should be able to unravel all of your vehicles
secrets. We have gathered together some
information to help you in this quest and all
of this is available to download from the
Elek-
tor Electronics
website under the following
headings:
– Communication between the PC and the
ELM323.
– AT commands.
– OBD bus initialisation.
– OBD commands.
– Diagnostic test modes.
– Read out and evaluation of fail codes.
– Clearing fail codes.
– ELM232 fail codes.
This detailed information should be of inter-
est not only to anyone building this project
but also to those of you thinking of develop-
ing an application based around the OBD sys-
tem. In a forthcoming article we concentrate
on the software for this project and include
tips and code examples along with the other
programs already mentioned above.
(020138-2)
Note:
Much of the material in this article is taken from a
data sheet from Elm Electronics, Canada. The
original can be downloaded from:
www.elmelectronics.com/dsheets.html
test the interface then a terminal
emulator program such as HyperTer-
minal will do. With any luck it should
already be loaded on your Windows
system, usually in ‘Program Files’
under ‘Accessories’. If not then the
program can be downloaded free of
charge from:
www.hilgraeve.com
HyperTerminal should be initialised
with the following communication
parameters: Data rate 9600 Baud, 8
Data bits, no Parity bit, 1 Stop bit
and no handshake (no hardware
handshake and no XOn/Xoff hand-
shake). This is abbreviated to:
9600,8N1
.
Assuming that the interface is
correctly connected you should see
the four red LEDs light up and the
following message will appear on
the screen:
ELM323 v1.0
>
This gives the version number of the
microcontroller software and also
indicates that the IC is working cor-
rectly, and that the communication
baud rate and receive path (the Rx
input to the PC) is in order. The ‘>’
character is a prompt issued by the
ELM323 indicating that it is ready to
accept data from the RS232 port.
Messages from the PC can be
intended for the internal use by the
ELM323 or for the vehicles diagnostic
system. The ELM323 handles all the
communication and determines the
message destination by analysing
its character strings. Commands for
the ELM323 are always prefixed
with ‘AT’ commands just like the
command set for Hayes compatible
modems, while instructions for the
OBD are ASCII coded hexadecimal
numbers (0 to 9 and A to F). As a
test, enter the command ATE1 (turn
echo on) followed by the enter key, if
there is no ‘OK’ response check the
earth connection (Pin 5) and check
the settings in the terminal emulator
program to ensure that ‘No hand-
shake’ is selected.
now for the car clinic
It is undoubtedly easier to communi-
cate with the OBD system by using
the Windows based program that
we referred to earlier. This program
will be featured in the next article on
this project. If you can’t wait, then
the terminal emulator can meet out
needs again; armed with a copy of
the OBD-2 interface documentation
and the ELM323 command set you
TEST
&MEASUREMENT
29
11/2002
Elektor Electronics
Figure 6.The fully populated PCB. The red LEDs indicate transmitting and receiving data through the ports.
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