ELM327LP datasheet(49/94 Pages) ELM

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Monitoring the Bus

Some vehicles use the OBD bus for information

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.

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.

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

and all messages that contain 10 in the third byte of

the header will be displayed.

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.

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.

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