ELM327P datasheet(35/76 Pages) ELM

35 / 76 page

OBD Message Formats

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,

35 of 76

ELM327

ELM327DSH

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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

to as header bytes. Figure 3 below shows a typical

OBD

message

structure

that

used

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

Figure 3. An OBD Message

up to 7 data bytes

checksum

3 header bytes

priority

receiver

transmitter

based on any active inputs that are present. If you

need to follow the SAE J1978 order, you should send

the ELM327 an AT SS command, or step through

each protocol with the TP command.

The automatic search works well with OBDII

systems, but may not be what you need if you are

experimenting. During a search, the ELM327 ignores

any headers that you have previously defined (since

there is always a chance that your headers may not

result in a response), and it uses the default OBD

header values for each protocol. It will also use

standard requests (ie 01 00) during the searches. If

this is not what you want, the results may be a little

frustrating.

To use your own header (and data) values when

attempting to connect to an ECU, do not tell the

ELM327 to use protocol 0. Instead, tell it to either use

Selecting Protocols (continued)

only your target protocol (ie. AT SP n), or else tell it to

use yours with automatic searches allowed on failure

(ie AT SP An). Then send your request, with headers

assigned as required. The ELM327 will then attempt to

connect using your headers and your data, and only if

that fails (and you have chosen the protocol with AT

SP An) will it search using the standard OBD default

values.

In general, 99% of all users find that enabling the

memory (setting pin 5 to 5V) and using the ‘Auto’

option when searching (you may need to send AT

SP 0) works very well. After the initial search, the

protocol used by your vehicle becomes the new

default, but it is still able to search for another, without

your having to say AT SP 0 again.

ELM327P Datasheet(PDF) 35 Page - ELM Electronics

ELM327P Datasheet(HTML) 35 Page - ELM Electronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76

Datasheet Download

Link URL