ELM327P datasheet(41/76 Pages) ELM

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CAN Messages and Filtering

Just prior to sending an OBD request, the ELM327

will make sure that the receive filters are set to only

accept certain types of messages, and to reject others.

With the older protocols (J1850, ISO 9141, etc.) the

filters only need to monitor a single byte (much like the

AT MR and MT commands), but with CAN protocols, a

decision typically requires looking at all of the ID bits.

The decision whether to receive a CAN message

or to ignore it is made by what is called an ‘acceptance

filter’. This filter works in conjunction with a ‘mask’,

which is actually a filter on the filter that says which

bits are relevant, and which ones can be ignored. A

few examples might be best to describe how to set the

filter and the mask.

By far the easiest way to set both the filter and the

mask is with the AT CRA command. You don’t really

need to know anything about filters and masks to use

it - simply send the command with the address that

you want to see, and the ELM327 does all the work for

you. For example, if the only messages that you wish

to see are those that have the CAN ID 7E9, then

simply send:

>AT CRA 7E9

and the ELM327 will set the necessary values so that

all that is displayed are messages with ID 7E9.

If you would like to see a range of values, rather

than a specific ID, then you will need to manually

define the filter and the mask. For example, consider

an application where you are trying to monitor for 29

bit CAN diagnostic messages, exactly like the ELM327

does. By definition, these messages will be sent to the

scan tool at address F1, so from ISO 15765-4, you

know that the ID portion of the reply is of the form:

18 DA F1 xx

where xx is the address of the ECU that is sending the

message. Clearly a filter is needed that requires the

first 21 bits to match, but does not care what the last

eight bits contain.

To create such a filter, we first use the CAN Filter

command to enter the ID values that correspond to the

values that you want to match, and put any value in for

the unknown portion (you will see why in a moment).

For this example, the command would be:

>AT CF 18 DA F1 00

Now, how do you tell the ELM327 to ignore those

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last two 0’s? You do that with the mask. The mask is a

bit pattern that tells the ELM327 which bits in the filter

are relevant. If the mask bit is 1, that filter bit is

relevant, and is required to match. If it is 0, then that

filter bit will be ignored. All bits in this filter are relevant,

except those of the last byte. To set the mask then,

you would need to use the CAN Mask command, and

put 1’s for all but the last byte:

>AT CM 1F FF FF 00

After setting the mask and filter in this way, only

IDs that start with 18 DA F1 will be accepted by the

ELM327, and all others will be ignored. Be careful

when experimenting with this, as you will override the

default settings, and you might stop ‘seeing’ any

replies to the requests.

The 11 bit CAN IDs are treated in much the same

manner. Recall that they are stored internally in the

right-most 11 bits of the locations used for 29 bit CAN,

which must be considered when creating a filter or

mask. As an example, assume that we wish to display

all messages that have a 6 as the first digit of the 11

bit ID. We need to set a filter to look for 6 in that digit:

>AT CF 00 00 06 00

The 11 bit ID is stored in the last three locations,

so the 6 would appear where it is shown. Now, to

make only that digit relevant, we create the mask:

>AT CM 00 00 0F 00

Technically, this one digit actually represents 3

bits of the 11 bit ID, so we should not use F (ie 1111)

for the mask, but the ELM327 will only look at the 11

bits, and does not care what we put in the 12th bit (so

we can be lazy).

Clearly, trying to insert 11 bits into the right spot in

a 29 bit ID can be quite cumbersome. To help with

that, the ELM327 offers some 11 bit versions of the CF

and CM commands, which can be used as follows for

the above commands:

>AT CF 600

>AT CM F00

Again, only the 11 least significant (right-most)

digits are actually used, and the ELM327 ignores the

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