ELM327
Monitoring the Bus
Some vehicles use the OBD bus for information
and all messages that contain 10 in the third byte of
the header will be displayed.
transfer during normal vehicle operation, passing a
great deal of information over it. A lot can be learned if
you have the good fortune to connect to one of these
vehicles, and are able to decipher the contents of the
messages.
To see how your vehicle uses the OBD bus, you
can enter the ELM327’s ‘Monitor All’ mode, by sending
the command AT MA from your terminal program. This
will cause the IC to display any information that it sees
on the OBD bus, regardless of transmitter or receiver
addresses (it will show all). Note that the ELM327
remains silent while monitoring, so periodic ‘wakeup’
messages are not sent (if you have an ISO 9141 or
ISO 14230 bus that had been initialized previously, it
may ‘go to sleep’), IFRs are not sent, and the CAN
module does not acknowledge messages.
The monitoring mode can be stopped by putting a
logic low level on the RTS pin, or by sending a single
RS232 character to the ELM327. Any convenient
character can be used to interrupt the IC – there are
no restrictions on whether it is printable, etc. Note that
any character that you send will be discarded, and will
have no effect on any subsequent commands.
Using this command with 11 bit CAN systems can
be a little confusing at first. Recall the way in which all
header bytes are stored within the ELM327. An 11 bit
CAN ID is actually stored as the least significant 11
bits in the 3 byte ‘header storage’ location. It will be
stored with 3 bits in the receiver’s address location,
and the remaining 8 bits in the transmitter’s address
location. For this example, we have requested that all
messages created by transmitter ‘10’ be printed, so all
11 bit CAN IDs that end in 10 will be displayed (ie all
that look like ‘x10’).
The other monitoring command that is very useful
is the AT MR command, which looks for specific
addresses in the middle byte of the header. Using this
command, you can look for all messages being sent to
a particular address. For example, to use it to look for
messages being sent to the ECU with address 10,
simply send:
>AT MR 10
and all messages that contain 10 in the second byte of
the header will be displayed.
The time it takes to respond to such an interrupt
will depend on what the ELM327 is doing when the
character is received. The IC will always finish a task
that is in progress (printing a line, for example) before
printing ‘STOPPED’ and returning to wait for your
input, so it is best to wait for the prompt character (‘>’)
to be sent, or the Busy line to go low, before beginning
to send a new command.
One unexpected result may occur if you have the
automatic protocol search feature enabled, and you
tell the ELM327 to begin monitoring. If the bus is quiet,
the ELM327 will begin searching for an active protocol,
which may not be what you were expecting. Be aware
also that the ISO 9141 and ISO 14230 protocols look
identical when monitoring, so the ELM327 may stop
searching at ISO 9141, even if the actual protocol is
ISO 14230. With the Automatic searching enabled, this
should correct itself, however, when an OBD request
is later made.
Using this command with the 11 bit CAN systems
will again need further explanation. It may be helpful to
first picture the hex number ‘10’ in the above example
as the binary number ‘0001 0000’. Recall from above
that 11 bit CAN IDs are actually stored as the least
significant 11 bits in the 3 byte ‘header storage’
locations, and only 3 bits are actually stored in the
middle byte (receiver’s address) position. When
comparing the received CAN ID to the address you
provide with the MR command then, only the right-
most 3 bits of your MR address are considered and
the other 5 bits are ignored. In this example, the
AT MR 10 effectively becomes AT MR 0 for 11 bit
CAN systems, and so all messages that begin with ‘0’
as the first digit will be displayed.
It is best not to use the AT MT or MR commands
when monitoring CAN systems. The ELM327 provides
another command (AT CRA) that allows much better
control over the received data - see the next section
for complete details.
If the ‘Monitor All’ command provides too much
information (it certainly does for most CAN systems),
then you can restrict the range of data that is to be
displayed. Perhaps you only want to see messages
that are being transmitted by the ECU with address 10.
To do that, you only need to type:
>AT MT 10
ELM327DSI
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