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PDF ELM320 Data sheet ( Hoja de datos )
| Número de pieza | ELM320 | |
| Descripción | OBD (PWM) TO RS232 INTERPRETER | |
| Fabricantes | ELM | |
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ELM320
OBD (PWM) to RS232 Interpreter
Description
Since the 1996 model year, North American
automobiles have been required to provide an OBD,
or On Board Diagnostics, port for the connection of
test equipment. Data is transferred serially between
the vehicle and the external equipment using these
connections, in a manner specified by the Society of
Automotive Engineers (SAE) standards. In addition
to operating at different voltage levels, these ports
also use a data format that is not compatible with the
standard used for personal computers.
The ELM320 is an 8 pin integrated circuit that is
able to change the data rate and reformat the OBD
signals into easily recognized ASCII characters. This
allows virtually any personal computer to
communicate with an OBD equipped vehicle using
only a standard serial port and a terminal program.
By also enhancing it with an interface program,
hobbyists can create their own custom ‘scan tool’.
This integrated circuit was designed to provide a
cost-effective way for experimenters to work with an
OBD system, so many features such as RS232
handshaking, variable baud rates, etc., have not
been implemented. In addition, this device is only
able to communicate using the 41.6KHz J1850 PWM
protocol that is commonly used in Ford Motor
Company vehicles.
Features
• Low power CMOS design
• High current drive outputs - up to 25 mA
• Crystal controlled for accuracy
• Configurable with AT commands
• Standard ASCII character output
• High speed RS232 communications
• 41.6KHz J1850 PWM Protocol
Connection Diagram
PDIP and SOIC
(top view)
VDD 1
XT1 2
XT2 3
OBDIn 4
8 VSS
7 OBDOut
6 Tx
5 Rx
Applications
• Diagnostic Trouble Code Readers
• Automotive Scan Tools
Block Diagram
3.58MHz
XT1 2
3 XT2
Timing and
Control
Tx 6
Rx 5
RS232
Interface
Interpreter
OBD
Interface
4 OBDIn
7 OBDOut
ELM320DSB
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
1 of 10
1 page  
ELM320
ELM320. Note that they are not case-sensitive, and
that the character ‘0’ is the number ‘zero’:
ATE0 and ATE1
These commands control whether characters
received on the RS232 port are retransmitted (or
echoed) back to the host computer. To reduce traffic
on the RS232 bus, users may wish to turn echoing
off by issuing ATE0. Echo is initially on at powerup
(default) and can be turned on at any time by issuing
ATE1.
ATH0 and ATH1
These commands control whether or not the header
information is shown in the responses. All OBD
messages have an initial (header) string of three
bytes and a trailing check digit (CRC character) that
is normally not displayed by the ELM320. To see this
extra information, users should turn headers on by
issuing ATH1. The default is H0 (headers off).
ATZ
This combination causes the chip to perform a
complete reset as if power were cycled off and then
on again. All settings are returned to their default
values, and the chip will be put in the idle state,
waiting for characters on the RS232 bus.
OBD Commands
If the bytes received on the RS232 bus do not
begin with the letters A and T, they are assumed to be
commands for the vehicle’s OBD bus. The bytes will
be tested to ensure that they are valid pairs of
hexadecimal digits and, if they are, will be combined
into bytes for transmitting. Recall that no checks are
made as to the validity of the OBD command – data is
simply retransmitted as received.
OBD commands are actually sent to the vehicle
embedded in a data message. The standards require
that every message begin with three header bytes and
end with a checksum byte, which the ELM320 adds
automatically for the user (the header bytes never
change in value, so are stored internally). To view the
extra bytes that are received with the vehicle’s
messages, issue an ATH1 internal command.
Most OBD commands to the vehicle are one or
two bytes in length, but some can be three or more
bytes long. As the ELM320 is considered an
experimenter’s circuit, it will only accept a maximum of
three command bytes (or six hexadecimal digits) per
message. Attempts to send more will result in a syntax
error, with the entire command being ignored and a
single question mark being printed.
The use of hexadecimal digits for all of the data
exchange was chosen as it is the most common data
format used in the relevant SAE standards. It is
consistent with mode request listings and is the most
frequently used format for displaying results. With a
little practice, it should not be very difficult to deal in
hex numbers, but some people may want to obtain a
conversion table or keep a calculator nearby. All users
will be required to manipulate the results in some way,
though (combine bytes and divide by 4 to obtain rpm,
divide by 2 to obtain degrees of advance, etc.), and
may find a software front-end helpful.
As an example of sending a command to the
vehicle, assume that A6 (or decimal 166) is the
command that is required to be sent. In this case, the
user would type the letter A, then the number 6, then
would press the return key. These three characters
would be sent to the ELM320 on the RS232 bus. The
ELM320 would store the characters as they are
received, and when the third character (the carriage
return) is received, begin to assess the other two. It
would see that they are both valid hex digits, and
would convert them to a one byte value (decimal value
is 166). Four header bytes would be added, and a total
of five bytes would be sent to the vehicle. Note that the
carriage return character is only a signal to the
ELM320, and is not sent to the vehicle.
After sending a command, the ELM320 listens on
the OBD bus for any responses that are directed to it.
Each received byte is converted to the equivalent
hexadecimal pair of ASCII characters and transmitted
on the RS232 port for the user. Rather than send
control characters which are unprintable on most
terminals, the digits are sent as numbers and letters
(eg. the hex digit ‘A’ is transmitted as decimal value
65, and not 10).
If there was no response from the vehicle, due to
no data being available, or because the command is
not supported, a ‘NO DATA’ message will be sent. See
the error messages section for a description of this
message and others.
ELM320DSB
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
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| PDF Descargar | [ Datasheet ELM320.PDF ] | |
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