ELM327_1 Datasheet, PDF (46/51 Pages) ELM Electronics – OBD to RS232 Interpreter

ELM327
Example Applications
The SAE J1962 standard dictates that all OBD
compliant vehicles must provide a standard connector
near the driver’s seat, the shape and pinout of which is
shown in Figure 8 below. The circuitry described here
can be used to connect to this J1962 plug without
modification to your vehicle.
The male J1962 connector required to mate with a
1
8
9
16
Figure 8. The J1962 Vehicle Connector
vehicle’s connector may be difficult to obtain in some
locations, and you could be tempted to improvise by
making your own connections to the back of your
vehicle’s connector. If doing so, we recommend that
you do nothing that would compromise the integrity of
your vehicle’s OBD network. The use of any connector
which could easily short pins (such as an RJ11 type
telephone connector) is definitely not recommended.
The circuit on page 48 (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 (for visual confirmation that power is present).
We have shown a 78L05 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 shut
down on over-temperature. Should you experience this
problem, you may want to consider using a 1 Amp
version of the regulator.
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
may have a lot of critical information on them, and you
can easily do more harm than good if you fail. It is
strongly recommended that you use a commercial
transceiver chip as shown. The Microchip MCP2551 is
used in our circuit, but most major manufacturers
produce CAN transceiver ICs – look at the NXP
82C251 (NXP was formerly Philips), the Texas
Instruments SN65LBC031, and the Linear Technology
LT1796, to name only a few. Be sure to pay attention
to the voltage limits – depending on the application,
you may have to tolerate 24V, 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.
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. We are often
asked about substitutes for these resistors – if you
need to substitute, you can either go up to 560Ω or
possibly make 510Ω from two resistors (1/4W 240Ω +
270Ω resistors work well), but we do not recommend
using a lower value as it stresses every device on the
bus. Note that 1/2W resistors should be used as a
short at 13.8V causes about 0.4W dissipation.
Data is received from the K Line of the OBD bus
and connected to pin 12 after being reduced by the
R20/R21 voltage divider shown. Because of the
Schmitt trigger input on pin 12, these resistors will give
typical threshold levels of 9.1V (rising) and 4.7V
(falling), providing a large amount of noise immunity
while also protecting the IC.
The final OBD interface shown is for the two
J1850 standards. The J1850 VPW standard needs a
positive supply of up to 8V while the J1850 PWM
needs 5V, so we have shown a two level supply that
can provide both. This dual voltage supply uses a
317L adjustable regulator as shown, controlled by the
pin 3 output. With the resistor values given, the
selected voltages will be about 7.5V and 5V, which
works well for most vehicles. The two J1850 outputs
are driven by the Q1-Q2 combination for the Bus+,
and Q3 for the Bus-.
The J1850 VPW input uses a resistor divider as
was used for the ISO input. Typical threshold voltages
with the resistors shown will be about 4.2V (rising) and
2.2V (falling). The J1850 PWM input is a little different
in that it must convert a differential input into a single-
ended one for use by the ELM327. In operation, Q4 is
actually used as the difference amplifier. The Q4-D3
series circuit sets a threshold voltage of about 1V (for
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