ELM327LSM datasheet(40/94 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 echoes 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

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encapsulated within a message, with ‘header’ bytes at

the beginning, and a ‘checksum’ at the end. The

J1850, ISO 9141-2, and ISO 14230-4 protocols all use

essentially the same structure, with three header

bytes, a maximum of seven data bytes and one

checksum byte.

The ISO 15765-4 (CAN) protocol uses a very

similar structure (see Figure 4), the main difference

really only relating to the structure of the header. CAN

header bytes are not referred to as header bytes –

they are called ‘ID bits’ instead. The initial CAN

standard defined the ID bits as being 11 in number,

while the more recent CAN standard now allows for

either 11 or 29 bit IDs.

The ELM327 does not normally show any of these

extra bytes unless you turn that feature on with the

Headers On command (AT H1). Issuing that allows

you to see the header bytes and the checksum byte

(for the J1850, ISO 9141 and ISO 14230 protocols).

For the CAN protocols, you will see the ID bits, and

other items which are normally hidden such as the PCI

byte for ISO 15765, or the data length codes (if they

are enabled with PP 29, or AT D1). Note that the

ELM327 does not display the checksum information

for CAN systems, or the IFR bytes for J1850 systems.

It is not necessary to ever have to set these

header byes, or to perform a checksum calculation, as

the ELM327 will always do this for you. The header

bytes are adjustable however, should you wish to

experiment with advanced messages such as those

for physical addressing. The next section provides a

discussion on how to do this…

OBD Message Formats (continued)

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