ELM329
Overview
The following describes how to use the ELM329 to
obtain information from your vehicle.
We begin by discussing just how to ‘talk’ to the IC
using a PC, then explain how to change options using
‘AT’ commands, and finally we show how to use the
ELM329 to obtain trouble codes (and reset them). For
the more advanced experimenters, there are also
sections on how to use some of the other features of
this product as well.
Using the ELM329 is not as daunting as it first
seems. 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 smart device with a
terminal program (such as HyperTerminal or ZTerm),
and a little knowledge of OBD commands, which we
will provide in the following sections…
Communicating with the ELM329
The ELM329 expects to communicate with a PC
through an RS232 serial connection. Although modern
computers do not usually provide a serial connection
such as this, there are several ways in which a ‘virtual
serial port’ can be created. The most common devices
are USB to RS232 adapters, but there are several
others such as PC cards, ethernet devices, or
Bluetooth to serial adapters.
No matter how you physically connect to the
ELM329, you will need a way to send and receive
data. The simplest method is to use one of the many
‘terminal’ programs that are available (HyperTerminal,
ZTerm, etc.), to allow typing the characters directly
from your keyboard.
To use a terminal program, you will need to adjust
several settings. First, ensure that your software is set
to use the proper ‘COM’ port, and that you have
chosen the proper data rate - this will be either 9600
baud (if pin 6 = 0V at power up), or 38400 baud (if
pin 6 = 5V and PP 0C has not been changed). If you
select the wrong ‘COM’ port, you will not be able to
send or receive any data. If you select the wrong data
rate, the information that you send and receive will be
all garbled, and unreadable by you or the ELM329.
Don’t forget to also set your connection for 8 data bits,
no parity bits, and 1 stop bit, and to set it for the proper
‘line end’ mode. All of the responses from the ELM329
are terminated with a single carriage return character
and, optionally, a linefeed character (depending on
your settings).
Properly connected and powered, the ELM329 will
energize the five LED outputs in sequence (as a lamp
test) and will then send the message:
ELM329 v2.0
>
In addition to identifying the version of this IC,
receiving this string is a good way to confirm that the
ELM329DSB
computer connections and terminal software settings
are correct (however, at this point no communications
have taken place with the vehicle, so the state of that
connection is still unknown).
The ‘>’ character that is shown on the second line
is the ELM329’s prompt character. It indicates that the
device is in the idle state, ready to receive characters
on the RS232 port. If you did not see the identification
string, you might try resetting the IC again with the AT
Z (reset) command. Simply type the letters A T and Z
(spaces are optional), then press the return key:
>AT Z
That should cause the LEDs to flash again, and
the identification string to be printed. If you see strange
looking characters, then check your baud rate - you
have likely set it incorrectly.
Characters sent from the computer can either be
intended for the ELM329’s internal use, or for
reformatting and passing on to the vehicle. The
ELM329 can quickly determine where the received
characters are to be directed by monitoring the
contents of the message. Commands that are
intended for the ELM329’s internal use will begin with
the characters ‘AT’, while OBD commands for the
vehicle are only allowed to contain the ASCII codes for
hexadecimal digits (0 to 9 and A to F).
Whether it is an ‘AT’ type internal command or a
hex string for the OBD bus, all messages to the
ELM329 must be terminated with a carriage return
character (hex ‘0D’) before it will be acted upon. The
one exception is when an incomplete string is sent and
no carriage return appears. In this case, an internal
timer will automatically abort the incomplete message
after about 20 seconds, and the ELM329 will print a
single question mark (‘?’) to show that the input was
not understood (and was not acted upon).
Messages that are not understood by the ELM329
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