Bedienungsanleitung Elm 327 (Seite 38 von 94) (Englisch)

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To this point we have only discussed the contents

(data portion) of an OBD message, and made only

passing mention of other parts such as headers and

checksums, which all messages use to some extent.

On Board Diagnostics systems are designed to be

very flexible, providing a means for several devices to

communicate with one another. In order for messages

to be sent between devices, it is necessary to add

information describing the type of information being

sent, the device that it is being sent to, and perhaps

which device is doing the sending. Additionally, the

importance of the message becomes a concern as

well – crankshaft position information is certainly of

considerably more importance to a running engine

than a request for the number of trouble codes stored,

or the vehicle serial number. So to convey importance,

messages are also assigned a priority.

The information describing the priority, the

intended recipient, and the transmitter are usually

needed by the recipient even before they know the

type of request that the message contains. To ensure

that this information is obtained first, OBD systems

transmit it at the start (or head) of the message. Since

these bytes are at the head, they are usually referred

38 of 94

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to as header bytes. Figure 3 below shows a typical

OBD message structure that is used by the

SAE J1850, ISO 9141-2, and ISO 14230-4 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).

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

Figure 3. An OBD Message

Figure 4. A CAN OBD Message

up to 7 data bytes checksum3 header bytes

priority receiver transmitter

TA SA

OBD Message Formats

ID bits (11 or 29) 7 data bytes checksumPCI

data bytes (8 in total)‘header’ bytes

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Elm 327 Seite 38

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