ELM320P datasheet(12/16 Pages) ELM

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ELM320

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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. By the same token, you can do a lot of

harm if you are careless, so be very careful.

To see how your vehicle uses the OBD bus, you

will have to enter one of the ELM320’s monitoring

modes. The simplest is the “Monitor All” mode which is

entered into by simply sending the command AT MA

from your terminal program. Once received, the IC will

continually display any information it sees on the OBD

bus, regardless of transmitter or receiver addresses.

Monitoring modes can only be stopped by sending

something over the RS232 connection to the ELM320.

It is not critical what you send - any single character

will interrupt the processor, and return it to the

command mode waiting for an input. Note that the

character you send is discarded and has no effect on

any subsequent commands. The IC will always finish a

task in progress (printing a line, for example) before

returning to wait for input, so always wait for the

prompt character (‘>’) before continuing to issue other

commands.

If the headers are not currently displayed, simply

typing ATMA shows only the contents of messages,

not the transmitter and receiver addresses. To show

who is sending to whom, you will need to first turn

headers on (AT H1) before beginning to monitor (AT

MA).

Either

way,

you

may

end

with

overwhelming amount of information that you may

want to filter, showing only specific messages.

If, for example, you find that the engine controller’s

address seems to be 10, you may want to restrict the

data displayed to only messages from that ECU. To do

so, you would monitor only for messages transmitted

from address 10, by issuing AT MT 10 from your

terminal program. Only messages with 10 in the third

byte of the header will be displayed. Similarly, you may

wish to only see messages which are being sent to

address 3B. To monitor for these, send AT MR 3B and

only messages with 3B as the second header byte will

be shown.

There

are

few

limitations

the

current

monitoring modes that you should be aware of. First,

there is no internal buffering of OBD messages as

data is being sent on the RS232 connection, so

information may be missed while the IC is busy. If

under computer control, you may want to consider the

‘packed data’ mode to reduce the chance of this. The

second limitation is that the data being printed only

extends up to the End Of Data symbol, and does not

include any In-Frame Response bytes that may be

present. However, for most users this will not be of

consequence.

Computer Control – Using Packed Data

If a person is simply asking a vehicle for the

current Diagnostic Trouble Codes, speed is normally

not an issue, as data is displayed (essentially) as

quickly as it can be read. If interfaced to a computer,

however, speed may be important.

The packed data mode is a convenient means to

effectively triple the ELM320’s data transfer rate while

maintaining the connection at 9600 baud. Once

entered (with AT PD), all OBD messages will be

returned as a single length byte followed by the actual

data bytes. There are no space characters sent

between bytes, no carriage returns or linefeeds – the

data is retransmitted exactly as received from the

vehicle (except for the change to 9600 baud). While no

longer

readable

terminal,

computers

will

understand the information just the same, and will gain

speed through both reduced transfer and conversion

times. The ELM320 does not function any differently

when in this mode – if the headers are to be displayed,

they are sent, if in monitoring mode, data is continually

sent, etc. The only difference is in the format in which

the OBD responses are returned to the controlling

computer.

Often there is no response from the vehicle for a

particular request. When in the default (formatted data)

mode, this is shown with ‘NO DATA’ being printed, but

while in the packed data mode you will only receive a

single length byte of value 0 (zero).

While rare, errors may occasionally be detected in

the vehicle’s data. Normally, a ‘<DATA ERROR’ would

be printed for this, but in the Packed Data mode, the

checksum (CRC) errors are identified by setting the

most significant bit of the length byte. Because of this,

one should always check the length byte for a value of

128 or greater before processing the remainder of the

message.

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