ELM327P datasheet(38/76 Pages) ELM

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Setting the Headers (continued)

38 of 76

ELM327

ELM327DSH

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Electronics does not maintain lists of this information,

and cannot provide any further details for you. Mode

22 and others are described in more detail in the SAE

standards document J2190, ‘Enhanced E/E Diagnostic

Test Modes’.

The ISO14230-4 standard defines its header bytes

a little differently. Advanced experimenters will be

aware that for ISO 14230-4, the first header byte must

always include the length of the data field, which

varies from message to message. From that, one

might assume that the you would need to redefine the

header for every message that is to be sent – not so!

The ELM327 always determines the number of bytes

that you are sending, and inserts that length for you, in

the proper place for the header that you are using. If

you are using the standard ISO 14230-4 header, the

length will be put into the first header byte, and you

need only provide the two (most significant) bits of this

byte when defining the header. What you place in the

rest of the byte will be ignored by the ELM327 unless

you set it to 0. If it is 0, it is assumed that you are

experimenting with KWP four byte headers, and the

ELM327 then creates the fourth header byte for you.

Again, you do not need to provide any length to be put

into this byte – it is done for you.

Addressing

within

the

CAN

(ISO

15765-4)

protocols is quite similar in many ways. First, consider

the 29 bit standard. The ELM327 splits the 29 bits into

a CAN Priority byte and the three header bytes that we

are now familiar with. This is how they are combined

for use by the ELM327:

>AT SH xx yy zz

>AT CP vv

5 bits only

29 bit ID

Setting a 29 bit (extended) CAN ID

The CAN standard states that for diagnostics, the

priority byte (‘vv’ in the diagram) will always be 18 (it is

the default value used by the ELM327).

Since it is

rarely changed, it is assigned separately from the

other header bytes, using the CP command.

The next byte (‘xx’) describes the type of message

that this is, and is set to hex DB for functional

addressing, and to DA if using physical addressing.

The next two bytes are as defined previously for the

other standards – ‘yy’ is the receiver (or Target

Address), and ‘zz’ is the transmitter (or Source

Address). For the functional diagnostic requests, the

receiver is always 33, and the transmitter is F1, which

is very similar to ISO 14230-4.

Those that are familiar with the SAE J1939

standard will likely find this header structure to be very

similar (J1939 is a CAN standard for use by ‘heavy-

duty vehicles’ such as trucks and buses). It uses

slightly different terminology, but there is a direct

parallel between the bytes used by J1939 for the

headers and the grouping of the bytes in the ELM327.

Pages 48 and 49 provide more details of the J1939

message structure.

The final header format to discuss is that used in

11 bit CAN systems. They also use a priority/address

structure, but shorten it into roughly three nibbles

rather than three bytes. The ELM327 uses the same

commands to set these values as for other headers,

except that it only uses the 11 least significant (‘right-

most’) bits of the provided header bytes, and ignores

the others, as shown here:

11 bit ID

>AT SH xx yy zz

Setting an 11 bit (standard) CAN ID

It quickly becomes inconvenient to have to enter

six digits when only three are required, so there is a

special ‘short’ version of the AT SH command that

uses only three hex digits. It actually operates by

simply adding the leading zeros for you.

The

bit

CAN

standard

typically

makes

functional requests (ID/header = 7DF), but receives

physical replies (7En). With headers turned on, it is a

simple matter to learn the address of the module that

is replying, then use that information to make physical

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