ELM327
AT Command Descriptions (continued)
command may be a better choice). Also, the command
has no effect on the addresses used by the J1939
protocols, as the J1939 routines set their own receive
addresses based on the ID bit (header) values.
This SR command is exactly the same as the RA
command, and can be used interchangeably with it.
Note that CAN Extended Addressing does not use this
value - it uses the one set by the AT TA command.
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.
SS
[ use the Standard Sequence for searches ]
SAE standard J1978 specifies a protocol search
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.
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.
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 any 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 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.
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.
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,
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
ELM327DSI
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