ELM327P datasheet(23/76 Pages) ELM

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ELM327

ELM327DSH

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SH xyz

[ Set the Header to 00 0x yz ]

Entering CAN 11 bit ID words (headers) normally

requires that extra leading zeros be added (eg. AT SH

00 07 DF), but this command simplifies doing so. The

AT SH xyz command accepts a three digit argument,

takes only the right-most 11 bits from that, adds

leading zeros, and stores the result in the header

storage locations for you. As an example, AT SH 7DF

is a valid command, and is quite useful for working

with 11 bit CAN systems. It actually results in the

header bytes being stored internally as 00 07 DF.

SH xx yy zz

[ Set the Header to xx yy zz ]

This command allows the user to manually control

the values that are sent as the three header bytes in a

message. These bytes are normally assigned values

for you (and are not required to be adjusted), but there

may be occasions when it is desirable to change them

(particularly if experimenting with physical addressing).

If experimenting, it is not necessary but may be better

to set the headers after a protocol is active. That way,

wakeup messages, etc. that get set on protocol

activation will use the default values.

The header bytes are defined with hexadecimal

digits - xx will be used for the first or priority/type byte,

yy will be used for the second or receiver/target byte,

and zz will be used for the third or transmitter/source

byte. These remain in effect until set again, or until

restored to their default values with the D, WS, or Z

commands.

If new values for header bytes are set before the

vehicle protocol has been determined, and if the

search is not set for fully automatic (ie other than

protocol 0), these new values will be used for the

header bytes of the first request to the vehicle. If that

first request should fail to obtain a response, and if the

automatic search is enabled, the ELM327 will then

continue to search for a protocol using default values

for the header bytes. Once a valid protocol is found,

the header bytes will revert to the values assigned with

the AT SH command.

This command is used to assign all header bytes,

whether they are for a J1850, ISO 9141, ISO 14230, or

a CAN system. The CAN systems will use these three

bytes to fill bits 0 to 23 of the ID word (for a 29 bit ID),

or will use only the rightmost 11 bits for an 11 bit CAN

ID (and any extra bits assigned will be ignored). The

additional 5 bits needed for a 29 bit system are set

with the AT CP command.

If assigning header values for the KWP protocols

(4 and 5), care must be taken when setting the first

header byte (xx) value. The ELM327 will always insert

the number of data bytes for you, but how it is done

depends on the values that you assign to this byte. If

the second digit of this first header byte is anything

other than 0 (zero), the ELM327 assumes that you

wish to have the length value inserted in that first byte

when sending. In other words, providing a length value

in the first header byte tells the ELM327 that you wish

to use a traditional 3 byte header, where the length is

stored in the first byte of the header.

If you provide a value of 0 for the second digit of

the first header byte, the ELM327 will assume that you

wish that value to remain as 0, and that you want to

have a fourth header (length) byte inserted into the

message. This is contrary to the ISO 14230-4 OBD

standard, but it is in use by many KWP2000 systems

for (non-OBD) data transfer, so may be useful when

experimenting. Support for 4 byte KWP headers was

added with v1.2 of the ELM327 IC, and is not available

in previous versions.

[ perform a Slow Initiation ]

Protocols 3 and 4 use what is sometimes called a

5 baud, or slow initiation sequence in order to begin

communications. Usually, the sequence is performed

when the first message needs to be sent, and then the

message is sent immediately after. Some ECUs may

need more time between the two however, and having

a separate initiation command allows you to control

this time. Simply send AT SI, wait a little, then send

the message. You may need to experiment a little to

get the right amount of delay. Protocol 3 or 4 must be

selected to use the AT SI command, or an error will

result.

SP h

[ Set Protocol to h ]

This command is used to set the ELM327 for

operation using the protocol specified by 'h', and to

also save it as the new default. Note that the protocol

will be saved no matter what the AT M0/M1 setting is.

The ELM327 supports 12 different protocols (two

can be user-defined). They are:

0 - Automatic

1 - SAE J1850 PWM (41.6 kbaud)

2 - SAE J1850 VPW (10.4 kbaud)

AT Command Descriptions (continued)

ELM327P Datasheet(PDF) 23 Page - ELM Electronics

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