Bedienungsanleitung Elm 327 (Seite 47 von 94) (Englisch)

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47 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

ELM327

Using the CAN Mask and Filter

Filtering of CAN messages (that is, deciding which

to keep and which to reject), is usually handled most

easily with the CRA command. The CRA command

only allows for definition to the nibble level, however -

if you need more selectivity (to the bit level), you must

program the mask and filter.

Internally, the ELM327 configures an ‘acceptance

filter’ with 1’s and 0’s based on the type of message

that it wishes to receive (OBD, J1939, etc.). This

pattern is then compared to the ID bits of all incoming

messages. If the two patterns match, then the entire

message is accepted, and if they do not, the message

is rejected.

Having to match all 11 or 29 bits of the ID may be

very restrictive in some cases (and would require a

very large number of filters for some applications). To

allow a little more flexibility in what to accept, and what

to reject, a mask is also defined, in addition to the

filter. This mask acts just like the type worn on your

face - some features are exposed and some are

hidden. If the mask has a ‘1’ in a bit position, that bit in

the filter must match with the bit in the ID, or the

message will be rejected. If the mask bit is a ‘0’, then

the ELM327 does not care if that filter bit matches with

the message ID bit or not.

As an example, consider the standard response to

an 11 bit OBD request. ISO15765-4 states that all

responses will use IDs in the range from 7E8 to 7EF.

That is:

1. There must always be a ‘7 ‘ (binary 111) as the

first nibble (so the filter should have the value 111

or 7). All 3 bits are relevant (so the mask should

be binary 111 or 7). Note that this first nibble is

only 3 bits wide for the 11 bit CAN ID.

2. There must always be an ‘E’ (binary 1110) in

the second position, so the filter needs to be of

value 1110 or E. Since all 4 bits are relevant, the

mask needs to be of value 1111 or F.

3. If you analyze the patterns for the binary

numbers from 8 to F, you will see that the only

thing in common is that the most significant bit is

always set. That is, the mask will have a value of

1000 since only that one bit is relevant, and you

do not care what the other bits are. The filter

needs to be assigned a value that has a 1 in the

first position, but we do not care what is in the

other three positions. In the following, we use 0’s

in the positions (but it doesn’t really matter).

Putting this together, the filter will have a value:

111 1110 1000 = 7E8

and the mask will have a value:

111 1111 1000 = 7F8

In order to make these active, you will need to

issue both a CAN Filter and a CAN Mask command:

>AT CF 7E8

>AT CM 7F8

From that point on, only the IDs from 7E8 to 7EF

will be accepted by the chip.

The 29 bit IDs work in exactly the same way. For

example, assume that you wish to receive only

messages of the form:

18 DA F1 XX

where XX is the address of the ECU that is sending

the message, but you do not care what the value is

(this is the standard OBD response format). Putting 0’s

in the mask for don’t care bits, then the mask needs to

be set as follows:

>AT CM 1F FF FF 00

(as every bit except those in the last byte are relevant)

while the filter may be set to:

>AT CF 18 DA F1 00

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

all CAN messages, so setting filters and masks may

cause standard OBD requests to be ignored, and you

may begin seeing ‘NO DATA’ replies. If this happens,

and you are unsure of why, you may want to reset

everything to the default values (with AT CRA, AT D,

or possibly AT WS) and start over.

Quite likely, you will never have to use the CM and

CF commands. If you do, then creating your own

masks and filters can be difficult. You may find it

helpful to draw the bit patterns first, and think about

which ones matter, and which ones do not. It may also

help to connect to a vehicle, apply test settings, and

send AT MA to see how the settings affect the

displayed data.

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Elm 327 Seite 47

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