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ELM320

ELM320DSC

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Advanced Data Retrieval – Setting the Headers

Prior to v2.0, the ELM320 used a fixed format for

the message headers, allowing only for the retrieval of

the mandated diagnostic codes, not allowing the user

to change them. The IC is now fully programmable,

however, allowing the headers to be changed and a

great deal more information to be obtained, if your

vehicle supports it. Note that only the OBDII diagnostic

codes

have

been

mandated,

there

requirement for all vehicles to support these extra

capabilities.

The diagnostic trouble codes that most people are

familiar with are described by SAE standard J1979

(ISO15031-5). This is really a specific instance of the

many modes allowed by the J2178-4 standard, which

allows for information transfer through what is known

as ‘functional addressing’. For the OBDII mandated

diagnostics,

requests

are

actually

made

the

functional address 6A, with whatever processor is

responsible for this function answering the request.

Theoretically many different processors can respond

to a single functional request, each contributing their

insight as to the information requested.

To retrieve some of this extra information, the

function being addressed needs to be known. For

example, consider that you have studied the J2178

standards and want to request that the processor

responsible for Engine Coolant provide the current

Fluid Temperature. You determine that Engine Coolant

is functional address 48, you know that your address

as a scan tool is normally F1, and that since the

ELM320 only supports single IFR responses (type 1),

you choose A1 as the initial priority/type byte.

Combining the above then, it is desirable to set the

three header bytes to A1 48 F1. This is done with the

Set Header command, which would be issued at the

prompt as follows:

>AT SH A1 48 F1

The three header bytes assigned in this manner

will stay in effect until changed with another AT SH

command, a reset, etc. If the default Auto Receive

mode has been selected, the receive address will

automatically be set to 49 (the second byte plus one).

This is consistent with the functional pairs assigned by

J2178-4. If you decide that this is not appropriate for

your case, you can always set the receive address to

what you wish using the AT SR command. For

example, if you wanted to obtain a response that is

being sent to address E2 instead, you would use AT

SR E2 to override the automatic receive mode. Any

receive address selected stays in effect until changed

by another AT SR, or reinstatement of the automatic

mode.

Having set the headers, all one needs to do is

issue the secondary ID for fluid temperature (10) at the

prompt. If the display of headers is set to on, the

conversation would typically look like this:

>10

81 49 10 10 2E 41

Ignoring the first three (header) bytes, and the final

check digit, one can see that the response to ID 10 is

the byte 2E. You may find that some requests, being

of a low priority, are not answered immediately,

possibly causing a NO DATA result. In these cases,

you may want to adjust the timeout value, perhaps first

trying the maximum (with AT ST FF).

Using the physical addressing modes described

by the J2190 standard involves an almost identical

process. The main difference is that you must know

the physical address of the device which you want to

speak to. This is always the third byte of a message

sent by any device, so can be determined by

monitoring the headers (for the above response, the

sender’s address is 10). Knowing that you wish to talk

to address 10, that your physical address is F1, and

that for type 1 IFR with physical addressing E4 may be

appropriate for the first byte, you would change the

header bytes using AT SH E4 10 F1. If Auto Receive

is enabled, the receive address will automatically be

set to F1, your physical address (the ELM320 knows

to do this from the first byte). As before, this header

will remain in effect for every message sent until

changed to something else.

One caution to note with physical addressing.

There are modes which initiate the constant sending of

data, and if the ELM320’s timeout is set longer than

the duration between responses, the ELM320 may

return messages forever. In these cases, just like in

the monitoring modes, a single character will have to

be sent to interrupt the process.

Finally, please note that while we have provided

some information on the SAE standards for the

examples, Elm Electronics will only reply to requests

for clarification on our product’s operation, and not on

the standards. It is the customer’s responsibility to

obtain their own information on the relevant standards,

and on their vehicle. Requests to Elm Electronics for

this information will go unanswered.

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