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CAN Messages and Filtering

The ELM327 monitoring commands (AT MA, MR

and MT) usually work very well with the ‘slower’

protocols – J1850, ISO 9141 and ISO 14230. The

CAN systems are a different story, however, as they

often have an order of magnitude more information

passing over them. The relatively small 256 byte buffer

that the ELM327 uses for sending can quickly fill up

when data is arriving at 500 Kbps and leaving at

38.4 Kbps, or even 115.2 Kbps.

To help reduce the amount of information seen by

the ELM327, the internal CAN module has a ‘filter’ that

can be used to pass only messages with specific ID

bits. A range of values can be passed when the filter is

used with what is called a ‘mask’ to say which bits are

relevant.

As an example, consider an application where you

are trying to monitor for 29 bit CAN diagnostic

messages,

exactly

the

ELM327

does.

definition, these messages will be sent to the scan tool

at address F1. From ISO 15765-4, you know then that

the ID portion of the reply must be of the form:

18 DA F1 xx

where xx is the address of the module that is sending

the message. To use the filter, then, enter what you

have into it, putting anything in for the unknown portion

(you will see why in a moment). The command to set

the CAN filter is AT CF…

>AT CF 18 DA F1 00

How, you ask, do you tell the ELM327 to ignore

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

mask is a set of bits that tell 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 the above message

are relevant, except those of the last two digits. To set

the mask for this example then, you would need to use

the CAN Mask command, as follows:

>AT CM 1F FF FF 00

If desired, you can convert the hexadecimal to

binary to see what has been done.

The 11 bit CAN IDs are treated in 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

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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 that digit relevant, we create the mask:

>AT CM 00 00 0F 00

The system only uses the 11 right-most bits in this

case, so we can be lazy and enter the F as shown (the

first bit of the F will be ignored, and it will be treated as

if we had entered a 7).

Clearly, this can be quite cumbersome if using 11

bit CAN systems routinely. To help with that, the

ELM327 offers some shorter versions of the CF and

CM commands. To use them for the example above,

you need only enter three digit arguments:

>AT CF 600

and

>AT CM F00

As for the full eight digit versions, only the 11 least

significant (right-most) digits are actually used, so you

do not need to take special care with the first bit.

With a little practice, these commands are fairly

easy to master. Initially, try entering the filter and mask

values, then use a command such as AT MA to see

what the results are. The ELM327 knows that you are

trying to filter, and combines the effects of both

commands (it will do that for MR and MT as well). The

MA, MR and MT commands all have the extra benefit

that while they are in effect, the ELM327 will remain

quiet, not sending acknowledgement or error signals,

so anything you do while monitoring should not disrupt

other devices that are on the bus.

Note that if a filter has been set, it will be used for

all CAN messages. Setting filters and masks may

cause standard OBD requests to be ignored though,

and you may begin seeing “NO DATA” responses. If

this happens, and you are unsure of why, you may

want to reset everything to their default values (AT D)

and start over.

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