ELM327P datasheet(11/51 Pages) ELM

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ELM327

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PCI bytes, and possibly the CAN data length code

(DLC) if it has been enabled with PP 29. The current

version of this IC does not display the CAN CRC code,

nor the special J1850 IFR bytes (which some protocols

use to acknowledge receipt of a message).

[ Identify yourself ]

Issuing this command causes the chip to identify

itself, by printing the startup product ID string (currently

“ELM327 v1.2”). Software can use this to determine

exactly which integrated circuit it is talking to, without

having to reset the IC.

IB 10

[set the ISO Baud rate to 10400 ]

This command restores the ISO 9141-2 and

ISO 14230-4 baud rates to the default value of 10400.

IB 96

[set the ISO Baud rate to 9600 ]

Several users have requested this command. It is

used to change the baud rate used for the ISO 9141-2

and ISO 14230-4 protocols (numbers 3, 4, and 5) to

9600 baud, while relaxing some of the requirements

for the initiation byte transfers. It may be useful for

experimenting with some vehicles. Normal 10,400

baud operation can be restored at any time by issuing

an IB 10 command.

IFR0, IFR1, and IFR2

[ IFR control ]

The SAE J1850 protocol allows for an In-Frame

Response (IFR) byte to be sent after each message,

usually to acknowledge the correct receipt of that

message. The ELM327 automatically generates and

sends this byte for you by default, but you can override

this behaviour with this command.

The AT IFR0 command will disable the sending of

all IFRs, no matter what the header bytes require.

AT IFR2 is the opposite - it will force an IFR byte to

always be sent, no matter what header bytes indicate.

The AT IFR1 command restores the response to

provide the automatic sending of IFRs, as determined

by the ‘K’ bit of the header byte. IFR1 is the default

setting of the ELM327.

IFR H and IFR S

[ IFR from Header or Source ]

The value sent in the J1850 In-Frame Response

(IFR) byte is normally the same as the value sent as

the Source (or Tester) Address byte that was in the

header of the request. There may be occasions when

it is desireable to use some other value, however, and

this set of commands allows for this.

If you send AT IFR S, the ELM327 will use the

value defined as the Source Address (usually F1, but it

can be changed by PP 06), even if another value was

sent in the Header bytes. This is not what is normally

required, and caution should be used when using

AT IFR S. AT IFR H restores the sending of the IFR

bytes to those provided in the Header. AT IFR H is the

default setting.

IIA hh

[ set the ISO Init Address to hh ]

The ISO 9141-2 and ISO 14230-4 standards state

that when beginning a session with an ECU, the

initiation sequence is to be directed to a specific

address ($33). If you wish to experiment by directing

the slow five baud sequence to another address, it is

done with this command. For example, if you prefer

that the initiation be performed with the ECU at

address $7A, then simply send:

>AT IIA 7A

and the ELM327 will use that address when called to

do so (protocols 3 or 4). The full eight bit value is used

exactly as provided – no changes are made to it (ie no

adding of parity bits, etc.)

Note that setting this value does not affect any

address values used in the header bytes, and that this

value is reset to to $33 whenever the defaults, or the

ELM327, are reset.

KW0 and KW1

[ Key Word checks off (0) or on (1) ]

The ELM327 looks for specific bytes (called Key

Words) to be sent to it during the ISO 9141-2 and

ISO14230-4 initiation sequences. If those bytes are

not found, the initiation is said to have failed (you

might see “UNABLE TO CONNECT” or perhaps “BUS

INIT: ...ERROR”). This may be because you are trying

to connect to a non-OBD compliant ECU, or perhaps

to an older one.

If you wish to experiment, but do not want the

ELM327 to check the values contained in the key

words, you can turn the checking off with:

>AT KW0

AT Commands (continued)

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