ELM322
Overview
The following describes how to use the ELM322 to
features of this product as well.
obtain a great deal of information from your vehicle. To
some, the quantity of information will be overwhelming,
and for others it will not be enough.
We begin by discussing just how to talk to the IC,
then how to adjust some options through the use of
‘AT’ commands, and finally go on to actually talk to the
vehicle, obtaining trouble codes and resetting them.
For the more advanced experimenters, there are also
sections on how to use some of the programmable
It is not as daunting as it first appears. Many users
will never need to issue an ‘AT’ command, adjust
timeouts or change the headers. For most, all that is
required is a PC or a PDA with a terminal program
(such as HyperTerminal or ZTerm), and knowledge of
one or two OBD commands, which we provide in the
following…
Communicating with the ELM322
The ELM322 relies on a standard RS232 type
serial connection to communicate with the user. The
data rate is fixed at 9600 baud, with 8 data bits, no
parity bit, 1 stop bit, and no handshaking (often
referred to as 9600 8N1). All responses from the IC
are terminated with a single carriage return character
and, by default, a line feed character as well. Make
sure your software is configured properly for the mode
you have chosen.
(hex ‘0D’) before they will be acted upon. If an
incomplete string is sent and no carriage return
appears, an internal timer will automatically abort the
incomplete message after about 10 seconds. Should
this happen, the ELM322 will print a single question
mark to show that the input was not understood (and
was ignored).
Messages that are misunderstood by the ELM322
(syntax errors) will always be signalled by a single
question mark (‘?’). These include incomplete
messages, invalid hexadecimal digit strings, or
incorrect AT commands. It is not an indication of
whether or not the message was understood by the
vehicle. (The ELM322 is a protocol interpreter that
makes no attempt to assess OBD messages for
validity – it only ensures that an even number of hex
digits were received, combined into bytes, and sent
out the OBD port, so it cannot determine if the
message sent to the vehicle is in error.)
Incomplete or misunderstood messages can also
occur if the controlling computer attempts to write to
the ELM322 before it is ready to accept the next
command (as there are no handshaking signals to
control the data flow). To avoid a data overrun, users
should always wait for the prompt character (‘>’)
before issuing the next command.
Finally, a few convenience items to note. The
ELM322 is not case-sensitive, so ‘ATZ’ is equivalent to
‘atz’, and to ‘AtZ’. The device ignores space characters
as well as control characters (tab, linefeed, etc.) in the
input, so they can be inserted anywhere to improve
readability, and finally, issuing only a single carriage
return character will repeat the last command (making
it easier to request updates on dynamic data such as
engine rpm).
Properly connected and powered, the ELM322 will
initially display the message:
ELM322 v2.0
>
In addition to identifying the version of the IC,
receipt of this string is a convenient way to be sure
that the computer connections and the settings are
correct. However, at this point no communications
have taken place with the vehicle, so the state of the
OBD connection is still unknown.
The ‘>’ character displayed above is the ELM322’s
prompt character. It indicates that the device is in its
idle state, ready to receive characters on the RS232
port. Characters sent from the computer can either be
intended for the ELM322’s internal use, or for
reformatting and passing on to the vehicle’s OBD bus.
Commands for the ELM322 are distinguished from
those to the vehicle by always beginning with the
characters ‘AT’ (as is common with modems), while
commands for the OBD bus can contain only the
ASCII characters for hexadecimal digits (0 to 9 and A
to F). This allows the ELM322 to quickly determine
where the received characters are to be directed.
Whether an ‘AT’ type internal command or a hex
string for the OBD bus, all messages to the ELM322
must be terminated with a carriage return character
ELM322DSF
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
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