Manual Elm 327 (page 19 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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

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

can be changed with 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, and is the

default setting.

IGN [ read the IgnMon input level ]

This command reads the signal level at pin 15. It

assumes that the logic level is related to the ignition

voltage, so if the input is at a high level, the response

will be ‘ON’, and a low level will report ‘OFF’.

This feature is most useful if you wish to perform

the power control functions using your own software. If

you disable the Low Power automatic response to a

low input on this pin (by setting bit 2 of PP 0E to 0),

then pin 15 will function as the RTS input. A low level

on the input will not turn the power off, but it will

interrupt any OBD activity that is in progress. All you

need to do is detect the ‘STOPPED’ message that is

sent when the ELM327 is interrupted, and then check

the level at pin 15 using AT IGN. If it is found to be

OFF, you can perform an orderly shutdown yourself.

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. The ISO init

address is restored to $33 whenever the defaults, or

the ELM327, are reset.

JE [ enables the J1939 ELM data format ]

The J1939 standard requires that PGN requests

be sent with the byte order reversed from the standard

‘left-to-right’ order, which many of us would expect. For

example, to send a request for the engine temperature

(PGN 00FEEE), the data bytes are actually sent in the

reverse order (ie EE FE 00), and the ELM327 would

normally expect you to provide the data in that order

for passing on to the vehicle.

When experimenting, this constant need for byte

reversals can be quite confusing, so we have defined

an ELM format that reverses the bytes for you. When

the J1939 ELM (JE) format is enabled, and you have a

J1939 protocol selected, and you provide three data

bytes to the ELM327, it will reverse the order for you

before sending them to the ECU. To request the

engine temperature PGN, you would send 00 FE EE

(and not EE FE 00). The ‘JE’ type of automatic

formatting is enabled by default.

JHF0 and JHF1 [ J1939 Header Formatting off or on ]

When printing responses, the ELM327 normally

formats the J1939 ID (ie Header) bits in such a way as

to isolate the priority bits and group all the PGN

information, while keeping the source address byte

separate. If you prefer to see the ID information as four

separate bytes (which a lot of the J1939 software

seems to do), then simply turn off the formatting with

JHF0. The CAF0 command has the same effect (and

overrides the JHF setting), but also affects other

formatting. The default setting is JHF1.

JS [ enables the J1939 SAE data format ]

The AT JS command disables the automatic byte

reordering that the JE command performs for you. If

you wish to send data bytes to the J1939 vehicle

without any manipulation of the byte order (ie in the

order specified by the SAE documents), then select JS

formatting.

As an example, when sending a request for

engine temperature (PGN 00FEEE) with the data

format set to JS, you must present the bytes to the

ELM327 as EE FE 00 (this is also known as little-

endian byte ordering).

The JS type of data formatting is off by default.

JTM1 [ set the J1939 Timer Multiplier to 1 ]

This used to set the AT ST time multiplier to 1, for

the SAE J1939 protocol. As of firmware v2.1, this

AT Command Descriptions (continued)

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