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Elm 327 Manual

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ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
AT Command Descriptions (continued)
stops monitoring and prints a prompt character. The
value ‘n’ may be any single hex digit.
MR hh [ Monitor for Receiver hh ]
This command is very similar to the AT MA
command except that it will only display messages that
were sent to the hex address given by hh. These are
messages which are found to have the value hh in the
second byte of a traditional three byte OBD header, in
bits 8 to 15 of a 29 bit CAN ID, or in bits 8 to 10 of an
11 bit CAN ID. Any single RS232 character aborts the
monitoring, as with the MA command.
Note that if this command is used with CAN
protocols, and if the CAN filter and/or mask were
previously set (with CF, CM or CRA), then the MR
command will over-write the previous values for these
bits only - the others will remain unchanged. As an
example, if the receive address has been set with
CRA 4B0, and then you send MR 02, the 02 will
replace the 4, and the CAN masks/filters will only allow
IDs that are equal to 2B0. This is often not what is
desired - you may want to reset the masks and filters
(with AT AR) first.
As with the AT MA command, this command
begins by performing an internal Protocol Close.
Subsequent OBD requests may show ‘SEARCHING’
or ‘BUS INIT’, etc. messages when the protocol is
reactivated.
MT hh [ Monitor for Transmitter hh ]
This command is also very similar to the AT MA
command, except that it will only display messages
that were sent by the transmitter with the hex address
given by hh. These are messages which are found to
have that value in the third byte of a traditional three
byte OBD header, or in bits 0 to 7 for CAN IDs. As with
the MA and MR monitoring modes, any RS232 activity
(single character) aborts the monitoring.
Note that if this command is used with CAN
protocols, and if the CAN filter and/or mask were
previously set (with CF, CM or CRA), then the MT
command will over-write the previous values for these
bits only - the others will remain unchanged. As an
example, if the receive address has been set with
CRA 4B0, and then you send MT 20, the 20 will
replace the B0, and the CAN masks/filters will only
allow IDs that are equal to 420. This is often not what
is desired - you may want to reset the masks and
filters (with AT AR) first.
As with the AT MA command, this command
begins by performing an internal Protocol Close.
Subsequent OBD requests may show ‘SEARCHING’
or ‘BUS INIT’, etc. messages when the protocol is
reactivated.
NL [ Normal Length messages ]
Setting the NL mode on forces all sends and
receives to be limited to the standard seven data bytes
in length, similar to the other ELM32x OBD ICs. To
allow longer messages, use the AL command.
Beginning with v1.2, the ELM327 does not require
a change to AL to allow longer message lengths for
the KWP protocols to be received (as determined by
the header length values). You can simply leave the IC
set to the default setting of NL, and all of the received
bytes will be shown.
PB xx yy [ set Protocol B parameters ]
This command allows you to change the protocol
B (USER1) options and baud rate without having to
change the associated Programmable Parameters.
This allows for easier testing, and program control.
To use this feature, simply set xx to the value for
PP 2C, and yy to the value for PP 2D, and issue the
command. The next time that the protocol is initialized
it will use these values. For example, assume that you
wish to try monitoring a system that uses 11 bit CAN at
33.3 kbps. If you do not want any special formatting,
this means a value of 11000000 or C0 hex for PP 2C,
and 15 decimal or 0F hex for PP 2D. Send both of
these values to the ELM327 in one command:
>AT PB C0 0F
then monitor:
>AT MA
if you see CAN ERRORs, and realize that you wanted
an 83.3 kbps baud rate, simply close the protocol, and
then send the new values:
>AT PC
OK
>AT PB C0 06
OK
>AT MA
… 22 …


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