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Elm 327 Manual

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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
ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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
… 38 …


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