Manual Elm 327 (page 27 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

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AT Command Descriptions (continued)

order that scan tools should use. It follows the number

order that we have assigned to the ELM327 protocols.

In order to provide a faster search, the ELM327 does

not normally follow this order, but it will if you

command it to with AT SS.

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 may also be used after a response

has been received, while waiting to see if any more is

coming (but this depends on the AT AT setting). The

AT ST command allows this timer to be adjusted, in

increments of 4 msec (or 20 msec if in a CAN protocol,

with CTM5 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 default setting

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. Also, during

protocol searches, an internally set minimum time is

used - you may select longer times with AT ST, but not

shorter ones.

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. Any replies to these messages are

ignored by the ELM327, and are not visible to the user.

(Currently, only protocols 3, 4, and 5 support these

messages - nothing is available for CAN. If you require

CAN periodic messages, you must use the ELM329.)

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 (92) provides a nominal delay of 3 seconds

between messages.

Note that the value 00 (zero) is special, as it will

stop the periodic (wakeup) messages. This provides a

control for experimenters to stop the messages while

keeping the rest of the protocol functioning normally,

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. Once periodic messages have been turned

off with AT SW, they can only be reestablished by

closing and reinitializing the protocol.

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 EEPROM 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 to initialize, the ELM327 will then

automatically sequence through the other protocols,

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