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PDF ELM325 Data sheet ( Hoja de datos )
| Número de pieza | ELM325 | |
| Descripción | J1708 Interpreter | |
| Fabricantes | ELM | |
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ELM325
J1708 Interpreter
Description
With the advent of electronic engine controls,
many vehicles also adopted some form of diagnostic
tools to help monitor their operation. As more
modules began to be used in vehicles, there was
also a need for the devices to share information
rather than each independently obtain this from
separate sensors.
In the 1980’s, the SAE J1708 standard was
created to provide a specification for a common data
bus to be used in heavy duty vehicles. It used
RS485 wiring (already proven to be reliable in noisy
environments), and a UART-based low speed data
format. The SAE J1587 standard followed a few
years later to describe the mechanism by which
messages and data should be sent between vehicle
modules.
The ELM325 allows a PC or similar device to be
used to monitor and query devices on a J1708 data
bus, using simple commands that can be sent from
almost any terminal program. It is able to work with
either the J1587 or the J1922 data formats.
Applications
• Diagnostic trouble code readers
• Heavy duty vehicle scan tools
• Teaching aids
• ECU Simulators
Features
• Supports both SAE J1587 and J1922
• High speed RS232 interface
• Works with standard RS485 transceivers
• Fully configurable with AT commands
• Wide operating voltage range (1.8 to 5.5V)
• Low power CMOS design
Connection Diagram
PDIP and SOIC
(top view)
VDD 1
XT1 2
XT2 3
InvDE 4
RS232 Rx 5
RS232 Tx 6
RS Rx LED 7
14 VSS
13 RO
12 RE
11 DE
10 J Tx LED
9 J Rx LED
8 RS Tx LED
Block Diagram
RS232 Rx 5
RS232 Tx 6
RS232
Interface
3.58 MHz
XT1 2
3 XT2
Timing and
Control
J1587/1922
Interpreter
InvDE
4
J1708
Interface
13 RO
11 DE
12 RE
RS Rx LED 7
RS Tx LED 8
10 J Tx LED
9 J Rx LED
ELM325DSA
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
1 of 31
Free Datasheet http://www.datasheet4u.com/
1 page  
ELM325
Overview
The following describes how to use the ELM325 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
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.
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…
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).
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
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).
Messages that are not understood by the ELM325
(syntax errors) will always be signalled by a single
ELM325DSA
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
5 of 31
Free Datasheet http://www.datasheet4u.com/
5 Page 
ELM325
Sending AT Commands
Before learning some J1587 Commands, we will
show a few examples of how to use an AT Command.
We will assume that you have built (or purchased) a
circuit that is similar to that of Figure 7 in the Example
Applications section.
Do not connect your circuit to a vehicle at this
time. Power it from a test source only (a 9V ‘transistor
radio’ battery works well), and connect it to your PC as
discussed in the Communicating with the ELM325
section.
For your first command, simply reset the IC by
sending AT Z. Try this a few ways (don’t forget to
press enter or return after each):
>AT Z
or
>atz
or
>a T
z
You should see the RS232 LEDs blink as you type
each letter, and after you press return (or enter) you
should see all four Tx/Rx LEDs blink, in order, followed
by a final blink of the RS232 Tx LED as the ELM325
sends ‘ELM325 v1.0’.
Most J1708 messages are continually sent on the
data bus, at a predetermined rate. Some messages
may be sent 10x per second, while others are only 1x
per 10 seconds. This means that you may need to
adjust the internal timeout setting depending on what
message you are attempting to receive. We will adjust
this timeout setting next.
Try this request for trouble codes:
>AT TC
After about 2 seconds, you should see a response
that looks like:
NO DATA
>
since there is no vehicle attached, there was no data
received.
Now, adjust the timeout to 10 seconds. If you look
at the AT command list, you will see that there is a Set
Timeout command that is used for this. Timing is in
increments of 100 msec (0.1 sec), so to obtain a 10
second delay, the AT ST setting should be 100. We
need to convert the 100 to hexadecimal, however, as
all numbers handled by the ELM325 must be in
hexadecimal. Converting then, 100 (= 6 x 16 + 4) is 64
in hexadecimal, so we send:
>AT ST 64
Again, don’t forget to press return (or enter). Now,
the timeout should be set to 10 seconds. To verify this,
repeat the Trouble Codes command:
>AT TC
It should be 10 seconds before you see the NO
DATA response this time. To try it again, you do not
need to enter the AT TC command again, you only
need to press enter, and the ELM325 will repeat your
last command (AT TC) for you.
As a final test, enter AT TC again, but before the
10 seconds is up, press any key on the keyboard. You
should see the ELM325 respond with:
STOP?
which means that it was interrupted and it thinks that
you wish to stop. If you ever see the ‘STOP?’
response, it means that the ELM325 thinks it has been
interrupted by you.
Now, restore the AT ST time to the default value,
with the Defaults command:
>AT D
You should see a response of ‘OK’ and then a
prompt character on a new line, to show that the
ELM325 is waiting for you. In this case, you might also
have sent AT ST 00, since that also restores the
timeout setting to its default value.
Experiment with these commands – you are not
connected to a vehicle, so can do no harm. Sending
AT Commands is not difficult, they just require a little
practice.
ELM325DSA
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
11 of 31
Free Datasheet http://www.datasheet4u.com/
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| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet ELM325.PDF ] | |
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