ELM329LP datasheet(35/87 Pages) ELM

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ELM329L

ELM329L DSA

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Figure 4. A CAN OBD Message

standards. It uses 3 header bytes as shown, to provide

details concerning the priority, the receiver, and the

transmitter. Note that many texts refer to the receiver

as the ‘Target Address’ (TA), and the transmitter as

the ‘Source Address’ (SA).

A concern when sending any message is that

errors might occur in the transmission, and the

received data may be falsely interpreted. To detect

errors, all of the protocols provide some form of check

on the received data. This may be as simple as a sum

calculation (ie a ‘running total’ of byte values) that is

sent at the end of a message. If the receiver also

calculates a sum as bytes are received, then the two

values can be compared and if they do not agree, the

receiver will know that an error has occurred. CAN

systems use a special kind of checksum called a

Cyclic Redundancy Check (or ‘CRC’).

The

OBD

data

bytes

are

thus

normally

encapsulated within a message, with ‘header’ bytes at

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

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

structure that is very similar to that of Figure 3 - see

Figure 4, below. The main difference between the two

is really only the structure of the header, as CAN does

not have distinct bytes, but rather has groups of bits.

For this reason, CAN headers are generally known as

‘ID bits’ and not headers. We use the terms

interchangeably, however, as so many people are

familiar with our other OBD chips (the ELM320,

ELM322, ELM323 and ELM327) which use the term

OBD Message Formats (continued)

‘header’ almost exclusively.

The initial CAN standard stated that there will be

11 ID bits for every message, but that has been

expanded and the latest CAN standards now allow for

either 11 or 29 bit IDs.

The ELM329 does not normally show anything

more than the relevant data bytes unless you turn that

feature on with the Headers On command (AT H1).

Issuing it allows you to see the header bytes (ID bits),

and other items which are normally hidden such as the

PCI byte or possibly the data length code. The current

version of the ELM329 does not display the checksum

(CRC) information.

It is not necessary to ever have to set the header

bytes, or to perform a checksum calculation, as the

ELM329 will always do this for you. The header bytes

(ID bits) are adjustable however, should you wish to

experiment with advanced messages such as those

for physical addressing.

ID bits (11 or 29)

7 data bytes

checksum

PCI

data bytes (8 in total)

‘header’ bytes

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