ELM323 데이터 시트보기 (PDF) - Elm Electronics
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ELM323 Datasheet PDF : 19 Pages
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ELM323
Overview
The following describes how to use the ELM323 to
obtain a great deal of information from your vehicle. To
some, the following information will be overwhelming,
and for others it will not be nearly enough.
We begin by discussing just how to talk to the IC
using a PC, then go on to explain how to change
options using ‘AT’ commands, and finally go on to
actually use the ELM323 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 product as well.
Using the ELM323 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 provide in the following…
Communicating with the ELM323
The ELM323 relies on a standard RS232 type
serial connection to communicate with the user. The
data rate is fixed at 9600 baud, with 8 data bits, no
parity bit, 1 stop bit, and no handshaking (often
referred to as 9600 8N1). All responses from the IC
are terminated with a single carriage return character
and, optionally, a linefeed character. Make sure your
software is configured properly for the mode you have
chosen.
Properly connected and powered, the ELM323 will
energize the four LED outputs in sequence (as a ‘lamp
test’) and will then send the message:
ELM323 v2.0
>
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
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 displayed above is the ELM323’s
prompt character. It indicates that the device is in its
idle state, ready to receive characters on the RS232
port. Messages sent from the computer can either be
intended for the ELM323’s internal use, or for
reformatting and passing on to the OBD bus.
The ELM323 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 ELM323’s internal use
will always begin with the characters ‘AT’ (as is
common with modems), while commands for the OBD
bus 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 ELM323
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 15 seconds, and the ELM323 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 ELM323
(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 ELM323 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, and
sent out the OBD port, and it does not know if the
message sent to the vehicle is in error.
Incomplete or misunderstood messages can also
occur if the controlling computer attempts to write to
the ELM323 before it is ready to accept the next
command (as there are no handshaking signals to
control the data flow). To avoid a data overrun, users
should always wait for the prompt character (‘>’)
before issuing the next command.
Finally, there are a few convenience items to note.
The ELM323 is not case-sensitive, so ‘ATZ’ is
equivalent to ‘atz’, and to ‘AtZ’. Also, it ignores space
characters and all control characters (tab, linefeed,
etc.) in the input, so they can be inserted anywhere to
improve readability. Another feature is that sending
only a single carriage return character will always
repeat the last command (making it easier to request
updates on dynamic data such as engine rpm).
ELM323DSD
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
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