ELM327
Example Applications
The SAE J1962 standard dictates that all OBD
compliant vehicles must use a standard connector, the
shape and pinout of which is shown in Figure 8 below.
The dimensions and pin connections for this ‘Type A’
connector are fully described in the SAE J1962
standard.
strongly recommended that you use a commercial
transceiver chip as shown. We show a Microchip
MCP2551 in this circuit, but most major manufacturers
produce CAN transceiver ICs – look at the NXP
PCA82C251, the Texas Instruments SN65LBC031,
and the Linear Technology LT1796, to name only a
few. Be sure to pay attention to the voltage limits as
depending on the application, you may have to tolerate
24V, and not just 12V.
The next interface shown is for the ISO 9141 and
ISO 14230 connections. We provide two output lines,
as required by the standards, but depending on your
vehicle, you may not need to use the ISO-L output.
(Many vehicles do not require this signal for initiation,
but some do, so it is shown here.) If your vehicle does
not require the L line, simply leave pin 22 unused, and
do not install Q6, R16 or R17.
The ELM327 controls both of the ISO outputs
through NPN transistors Q6 and Q7 as shown. These
transistors have 510W pullup resistors connected to
their collectors, as the standard requires. We are often
asked about substitutes for these resistors – the
standard specifies 510W but in a pinch you might be
able to use 560W. A better solution would be to make
510W from 240W and 270W 1/4W resistors in series.
We do not recommend using a lower value for the
resistance as it stresses every device on the bus. Note
that 1/2W resistors are specified in Figure 10 as a
short at 13.8V causes about 0.4W dissipation.
Be careful if you are designing a circuit that might
monitor other scan tools. Both the ELM327 and the
other scan tool would present 510W resistors, so the
vehicle would see 255W. This would very likely cause
data errors, and might even damage some circuitry. To
avoid this, you might wish to build in a way to switch
the ELM327’s 510W resistors out, and replace them
with a larger value. For example, you might put 10KW
resistors in series with the 510W ones, and add
jumpers or switches across the 10KW resistors. The
jumpers could be removed or the switches opened if
you are monitoring a scan tool.
1
9
8
16
Figure 8. The J1962 Vehicle Connector
The circuit that you build with the ELM327 will be
required to connect by way of a matching male J1962
connector. Fortunately these are available from
several sources, easily found with a web search.
Note that before OBDII was adopted, several
vehicles (most notably those made by General Motors)
used a connector like that shown above for factory
communications. These vehicles typically used what is
known as the ALDL protocol, which the ELM327 does
not support. Check that your vehicle is actually OBDII
compliant before building your ELM327 circuit.
The circuit on page 73 (Figure 9) shows how the
ELM327 might typically be used. Circuit power is
obtained from the vehicle via OBD pins 16 and 5 and,
after a protecting diode and some capacitive filtering,
is presented to a five volt regulator. (Note that a few
vehicles have been reported to not have a pin 5 – on
these you will use pin 4 instead of pin 5.) The regulator
powers several points in the circuit as well as an LED
(L5) for visual confirmation that power is present. We
have used an LP2950 for the regulator as that limits
the current available to about 100mA which is a safe
value for experimenting. The CAN interface is a low
impedance circuit however, and if doing sustained
transmissions on CAN, this type of regulator may
cause LV RESETs or possibly shut down on over-
temperature. Should you experience either of these
problems, you may want to consider using a device
that is capable of more current (such as a 78M05 or
7805).
Data is both sent and received on the ISO-K line.
When receiving, the data signal is presented to pin 12
after being reduced by the R20-R21 voltage divider.
Because of the Schmitt trigger input on pin 12, these
resistors will give typical threshold levels of 7.0V
(rising) and 3.6V (falling), providing a large amount of
noise immunity while also protecting the IC. If you
connect test equipment in parallel with R21, it will
cause these thresholds to increase, so be conscious of
The top left corner of Figure 9 shows the CAN
interface circuitry. We do not advise making your own
interface using discrete components – CAN buses
typically have a lot of critical information on them, and
you can easily do more harm than good here. It is
ELM327DSI
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