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Elm 327 Manual

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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
OK
>AT CM 7F8
OK
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
OK
(as every bit except those in the last byte are relevant)
while the filter may be set to:
>AT CF 18 DA F1 00
OK
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.
… 47 …


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