ELM325
Overview
The following describes how to use the ELM325 to
obtain information from your vehicle.
Using the ELM325 is not as daunting as it first
seems. Many users may never need to issue an ‘AT’
command, adjust timeouts, or change the MID. For
those that do want to make changes, all that is
required is a PC or smart device with a terminal
program (such as HyperTerminal or ZTerm), and a
little knowledge…
We begin by discussing just how to ‘talk’ to the IC
using a PC, then explain how to change options using
the ‘AT’ commands, and finally we show how to
communicate with a vehicle. For the more advanced
experimenters, there are also sections on how to use
some of the other features of this product as well.
Communicating with the ELM325
The ELM325 expects to communicate with the
controlling device through an RS232 serial connection.
Although most modern devices 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 Wi-Fi modules,
ethernet devices, or Bluetooth to serial adapters.
No matter how you physically connect to the
ELM325, 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 – the ELM325 can only
communicate at 57600 bps. 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, but the right
‘COM’ port, the information that you send and receive
will be unreadable by you or the ELM325. 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 ELM325 are
terminated with a single carriage return character and,
optionally, a linefeed character (depending on your
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 ELM325’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, 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 only 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 ELM325’s internal use, or for
reformatting and passing on to the vehicle. The
ELM325 can quickly determine where the received
characters are to be directed by monitoring the
contents of the message. Commands that are
intended for the ELM325’s internal use will begin with
the characters ‘AT’, while messages 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 J1708 bus, all messages to the
ELM325 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 ELM325 will print a
single question mark (‘?’) to show that the input was
not understood (and was not acted upon).
Properly connected and powered, the ELM325 will
energize the four LED outputs in sequence (as a ‘lamp
test’) and will then send the message:
ELM325 v1.0
>
In addition to identifying the version of this IC,
receiving this string is a good way to confirm that the
computer connections and terminal software settings
Messages that are not understood by the ELM325
(syntax errors) will always be signalled by a single
ELM325DSA
Elm Electronics – Circuits for the Hobbyist
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