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Elm 327 Manual

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ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
Reading the Battery Voltage
Before learning the OBD Commands, we will show
an example of how to use an AT Command. We will
assume that you have built (or purchased) a circuit
which is similar to that of Figure 9 in the Example
Applications section (page 80). This circuit provides a
connection to read the vehicle’s battery voltage, which
many will find very useful.
If you look in the AT Command list, you will see
there is one command that is listed as RV [Read the
input Voltage]. This is the command which you will
need to use. First, be sure that the prompt character is
shown (that is the ‘>’ character), then simply enter ‘AT’
followed by RV, and press return (or enter):
>AT RV
Note that we used upper case characters for this
request, but it was not required, as the ELM327 will
accept upper case (AT RV) as well as lower case
(at rv) or any combination of these (At rV). It does not
matter if you insert space characters (‘ ’) within the
message either, as they are ignored by the ELM327.
A typical response to this command will show a
voltage reading, followed by another prompt character:
12.6V
>
The accuracy of this reading depends on several
factors. As shipped from the factory, the ELM327
voltage reading circuitry will typically be accurate to
about 2%. For many, this is all that is needed. Some
people may want to calibrate the circuitry for more
accurate readings, however, so we have provided a
special ‘Calibrate Voltage’ command for this.
To change the internal calibration constants, you
will need to know the actual battery voltage to more
accuracy than the ELM327 shows. Many quality digital
multimeters can do this, but you should verify the
accuracy before making a change.
Let us assume that you have connected your
accurate multimeter, and you find that it reads 12.47V.
The ELM327 is a little high at 12.6V, and you would
like it to read the same as your meter. Simply calibrate
the ELM327 to the measured voltage using the CV
command:
>AT CV 1247
OK
Note that you should not provide a decimal point in
the CV value, as the ELM327 knows that it should be
between the second and the third digits.
At this point, the internal calibration values have
been changed (ie. written to EEPROM), and the
ELM327 now knows that the voltage at the input is
actually 12.47V. To verify that the changes have taken
place, simply read the voltage again:
>AT RV
12.5V
The ELM327 always rounds off the measurement
to one decimal place, so the 12.47V actually appears
as 12.5V (but the second decimal place is maintained
internally for accuracy and is used in the calculations).
The ELM327 may be calibrated with any reference
voltage that you have available, but note that the CV
command always expects to receive four characters
representing the voltage at the input. If you had used a
9V battery for your reference, and it is actually 9.32V,
then you must add a leading zero to the actual voltage
when calibrating the IC:
>AT CV 0932
OK
If you should get into trouble with this command
(for example, if you set calibration values to something
arbitrary and do not have a voltmeter on hand to
provide accurate values), you can restore the settings
to the original (factory) values with the CV 0000
command. Simply send:
>AT CV 0000
OK
The other AT Commands are used in the same
manner. Simply type the letters A and T, then follow
with the command you want to send and any
arguments that are required. Then press return (or
enter, depending on your keyboard). Remember - you
can always insert space characters as often as you
wish if it improves the readability for you, as they are
ignored by the ELM327.
… 29 …


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