Manual Elm 327 (page 20 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

AT Command Descriptions (continued)

command now simply calls the CTM1 command.

JTM5 [ set the J1939 Timer Multiplier to 5 ]

This used to set the AT ST time multiplier to 5, for

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

command now simply calls the CTM5 command.

KW [ display the Key Words ]

When the ISO 9141-2 and ISO 14230-4 protocols

are initialized, two special bytes (key words) are

passed to the ELM327 (the values are used internally

to determine whether a particular protocol variation

can be supported by the ELM327). If you wish to see

what the value of these bytes were, simply send the

AT KW command.

KW0 and KW1 [ Key Word checks off or on ]

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 the bytes are not

found, the initiation is said to have failed (you might

see ‘UNABLE TO CONNECT’ or perhaps ‘BUS INIT:

...ERROR’). This might occur if you are trying to

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

an older one.

If you wish to experiment with non-standard

systems, you may have to tell the ELM327 to perform

the initiation sequence, but ignore the contents of the

bytes that are sent and received. To do this, send:

>AT KW0

After turning keyword checking off, the ELM327

will still require the two key word bytes in the

response, but will not look at the actual values of the

bytes. It will also send an acknowledgement to the

ECU, and will wait for the final response from it (but

will not stop and report an error if none is received).

This may allow you to make a connection in an

otherwise ‘impossible’ situation. Normal behaviour can

be returned with AT KW1, which is the default setting.

L0 and L1 [ Linefeeds off or on ]

This option controls the sending of linefeed

characters after each carriage return character. For

AT L1, linefeeds will be generated after every carriage

return character, and for AT L0, they will be off. Users

will generally wish to have this option on if using a

terminal program, but off if using a custom computer

interface (as the extra characters transmitted will only

serve to slow the communications down). The default

setting is determined by the voltage at pin 7 during

power on (or reset). If the level is high, then linefeeds

are on by default; otherwise they will be off.

LP [ go to the Low Power mode ]

This command causes the ELM327 to shut off all

but ‘essential services’ in order to reduce the power

consumption to a minimum. The ELM327 will respond

with an ‘OK’ (but no carriage return) and then, one

second later, will change the state of the PwrCtrl

output (pin 16) and will enter the low power (standby)

mode. The IC can be brought back to normal operation

through a character received at the RS232 input or a

rising edge at the IgnMon (pin 15) input, in addition to

the usual methods of resetting the IC (power off then

on, a low on pin 1, or a brownout). See the Power

Control section (page 64) for more information.

M0 and M1 [ Memory off or on ]

The ELM327 has internal ‘non-volatile’ memory

that is capable of remembering the last protocol used,

even after the power is turned off. This can be

convenient if the IC is often used for one particular

protocol, as that will be the first one attempted when

next powered on. To enable this memory function, it is

necessary to either use an AT command to select the

M1 option, or to have chosen ‘memory on’ as the

default power on mode (by connecting pin 5 of the

ELM327 to a high logic level).

When the memory function is enabled, each time

that the ELM327 finds a valid OBD protocol, that

protocol will be memorized (stored) and will become

the new default. If the memory function is not enabled,

protocols found during a session will not be

memorized, and the ELM327 will always start at power

up using the same (last saved) protocol.

If the ELM327 is to be used in an environment

where the protocol is constantly changing, it would

likely be best to turn the memory function off, and

issue an AT SP 0 command once. The SP 0 command

tells the ELM327 to start in an 'Automatic' protocol

search mode, which is the most useful for an unknown

environment. ICs come from the factory set to this

mode. If, however, you have only one vehicle that you

regularly connect to, storing that vehicle’s protocol as

the default would make the most sense.

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