ELM327P datasheet(25/76 Pages) ELM

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ELM327

ELM327DSH

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AT Command Descriptions (continued)

ST hh

[ Set Timeout to hh ]

After sending a request, the ELM327 waits a

preset time for a response before it can declare that

there was ‘NO DATA’ received from the vehicle. The

same timer setting is also used after a response has

been received, while waiting to see if more are

coming. The AT ST command allows this timer to be

adjusted, in increments of 4 msec (or 20 msec if in the

J1939 protocol, with JTM5 selected).

When Adaptive Timing is enabled, the AT ST time

sets the maximum time that is to be allowed, even if

the adaptive algorithm determines that the setting

should be longer. In most circumstances, it is best to

simply leave the AT ST time at the default setting, and

let the adaptive timing algorithm determine what to use

for the timeout.

The ST timer is set to 32 by default (giving a time

of approximately 200 msec), but this value can be

adjusted by changing PP 03. Note that a value of 00

does not result in a time of 0 msec – it will restore the

timer to the default value.

SW hh

[ Set Wakeup to hh ]

Once a data connection has been established,

some protocols require that there be data flow every

few seconds, just so that the ECU knows to maintain

the communications path open. If the messages do not

appear, the ECU will assume that you are finished,

and will close the channel. The connection will need to

be initialized again to reestablish communications.

The ELM327 will automatically generate periodic

messages, as required, in order to maintain a

connection. The replies to these messages are always

ignored, and are not visible to the user. (Currently,

only protocols 3, 4, and 5 support these messages -

nothing is available for CAN.)

The time interval between these periodic ‘wakeup’

messages can be adjusted in 20 msec increments

using the AT SW hh command, where hh is any

hexadecimal value from 00 to FF. The maximum

possible time delay of just over 5 seconds results

when a value of FF (decimal 255) is used. The default

setting provides a nominal delay of 3 seconds between

messages.

Note that the value 00 (zero) is treated as a very

special case, and must be used with caution, as it will

stop all periodic messages. This way of stopping the

messages while keeping the rest of the protocol

functioning normally, is for experimenters, and is not

intended to be used regularly. Issuing AT SW 00 will

not change a prior setting for the time between

wakeup messages, if the protocol is re-initialized.

TA hh

[ set the Tester Address to hh ]

This command is used to change the current

tester (ie. scan tool) address that is used in the

headers, periodic messages, filters, etc. The ELM327

normally uses the value that is stored in PP 06 for this,

but the TA command allows you to temporarily

override that value.

Sending AT TA will affect all protocols, including

J1939. This provides a convenient means to change

the J1939 address from the default value of F9,

without affecting other settings.

Although this command may appear to work ‘on

the fly’, it is not recommended that you try to change

this address after a protocol is active, as the results

may be unpredictable.

TP h

[ Try Protocol h ]

This command is identical to the SP command,

except that the protocol that you select is not

immediately saved in internal memory, so does not

change the default setting. Note that if the memory

function is enabled (AT M1), and this new protocol that

you are trying is found to be valid, that protocol will

then be stored in memory as the new default.

TP Ah

[ Try Protocol h with Auto ]

This command is very similar to the AT TP

command above, except that if the protocol that is tried

should

fail

initialize,

the

ELM327

will

then

automatically sequence through the other protocols,

attempting to connect to one of them.

V0 and V1

[ Variable data lengths off or on ]

These commands modify the current CAN protocol

settings to allow the sending of variable data length

messages, just as bit 6 of PP 2C and PP 2E do for

protocols B and C. This allows experimenting with

variable data length messages for any of the CAN

protocols (not just B and C). The V1 command will

always override any protocol setting, and force a

variable data length message. The default setting is

V0, providing data lengths as determined by the

protocol.

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