ELM327LSM datasheet(47/94 Pages) ELM

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CAN Receive Filtering - the CRA Command

When receiving CAN data, the ELM327 actually

retrieves every message from the CAN bus, and then

decides whether or not to display it, based on criteria

that you or the firmware set. The ‘CAN Receive

Address’ or CRA

command may be used to define

this criteria for you, in one simple step.

As an example, assume that the only messages

that you wish to see are those that have the CAN ID

7E9. To see only them, tell the ELM327 that the

receive address should be 7E9:

>AT CRA 7E9

and the ELM327 will set the necessary values so that

the only messages that are accepted are the ones with

ID 7E9.

If you do not want an exact address, but would

prefer to see a range of values, for example all the

OBD addresses (those that begin with 7E), then simply

use an ‘X’ for the digit that you do not want the

ELM327 to be specific about. That is, to see all

messages with CAN IDs that start with 7E (7E0, 7E1,

7E2,... 7EE, and 7EF), send:

>AT CRA 7EX

and the ELM327 will set the necessary values for you.

This command works exactly the same way for the

29 bit IDs. For example, if you wish to see all

messages that are being sent from the engine ECU

(address 10) to the scan tool (address F1), then you

can send:

>AT CRA XX XX F1 10

and all the settings will be taken care of for you.

If you wish to be more specific and see only the

OBD replies sent by the engine to the scan tool, you

would say:

>AT CRA 18 DA F1 10

and again, the ELM327 makes the necessary changes

for you.

Perhaps you do not care which device is sending

the information, but you want to see all messages that

start with 18 DA and are being sent to the scan tool.

For this, use the character ‘X’ to tell the ELM327 that

you do not care what value a digit has:

>AT CRA 18 DA F1 XX

and the ELM327 takes care of the details for you.

When working with J1939 data, the ELM327

normally formats the data for you, in order to separate

the priority from the PGN information. This is usually

not a concern when using the CRA command, except

when you are trying to filter for a specific priority. For

example, you might typically see:

>AT MA

3 0FE6C 00 FF FF FF FF FF FF 40 B5

6 0FEEE 00 15 50 FF FF FF FF FF FF

6 0FEF5 00 FE FF FF FF 19 00 23...

The single priority digit out front (the 3 or 6 above)

as well as the leading 0 with the PGN information are

actually part of the first two digits (5 bits) of the ID, and

need to be interpreted as such, in order to use the

CRA command. It may be easier if you turn off the

J1939 header formatting in order to see this:

>AT JHF0

>AT MA

0C FE 6C 00 FF FF FF FF FF FF 40 B5

18 FE EE 00 15 50 FF FF FF FF FF FF

18 FE F5 00 FE FF FF FF 19 00 23...

This more clearly shows the four bytes that need

to be defined for the CRA command to be set. To

search for all messages that begin with 6 0FEF5, you

would actually need to send the command:

>AT CRA 18 FE F5 XX

In summary then, the CRA command allows you

to tell the ELM327 what ID codes to look for, and the

letter ‘X’ may be used in it to represent any single digit

that you do not want the ELM327 to be specific about.

This is usually selective enough for most applications,

but occasionally, there is a need to be specific down to

the bit level, rather than to the nibble. For those

applications, you will need to program a separate

mask and filter, as we show in the next section.

ELM327L

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ELM327L DSA

Elm Electronics – Circuits for the Hobbyist

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