 (when at
idle, the level should be high), but can be used with
other interfaces as well, since the input has Schmitt
trigger waveshaping.
If either bit 7 or bit 2 of PP 0E are ‘0’, this pin will
function as an active low ‘Request To Send’ input.
This can be used to interrupt the OBD processing in
order to send a new command, or as previously
mentioned, to highlight the fact that the ignition has
been turned off. Normally kept at a high level, this
input is brought low for attention, and should remain
so until the Busy line (pin 16) indicates that the
ELM327 is no longer busy, or until a prompt
character is received (if pin 16 is being used for
power control).
VSS (pin 19)
Circuit common must be connected to this pin.
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 ELM327 processor, so an
external reset signal is not required. Refer to the
Electrical Characteristics section for further
information.
By default, pin 15 acts as the RTS interrupt input.
This input has Schmitt trigger waveshaping.
PwrCtrl / Busy (pin 16)
This output pin can serve one of two functions,
depending on how the Power Control options
(PP 0E) are set.
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.
If bit 7 of PP 0E is a ‘1’ (the default), this pin will
function as a Power Control output. The normal state
of the pin will be as set by PP 0E bit 6, and the pin
will remain in that state until the ELM327 switches to
the Low Power mode of operation, when the output
changes to the opposite level. This output is typically
used to control enable inputs, but may also be used
for relay circuits, etc. with suitable buffering. The
CAN Tx (pin 23) and CAN Rx (pin 24)
These are the two CAN interface signals that must
be connected to a CAN transceiver IC (see the
Example Applications section for more information).
5 of 94
ELM327L DSA
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