ELM327P datasheet(12/51 Pages) ELM

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after which the IC will look for a response, but will not

look at the actual values of the bytes in the response.

This

may

allow

connection

otherwise

‘impossible’

situation.

Normal

behaviour

can

returned with AT KW1, which is the default setting.

Caution should be used with this command, as

you are bypassing the checks that are normally

performed on the keyword bytes. The ELM327 sends

an acknowledgement to the ECU for these bytes, but

that is without considering what the bytes actually are.

You could be incorrectly activating an ISO 9141, or

KWP 2000 protocol, so should be very careful.

L0 and L1

[ Linefeeds off (0) or on(1) ]

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

on will be the default; otherwise it will be linefeeds off.

M0 and M1

[ Memory off (0) or on(1) ]

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

session

will

not

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 always 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.

As mentioned, the default setting for the memory

function is determined by the voltage level at pin 5 at

power up (or system reset). If it is connected to a high

level (VDD), then the memory function will be on by

default. If pin 5 is connected to a low level, the

memory saving will be off by default.

[ Monitor All messages ]

Using this command places the ELM327 into a

bus monitoring mode, in which it displays all messages

as it sees them on the OBD bus. It remains a quiet

monitor on the bus, not sending In Frame Responses

for J1850 systems or Acknowledges for CAN systems.

This continues indefinitely until stopped by activity on

the RS232 input, or the RTS pin.

To stop the monitoring, one can send a single

character then wait for the ELM327 to respond with a

prompt character (‘>’). Alternatively, the RTS input can

be brought to a low level to interrupt the device as

well. Waiting for the prompt is necessary as the

response time is unpredictable, varying depending on

what the IC was doing when interrupted. If for instance

it is in the middle of printing a line, it will first complete

that line then return to the command state, issuing the

prompt character. If it were simply waiting for input, it

would return immediately. Note that the character

which stops the monitoring will always be discarded,

and will not affect subsequent commands.

MP hhhh

[ Monitor for PGN hhhh ]

The AT MA, MR and MT commands are quite

useful for when you wish to monitor for a specific byte

in the header of a typical OBD message. For the SAE

J1939 Protocol, however, it is often desireable to

monitor for the multi-byte Parameter Group Numbers

(or PGNs), which can appear in either the header, or

the data bytes. The MP command is a special J1939

only command that is used to look for responses to a

particular PGN request, and follow any multi-segment

occurances of them.

Note that the MP command provides no means to

set the first two digits of the requested PGN, and they

AT Commands (continued)

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