ELM327
Overview
The following describes how to use the ELM327 to
features of this product as well.
obtain information from your vehicle.
Using the ELM327 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 a PDA with a
terminal program (such as HyperTerminal or ZTerm),
and knowledge of one or two OBD commands, which
we will provide in the following sections…
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
ELM327 to obtain trouble codes (and reset them). For
the more advanced experimenters, there are also
sections on how to use some of the programmable
Communicating with the ELM327
The ELM327 expects to communicate with the
host through an RS232 serial connection. Modern
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 ELM327’s prompt
character. It indicates that the device is in the idle
state, ready to receive characters on the RS232 port.
Characters sent from the computer can either be
intended for the ELM327’s internal use, or for
reformatting and passing on to the vehicle. The
ELM327 can quickly determine where the received
characters are to be directed by analyzing the entire
string once the complete message has been received.
Commands for the ELM327’s internal use will always
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 an ‘AT’ type internal command or a hex
string for the OBD bus, all messages to the ELM327
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 ELM327 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 ELM327
(syntax errors) will always be signalled by a single
question mark. These include incomplete messages,
incorrect AT commands, or invalid hexadecimal digit
strings, but are not an indication of whether or not the
message was understood by the vehicle. One must
keep in mind that the ELM327 is a protocol interpreter
that makes no attempt to assess the OBD messages
for validity – it only ensures that an even number of
hex digits were received, combined into bytes, then
sent out the OBD port, and it does not know if the
computers do not usually provide
a
physical
connection such as this, but 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 ethernet to RS232
devices, or Bluetooth to serial adapters.
No matter how you physically connect to the
ELM327, you will need a way to send and receive
characters. To do this, 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 make
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
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 ELM327. Don’t forget to also
set your connection for 8 data bits, no parity bits, and 1
stop bit, and to also set it for the proper “line end”
mode. All of the responses from the ELM327 are
terminated with a single carriage return character and,
optionally, a linefeed character (depending on your
settings).
Properly connected and powered, the ELM327 will
energize the four LED outputs in sequence (as a lamp
test) and will then send the message:
ELM327 v1.2
>
In addition to identifying the version of this IC,
receiving this string is a good way to confirm that the
ELM327DSC
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