ELM327
AT Command Descriptions (continued)
discarded. This three nibble version of the CAN Filter
command makes it a little easier to set filters with 11
bit ID CAN systems. Only the rightmost 11 bits of the
provided nibbles are used, and the most significant bit
is ignored. The data is actually stored as four bytes
internally however, with this command adding leading
zeros for the other bytes. See the CM command(s) for
more details.
to provide mask values for 11 bit ID systems (the most
significant bit is always ignored).
Note that a common storage location is used
internally for the 29 bit and 11 bit masks, so an 11 bit
mask could conceivably be assigned with the next
command (CM hh hh hh hh), should you wish to do the
extra typing. The values are right justified, so you
would need to provide five leading zeros followed by
the three mask bytes.
CF hh hh hh hh [ set the CAN ID Filter to hhhhhhhh ]
CM hh hh hh hh [ set the CAN ID Mask to hhhhhhhh ]
This command allows all four bytes (actually 29
bits) of the CAN Filter to be set at once. The 3 most
significant bits will always be ignored, and may be
given any value. This command may be used to enter
11 bit ID filters as well, since they are stored in the
same locations internally (entering AT CF 00 00 0h hh
is exactly the same as entering the shorter AT CF hhh
command).
This command is used to assign mask values for
29 bit ID systems. See the discussion under the
CM hhh command as it is essentially identical, except
for the length. Note that the three most significant bits
that you provide in the first digit will be ignored.
CP hh
[ set CAN Priority bits to hh ]
This command is used to assign the five most
significant bits of the 29 bit CAN ID that is used for
sending messages (the other 24 bits are set with the
AT SH command). Many systems use these bits to
assign a priority value to messages, and to determine
the protocol. Any bits provided in excess of the five
required are ignored, and not stored by the ELM327 (it
only uses the five least significant bits of this byte).
The default value for these priority bits is hex 18,
which can be restored at any time with the AT D
command.
CFC0 and CFC1
[ CAN Flow Control off or on ]
The ISO 15765-4 CAN protocol expects a ‘Flow
Control’ message to always be sent in response to a
‘First Frame’ message, and the ELM327 automatically
sends these without any intervention by the user. If
experimenting with a non-OBD system, it may be
desirable to turn this automatic response off, and the
AT CFC0 command has been provided for that
purpose.
As of firmware version 2.0, these commands also
enable or disable the sending of J1939 TP.CM_CTS
messages in response to TP.CM_RTS requests.
During monitoring (AT MA, MR, or MT), there are
never any Flow Controls sent no matter what the CFC
option is set to. The default setting is CFC1 - Flow
Controls on.
CRA
[reset the CAN Rx Addr]
The AT CRA command is used to restore the CAN
receive filters to their default values. Note that it does
not have any arguments (ie no data).
CRA hhh
[set the CAN Rx Addr to hhh]
CM hhh
[ set the CAN ID Mask to hhh ]
Setting the CAN masks and filters can be difficult
at times, so if you only want to receive information
from one address (ie. one CAN ID), then this
command may be very welcome. For example, if you
only want to see information from 7E8, simply send AT
CRA 7E8, and the ELM327 will make the necessary
adjustments to both the mask and the filter for you.
If you wish to allow the reception of a range of
values, you can use the letter X to signify a ‘don’t care’
condition. That is, AT CRA 7EX would allow all IDs
that start with 7E to pass (7E0, 7E1, etc.). For a more
There can be a great many messages being
transmitted in a CAN system at any one time. In order
to limit what the ELM327 views, there needs to be a
system of filtering out the relevant ones from all the
others. This is accomplished by the filter, which works
in conjunction with the mask. A mask is a group of bits
that show the ELM327 which bits in the filter are
relevant, and which ones can be ignored. A ‘must
match’ condition is signalled by setting a mask bit to
'1', while a 'don't care' is signalled by setting a bit to '0'.
This three digit variation of the CM command is used
ELM327DSI
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