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Elm 327 Manual

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Talking to the Vehicle
The standards require that each OBD command or
request that is sent to the vehicle must adhere to a set
format. The first byte sent (known as the ‘mode’)
describes the type of data being requested, while the
second byte (and possibly a third or more) specifies
the actual information that is required. The bytes which
follow after the mode byte are known as the
‘parameter identification’ or PID number bytes. The
modes and PIDs are described in detail in documents
such as the SAE J1979, or ISO 15031-5 standards,
and may also be defined by the vehicle manufacturers.
The SAE J1979 standard currently defines ten
possible diagnostic test modes, which are:
01 - show current data
02 - show freeze frame data
03 - show diagnostic trouble codes
04 - clear trouble codes and stored values
05 - test results, oxygen sensors
06 - test results, non-continuously monitored
07 - show ‘pending’ trouble codes
08 - special control mode
09 - request vehicle information
0A - request permanent trouble codes
Vehicles are not required to support all of the
modes, and within modes, they are not required to
support all possible PIDs (some of the first OBDII
vehicles only supported a very small number of them).
Within each mode, PID 00 is reserved to show which
PIDs are supported by that mode. Mode 01, PID 00
must be supported by all vehicles, and can be
accessed as follows…
Ensure that your ELM327 interface is properly
connected to the vehicle, and powered. Most vehicles
will not respond without the ignition key in the ON
position, so turn the ignition to on, but do not start the
engine. If you have been experimenting, the state of
your interface may be unknown, so reset it by sending:
>AT Z
You will see the interface leds flash, and then the
IC should respond with ‘ELM327 v2.1’, followed by a
prompt character. Now, you may choose a protocol
that the ELM327 should connect with, but it is usually
easier to simply select protocol ‘0’ which tells the IC to
search for one:
>AT SP 0
That’s all that you need to do to prepare the
ELM327 for communicating with a vehicle. At the
prompt, issue the mode 01 PID 00 command:
>01 00
The ELM327 should say that it is ‘SEARCHING...’
for a protocol, then it should print a series of numbers,
similar to these:
41 00 BE 1F B8 10
The 41 in the above signifies a response from a
mode 01 request (01 + 40 = 41), while the second
number (00) repeats the PID number requested. A
mode 02, request is answered with a 42, a mode 03
with a 43, etc. The next four bytes (BE, 1F, B8, and
10) represent the requested data, in this case a bit
pattern showing the PIDs that are supported by this
mode (1=supported, 0=not). Although this information
is not very useful for the casual user, it does prove that
the connection is working.
Another example requests the current engine
coolant temperature (ECT). Coolant temperature is
PID 05 of mode 01, and can be requested as follows:
>01 05
The response will be of the form:
41 05 7B
The 41 05 shows that this is a response to a
mode 1 request for PID 05, while the 7B is the desired
data. Converting the hexadecimal 7B to decimal, one
gets 7 x 16 + 11 = 123. This represents the current
temperature in degrees Celsius, but with the zero
offset to allow for subzero temperatures. To convert to
the actual coolant temperature, you need to subtract
40 from the value obtained. In this case, then, the
coolant temperature is 123 - 40 or 83°C.
A final example shows a request for the engine
rpm. This is PID 0C of mode 01, so at the prompt type:
>01 0C
If the engine is running, the response might be:
41 0C 1A F8
The returned value (1A F8) is actually a two byte
hex number that must be converted to a decimal value
to be useful. Converting it, we get a value of 6904,
which seems like a very high value for engine rpm.
31 of 94
ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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