ELM327P datasheet(27/51 Pages) ELM

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Setting the Headers

The emissions related diagnostic trouble codes

that most people are familiar with are described in the

SAE J1979 standard (ISO15031-5). They represent

only a portion of the data that a vehicle may have

available – much more can be obtained if you are able

to direct the requests elsewhere.

Accessing most OBDII diagnostics information

requires that requests be made to what is known as a

a ‘functional address.’ Any processor that supports the

function will respond to the request (and theoretically,

many different processors can respond to a single

functional request). In addition, every processor (or

ECU) will also respond to what is known as their

physical address. It is the physical address that

uniquely identifies each module in a vehicle, and

permits you to direct more specific queries to only one

particular module.

To retrieve information beyond that of the OBDII

requirements then, it will be necessary to direct your

requests to either a different functional address, or to

an ECU’s physical address. This is done by changing

the data bytes in the message header.

As an example of functional addressing, let us

assume that you want to request that the processor

responsible for Engine Coolant provide the current

Fluid Temperature. You do not know its address, so

you consult the SAE J2178 standard and determine

that Engine Coolant is functional address 48. SAE

standard J2178 also tells you that for your J1850 VPW

vehicle, a priority byte of A8 is appropriate. Finally,

knowing that a scan tool is normally address F1, you

have enough information to specify the three header

bytes (A8 48 and F1). To tell the ELM327 to use these

new header bytes, all you need is the Set Header

command:

>AT SH A8 48 F1

The three header bytes assigned in this manner

will stay in effect until changed by the next AT SH

command, a reset, or an AT D.

Having set the header bytes, you now need only

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

prompt. If the display of headers is turned off, the

conversation could look like this:

>10

10 2E

The first byte in the response echos the request,

as usual, while the data that we requested is the 2E

byte. You may find that some requests, being of a low

priority, may not be answered immediately, possibly

causing a “NO DATA” result. In these cases, you may

want to adjust the timeout value, perhaps first trying

the maximum (ie use AT ST FF). Many vehicles will

simply not support these extra addressing modes.

The other, and more common method of obtaining

information is by physical addressing, in which you

direct your request to a specific device, not to a

functional group. To do this, you again need to

construct a set of header bytes that direct your query

to the physical address of the processor, or ECU. If

you do not know the address, recall that the sender of

information is usually shown in the third byte of the

header. By monitoring your system for a time with the

headers turned on (AT H1), you can quickly learn the

main addresses of the senders. The SAE document

J2178 assigns address ranges to these devices if you

are unsure of which might be most appropriate.

When you know the address that you wish to

‘speak to,’ simply use it for the second byte in the

header (assume an address of 10 for this example).

Combine that with your knowledge of SAE J2178 to

choose a priority/type byte (assume a value of E4 for

this example, as if the vehicle is J1850 PWM). Finally,

you need to identify yourself to the target, so that

responses can be returned to you. As is customary for

diagnostic tools, we’ll use an address of F1. As before,

these three bytes are then assigned to the header with

the set header command:

>AT SH E4 10 F1

From this point on, all messages that the ELM327

sends will use these three bytes for the header. All that

needs to be done now is to request data from the

vehicle. For physical addressing, this is often done

using mode 22:

>22 11 6B

62 11 6B 00 00

The response to this command is of the same

format to those seen for ‘standard’ OBD requests. The

request has been repeated (with 40 added to the

mode value in order to show that this is a response),

and this is followed by the actual data (0000 in this

case). The PIDs used with mode 22 are usually

proprietary to each manufacturer and are generally not

published widely, so you may have difficulty in

determining the ones to use with your vehicle. Elm

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