Operation Manual

Figure 8. The J1962 Vehicle Connector
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ELM327
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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.
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 very similar connector (mostly) for their 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 80 (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 a capacitor for 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 a 7805 regulator for this circuit as it is very
common, and usually easy to obtain.
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, so it is
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. Also, if you are considering
using the Activity Monitor to wake the ELM327 from a
low power sleep mode, be sure that the transceiver
chip that you pick keeps the receiver functional when
in standby mode (if it does not pass the signals on, the
ELM327 can not see them).
The circuit shown directly below the CAN interface
is used for the ISO 9141 and ISO 14230 signals. 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
do not connect anything to pin 22, 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 510Ω pullup resistors connected to
their collectors, as the standard requires. Occasionally,
we are asked about substitutes for these resistors –
the standard specifies 510Ω but in a pinch you might
be able to use 560Ω. A better solution would be to
make 510Ω from 240Ω and 270Ω 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 510Ω resistors, so the
vehicle would see 255Ω connected externally. This
would very likely cause data errors, and might even
damage some circuitry. To avoid this, you might wish
to build your circuit in such a way that you can switch
the ELM327’s 510Ω resistors out, and replace them
with a larger value. For example, you might put 10KΩ
resistors in series with the 510Ω ones, and add
removable jumpers or switches across the 10KΩ
resistors. Most people do not need to do this – we only
mention it because of the questions that we do
receive.
The ISO data is both sent and received on the
ISO-K line. Pin 12 of the ELM327 reads this data
through 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), which provides a large amount of noise
immunity while also protecting the IC. If you should
connect test equipment to pin 12 (ie in parallel with
R21), the thresholds will increase, so be conscious of