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ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
AT Command Descriptions (continued)
RA hh [ set the Receive Address to hh ]
Depending on the application, users may wish to
manually set the address to which the ELM327 will
respond. Issuing this command will turn off the AR
mode, and force the IC to only accept responses
addressed to hh. Use caution with this setting, as
depending on what you set it to, you may end up
accepting (ie. acknowledging with an IFR) a message
that was actually meant for another module. To turn off
the RA filtering, simply send AT AR.
This command is not very effective for use with the
CAN protocols, as it only monitors for one portion of
the ID bits, and that is not likely enough for most CAN
applications - the CRA command may be a better
choice. Also, this command has no effect on the
addresses used by the J1939 protocols, as the J1939
routines derive them from the header values, as
required by the SAE standard.
The RA command is exactly the same as the SR
command, and can be used interchangeably. Note that
CAN Extended Addressing does not use this value - it
uses the one set by the AT TA command.
RD [ Read the Data in the user memory ]
The byte value stored with the SD command is
retrieved with this command. There is only one
memory location, so no address is required.
RTR [ send an RTR message ]
This command causes a special ‘Remote Frame’
CAN message to be sent. This type of message has
no data bytes, and has its Remote Transmission
Request (RTR) bit set. The headers and filters will
remain as previously set (ie the ELM327 does not
make any assumptions as to what format a response
may have), so adjustments may need to be made to
the mask and filter. This command must be used with
an active CAN protocol (one that has been sending
and receiving messages), as it can not initiate a
protocol search. Note that the CAF1 setting normally
eliminates the display of all RTRs, so if you are
monitoring messages and want to see the RTRs, you
will have to turn off formatting, or else turn the headers
on.
The ELM327 treats an RTR just like any other
message sent, and will wait for a response from the
vehicle (unless AT R0 has been chosen).
RV [ Read the input Voltage ]
This initiates the reading of the voltage present at
pin 2, and the conversion of it to a decimal voltage. By
default, it is assumed that the input is connected to the
voltage to be measured through a 47KΩ and 10KΩ
resistor divider (with the 10KΩ connected from pin 2 to
Vss), and that the ELM327 supply is a nominal 5V.
This will allow for the measurement of input voltages
up to about 28V, with an uncalibrated accuracy of
typically about 2%.
S0 and S1 [ printing of Spaces off or on ]
These commands control whether or not space
characters are inserted in the ECU response.
The ELM327 normally reports ECU responses as
a series of hex characters that are separated by space
characters (to improve readability), but messages can
be transferred much more quickly if every third byte
(the space) is removed. While this makes the message
less readable for humans, it can provide significant
improvements for computer processing of the data. By
default, spaces are on (S1), and space characters are
inserted in every response.
SD hh [ Save Data byte hh ]
The ELM327 is able to save one byte of
information for you in a special nonvolatile memory
location, which is able to retain its contents even if the
power is turned off. Simply provide the byte to be
stored, then retrieve it later with the read data (AT RD)
command. This location is ideal for storing user
preferences, unit ids, occurrence counts, or other
information.
SH xyz [ Set the Header to 00 0x yz ]
Entering CAN 11 bit ID words (headers) normally
requires that extra leading zeros be added (eg. AT SH
00 07 DF), but this command simplifies doing so. The
AT SH xyz command accepts a three digit argument,
takes only the right-most 11 bits from that, adds
leading zeros, and stores the result in the header
storage locations for you. As an example, AT SH 7DF
is a valid command, and is quite useful for working
with 11 bit CAN systems. It actually results in the
header bytes being stored internally as 00 07 DF.
… 24 …

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