ELM327P datasheet(36/76 Pages) ELM

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ELM327

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Figure 4. A CAN OBD Message

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

the ‘Source Address’ (SA).

Another concern when sending any message is

that errors might occur in the transmission, and the

received data may be falsely interpreted. To detect

errors, the various protocols all 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.

Since simple sums might not detect multiple errors, a

more reliable (and more complicated) sum called a

Cyclic Redundancy Check (or ‘CRC’) is often used. All

of the protocols specify how errors are to be detected,

and the various ways of handling them if they occur.

The

OBD

data

bytes

are

thus

normally

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, below), the main

OBD Message Formats (continued)

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…

ID bits (11 or 29)

7 data bytes

checksum

PCI

data bytes (8 in total)

‘header’ bytes

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