Manual Elm 327 (page 78 of 94) (English)

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Figure 8. The J1962 Vehicle Connector

78 of 94

ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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

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