ELM327P datasheet(46/51 Pages) ELM

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Figure 8. The J1962 Vehicle Connector

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

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 510

Ω 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 560

Ω or

possibly make 510

Ω from two resistors (1/4W 240Ω +

270

Ω 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

ELM327

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