Manual Elm 327 (page 8 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Communicating with the ELM327

The ELM327 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

ELM327, 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

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 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 v2.1

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

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. 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 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 monitoring the

contents of the message. Commands that are

intended for the ELM327’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

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

Overview

The following describes how to use the ELM327 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

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

features of this integrated circuit as well.

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 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…

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Elm 327 Manual

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