ELM320DSC_技术文档

ELM320

OBD (PWM) to RS232 Interpreter

Description

Features

Since the 1996 model year, North American

• Low power CMOS design

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 this

connection, 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.

• High current drive outputs - up to 25 mA

• Crystal controlled for accuracy

• Fully configurable using AT commands

• Standard ASCII character output

• High speed RS232 communications

• 41.6KHz J1850 PWM protocol

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.

Connection Diagram

PDIP and SOIC

(top view)

This integrated circuit was designed to provide a

cost-effective way for experimenters to work with an

OBD system, so a few 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.

1

2

3

4

8

7

6

5

VDD

XT1

VSS

OBDOut

Tx

XT2

OBDIn

Rx

Applications

• Diagnostic trouble code readers

• Automotive scan tools

3.58MHz

XT2

Block Diagram

2

3

XT1

Timing and

Control

Tx

6

5

4

OBDIn

RS232

Interface

OBD

Interface

Interpreter

7

Rx

OBDOut

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

1 of 16

< http://www.elmelectronics.com/ >

ELM320

Pin Descriptions

VDD (pin 1)

Rx (pin 5)

This pin is the positive supply pin, and should

always be the most positive point in the circuit.

Internal circuitry connected to this pin is used to

provide power on reset of the microprocessor, so

an external reset signal is not required. Refer to

the Electrical Characteristics section for further

information.

The computer’s RS232 transmit signal can be

directly connected to this pin from the RS232

line as long as a current limiting resistor

(typically about 47KW) is installed in series.

(Internal protection diodes will pass the input

currents safely to the supply connections,

protecting the ELM320.) Internal signal inversion

and Schmitt trigger waveshaping provide the

necessary signal conditioning.

XT1 (pin 2) and XT2 (pin 3)

A 3.579545 MHz NTSC television colourburst

crystal is connected between these two pins.

Crystal loading capacitors (typically 27pF) will

also normally be connected between each of the

pins and the circuit common (Vss).

Tx (pin 6)

The RS232 data output pin. The signal level is

compatible with most interface ICs, and there is

sufficient current drive to allow interfacing using

only a single PNP transistor, if desired.

OBDIn (pin 4)

OBDOut (pin 7)

The OBD data is input to this pin, with a high

logic level representing the active state (and a

low, the passive). No Schmitt trigger input is

provided, so the OBD signal should be buffered

to minimize transition times for the internal

CMOS circuitry. The external level shifting

circuitry is usually sufficient to accomplish this –

see the Example Application section for a typical

circuit.

This is the active low output signal which is used

to drive the OBD bus to its active state. Since the

J1850 PWM standard requires a differential bus

signal, the user must create the complement of

this signal to drive the other bus line. See the

Example Application section for more details.

VSS (pin 8)

Circuit common is connected to this pin. This is

the most negative point in the circuit.

Ordering Information

These integrated circuits are available in either the 300 mil plastic DIP format, or in the 208 mil SOIC surface

mount type of package. To order, add the appropriate suffix to the part number:

300 mil Plastic DIP............................... ELM320P

208 mil SOIC..................................... ELM320SM

Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be

given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information

described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these

products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or

systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve

reliability, function, or design.

2 of 16

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

ELM320

Absolute Maximum Ratings

Storage Temperature....................... -65°C to +150°C

Note:

Stresses beyond those listed here will likely damage

the device. These values are given as a design

guideline only. The ability to operate to these levels

is neither inferred nor recommended.

Ambient Temperature with

Power Applied....................................-40°C to +85°C

Voltage on VDD with respect to VSS............0 to +7.5V

Voltage on any other pin with

respect to VSS........................... -0.6V to (VDD + 0.6V)

Electrical Characteristics

All values are for operation at 25°C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.

Characteristic

Minimum Typical

Maximum Units

Conditions

Supply voltage, VDD

VDD rate of rise

4.5

5.0

5.5

V

see note 2

see note 3

0.05

V/ms

mA

Average supply current, IDD

1.0

2.4

Input low voltage

Input high voltage

Output low voltage

Output high voltage

Rx pin input current

RS232 baud rate

VSS

0.15 VDD

VDD

V

0.85 VDD

0.6

V

Current (sink) = 8.7mA

Current (source) = 5.4mA

see note 4

VDD - 0.7

-0.5

V

+0.5

mA

baud

see note 5

9600

Notes:

1. This integrated circuit is produced with a Microchip Technology Inc.’s PIC12C5XX as the core embedded

microcontroller. For further device specifications, and possibly clarification of those given, please refer to the

appropriate Microchip documentation (available at http://www.microchip.com/).

2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved

using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as

may be obtained through direct connection to solar cells, or some charge pump circuits.

3. Device only. Does not include any load currents.

4. This specification represents the current flowing through the protection diodes when applying large voltages

to the Rx input (pin 5) through a current limiting resistance. Currents quoted are the maximum that should be

allowed to flow continuously.

5. Nominal data transfer rate when a 3.58 MHz crystal is used as the frequency reference. Data is transferred

to and from the ELM320 with 8 data bits, no parity, and 1 stop bit (8 N 1).

3 of 16

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

ELM320

Overview

The following describes how to use the ELM320 to

features of this product as well.

obtain a great deal of information from your vehicle. To

many, the quantity of information will be overwhelming,

and to others it is not nearly enough.

We begin by discussing just how to talk to the IC,

then how to adjust some options through the use of

‘AT’ commands, and finally go on to actually talk to the

vehicle, obtaining trouble codes and resetting them.

For the more advanced experimenters, there are also

sections on how to use some of the programmable

It is not as daunting as it first appears. 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 ELM320

The ELM320 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, by default, a line feed character as well. Make

sure your software is configured properly for the mode

you have chosen.

(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 10 seconds, and the ELM320 will print a single

question mark to show that the input was not

understood (and was ignored).

Messages that are misunderstood by the ELM320

(syntax errors) will always be signalled by a single

question mark (‘?’). These include incomplete

messages, invalid hexadecimal digit strings, or

incorrect AT commands. It is not an indication of

whether or not the message was understood by the

vehicle. (The ELM320 is a protocol interpreter that

makes no attempt to assess 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, so it cannot determine 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 ELM320 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, a few convenience items to note. The

ELM320 is not case-sensitive, so ‘ATZ’ is equivalent to

‘atz’, and to ‘AtZ’. The device ignores space characters

as well as control characters (tab, linefeed, etc.) in the

input, so they can be inserted anywhere to improve

readability and, finally, issuing only a single carriage

return character will repeat the last command (making

it easier to request updates on dynamic data such as

engine rpm).

Properly connected and powered, the ELM320 will

initially display the message:

ELM320 v2.0

>

In addition to identifying the version of the IC,

receipt of this string is a convenient way to be sure

that the computer connections and the 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 ELM320’s

prompt character. It indicates that the device is in its

idle state, ready to receive characters on the RS232

port. Characters sent from the computer can either be

intended for the ELM320’s internal use, or for

reformatting and passing on to the vehicle’s OBD bus.

Commands for the ELM320 are distinguished from

those to the vehicle by always beginning with the

characters ‘AT’ (as is common with modems), while

commands for the OBD bus must contain only the

ASCII characters for hexadecimal digits (0 to 9 and A

to F). This allows the ELM320 to quickly determine

where the received characters are to be directed.

Whether an ‘AT’ type internal command or a hex

string for the OBD bus, all messages to the ELM320

must be terminated with a carriage return character

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

4 of 16

ELM320

AT Commands

Several parameters within the ELM320 can be

‘OK’ on the successful completion of a command, so

the user knows that it has been executed.

adjusted in order to modify its behaviour. These do not

normally have to be changed before attempting to talk

to the vehicle, but occasionally the user may wish to

customize the settings, for example by turning the

character echo off, adjusting the timeout value, or

changing the header addresses. In order to do this,

internal ‘AT’ commands must be issued.

Those familiar with PC modems will immediately

recognize AT commands as a standard way in which

modems are internally configured. The ELM320 uses

essentially the same method, always watching the

data sent by the PC, looking for messages that begin

with the character ‘A’ followed by the character ‘T’. If

found, the next characters will be interpreted as

internal configuration or ‘AT’ commands, and will be

executed upon receipt of a terminating carriage return

character. The ELM320 will reply with the characters

Some of the following commands allow passing

numbers as arguments in order to set the internal

values. These will always be hexadecimal numbers

which must be provided in pairs. The hexadecimal

conversion chart in the next section may prove useful

if you wish to interpret the values. Also, one should be

aware that for the on/off types of commands, the

second character is a number (1 or 0), the universal

terms for on and off, respectively.

The following is a summary of all of the AT

commands that are recognized by the current version

of the ELM320, sorted alphabetically. Users of

previous versions of this product (v1.x) should note

that their ICs will only support the E, H and Z options.

AR

[ Automatically set the Receive address ]

E0 and E1

[ Echo off (0) or on(1) ]

Responses from the vehicle will be acknowledged

and displayed by the ELM320, if its internally stored

receive address matches the address that the

message is being sent to. With the auto receive

mode in effect, the value used for the receive

address will be chosen based on the current header

bytes, and will automatically be updated whenever

the header bytes are changed.

These commands control whether or not 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. The default is E1 (echo on).

FD

[ send Formatted Data ]

This command requests that all responses be

returned as standard ASCII characters which are

readable on virtually any standard terminal program.

Hex digits are shown as two ASCII characters, and

spaces are provided between each byte as a

separator. Also, every line will end with a carriage

The value that is used for the receive address is

determined based on the contents of the first header

byte. If it shows that the message uses physical

addressing, the third header byte of the header is

used for the receive address, otherwise (for

functional addressing) the second header byte,

increased in value by 1, will be used. Auto Receive

is turned on by default.

return character and (optionally)

a

linefeed

character, ensuring that every response appears on

a new line. This is the default mode.

D

[ set all to Defaults ]

H0 and H1

[ Headers off (0) or on(1) ]

This command is used to set the E, H, L, and R

options to their default (or factory) settings, as when

power is first applied. Additionally, the Auto Receive

mode (AR) will be selected, data will be transmitted

in the standard formatted way (as if chosen by FD),

the ‘NO DATA’ timeout will be set to its default value,

and the header bytes will be set to the proper values

for the OBDII operation.

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 which are normally

not displayed by the ELM320. To see this extra

information, users can turn the headers on by

issuing an ATH1. The default is H0 (headers off).

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

5 of 16

ELM320

AT Commands (continued)

I

[ Identify yourself ]

the MA and MR monitoring modes, any RS232

activity (single character) aborts the monitoring.

Issuing this command causes the chip to identify

itself, by printing the startup product ID string (this is

currently ‘ELM320 v2.0’). Software can use this to

determine exactly which integrated circuit it is talking

to, without resorting to resetting the entire IC.

PD

[ send Packed Data ]

This option is for those that are building a computer

interface and want the fastest data transfer rate

possible while still operating at 9600 baud. When

selected, responses from the vehicle will be

formatted as an initial length byte followed by the

actual response bytes from the vehicle, with no

trailing carriage returns or linefeed characters. The

data will not be altered in any way, except for the

conversion to standard RS232 bytes.

L0 and L1

[ Linefeeds off (0) or on(1) ]

Whether the ELM320 transmits a linefeed character

after each carriage return character is controlled by

this option. If an ATL1 is issued, linefeed generation

will be turned on, and for ATL0, it will be off. Users

may wish to have this option on if using a terminal

program, but off if using a custom interface (as the

extra characters transmitted will only serve to slow

the vehicle polling down). The default setting is L1

(linefeeds on)

Note that the length byte only represents the total

number of data bytes following, and does not include

itself. Also, if there was a data (checksum) error, the

length byte will have its most significant bit set, so

the user should always check first to see if the length

is greater than 127. (The other 7 bits still provide a

valid byte count if there is an error, so one need only

ignore the msb, or subtract 128 from the value.)

MA

[ Monitor All messages ]

Using this command places the ELM320 into a bus

monitoring mode, in which it displays all messages

as it sees them on the OBD bus. This continues

indefinitely until stopped by activity on the RS232

input. To stop the monitoring, one should send any

single character then wait for the ELM320 to respond

with a prompt character (‘>’). Waiting for the prompt

is necessary as the response time is unpredictable,

varying depending on the IC was doing when

interrupted. If for instance it is in the middle of

printing a line, it will first complete the line then

return to the command state, issuing the prompt

character. If it were simply waiting for input, it would

return immediately. The character which stops the

monitoring will always be discarded, and will not

affect subsequent commands.

A ‘NO DATA’ response has no data bytes, but still

sends a length byte with value ‘0’.

R0 and R1

[ Responses off (0) or on(1) ]

These commands control the ELM320’s automatic

display of responses. If responses have been turned

off, the IC will not wait for anything to be returned

from the vehicle after sending a request, and will

return immediately to waiting for RS232 commands.

This is useful if sending commands blindly when

using the IC for a non-OBD network application, or

simulating an ECU, in a basic learning environment.

It is not recommended that this option normally be

used, however, as the vehicle may have difficulty if it

is expecting an acknowledgement byte and never

receives one. The default is R1 (responses on).

MR hh

[ Monitor for Receiver hh ]

This command also places the IC in a bus monitoring

mode, displaying only messages that were sent to

the hex address given by hh (i.e. messages which

are found to have that value in their second byte).

Any RS232 activity (single character) aborts the

monitoring, as with the MA command.

SH xx yy zz

[ Set the Header to xx yy zz ]

This command allows the user to control the values

that are sent as the three header bytes in the

message. The value of hex digits xx will be used for

the first or priority/type byte, yy will be used for the

second or target byte, and zz will be used for the

third or source byte. These remain in effect until set

again, or until restored to the default values with the

AT D, or AT Z commands. The default header values

MT hh

[ Monitor for Transmitter hh ]

Another monitoring command, which displays only

messages sent by Transmitter address hh. As with

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

6 of 16

ELM320

AT Commands (continued)

are 61 6A F1, as required by the SAE J1979

Diagnostic Test Modes (OBDII) standard.

SR hh

[Set the Receive address to hh ]

Depending on the application, users may wish to

manually set the address to which the ELM320 will

respond. Issuing this command will turn off the AR

mode, and force the IC to only accept responses

addressed to hh. Use caution with this setting, as

depending on what you set it to, you may end up

accepting (acknowledging with an IFR) a message

that was actually meant for another module.

ELM320 AT Commands

general

D set all to Defaults

I show the ID string

Z reset all

ST hh

[ Set Timeout to hh ]

<CR> repeat last command

After sending a request, the ELM320 waits a preset

time before declaring that there was no response

from the vehicle (the ‘NO DATA’ response).

Depending on the application (and priority of the

request), users may want modify this timeout period

before declaring the request a failure. The ST

command is used to do that.

responses

E1/0 Echo on/off

H1/0 Headers on/off

L1/0 Linefeeds on/off

R1/0 Responses on/off

PD use Packed Data

FD use Formatted Data

ST hh Set Timeout (hh*4ms)

The actual time used is (approximately) 4 ms x the

byte value passed as the hexadecimal argument.

Passing a value of FF thus results in a maximum

time of about 1020 ms. Values less than 08 will be

ignored and forced to a value of 8, providing a

minimum time of 32ms. The default value is 32

(decimal 50) providing a timeout of 200 ms.

requests

SH xx yy zz Set Header

SR hh Set Rx address

AR Auto Receive

MA Monitor All

MR hh Monitor for Rxer hh

MT hh Monitor for Txer hh

Z

[ reset all ]

This command 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.

Figure 1. ELM320 AT Commands

AT Command Summary

Figure 1 (at the right) shows all of the ELM320

commands in one convenient chart. In order to help

with the understanding of these, we have grouped

the commands into three functional areas, but this

has no bearing on how the commands are to be

used, it is only for clarity. You may find this chart to

be useful when experimenting with the IC.

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

7 of 16

ELM320

OBD Commands

If the bytes received on the RS232 bus do not

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 (with a

decimal value of 166). Three header bytes and a

checksum byte would be added, so 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 on 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

(e.g. the hex digit ‘A’ is transmitted as decimal value

65, and not 10).

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 to the vehicle. 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 to every message. The ELM320 powers-

on expecting to be used for the OBDII mandated

emissions diagnostics, and sets the header bytes

accordingly. If you wish to perform more advanced

functions, these bytes may be changed through the

use of AT commands. To view the extra bytes that are

received with the vehicle’s messages, turn the header

display on by issuing an ATH1 command.

The command portion of most OBD messages is

usually only one or two bytes in length, but can

occasionally be longer as the standard allows for as

many as seven. The current version of the ELM320

will accept the maximum seven command bytes (or 14

hexadecimal digits) per message, while users of

previous versions (v1.x) were limited to only three

command bytes. In either case, attempts to send more

than the maximum number of bytes allowed will result

in a syntax error, with the entire command being

ignored and a single question mark 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 may initially find the table in

Figure 2 or a calculator to be invaluable. All users will

eventually 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 more 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

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.

Hexadecimal

Number

Decimal

Equivalent

0

1

2

3

4

5

6

7

8

9

A

B

C

D

E

F

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Figure 2. Hex to Decimal Conversion

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

8 of 16

ELM320

Talking to the Vehicle

The ELM320 cannot be directly connected to a

vehicle as it is, but needs support circuitry as shown in

the Example Applications section. Once incorporated

into such a circuit, you only need to use a terminal

program to send bytes to, and receive them from, the

vehicle.

Although this information is not very useful for the

casual user, it does serve to show that you are

communicating with the vehicle.

Another example requests the current engine

coolant temperature (ECT). This is PID 05 in mode 01,

and is requested as follows:

SAE standards specify that command bytes sent

to the vehicle must adhere to a set format. The first

byte (known as the ‘mode’) always describes the type

of data being requested, while the second, third, etc.

bytes specify the actual information required (given by

a ‘parameter identification’ or PID number). The

modes and PIDs are described in detail in the SAE

standard documents J1979 and J2190, and may also

be expanded on by the vehicle manufacturers.

Normally, one is only concerned with the nine

diagnostic test modes described in J1979 (although

there is provision for more). Note that it is not a

requirement for all of them to be supported. These are

the nine modes:

>01 05

The response will be of the form:

41 05 7B

This shows a mode 1 response (41) from PID 05,

with value 7B. Converting the hexidecimal 7B to

decimal, one gets 7 x 16 + 11 = 123. This represents

the current temperature in degrees Celsius, with the

zero value offset by 40 degrees to allow operation at

subzero temperatures. To convert to the actual coolant

temperature, simply subtract 40 from the value. In this

case, then, the ECT is 123 - 40 = 83 degrees Celsius.

A final example shows a request for the OBD

requirements to which this vehicle was designed. This

is PID 1C of mode 01, so at the prompt, type:

01 : show current data

02 : show freeze frame data

>01 1C

03 : show diagnostic trouble codes

04 : clear trouble codes and stored values

05 : test results, oxygen sensors

06 : test results, non-continuously monitored

07 : test results, continuously monitored

08 : special control mode

A typical response would be:

41 1C 01

The returned value (01) shows that this vehicle

conforms to OBDII (California ARB) standards. The

presently defined responses are :

09 : request vehicle information

01 : OBDII (California ARB)

02 : OBD (Federal EPA)

03 : OBD and OBDII

Within each mode, PID 00 is normally reserved to

show which PIDs are supported by that mode. Mode

01, PID 00 is required to be supported by all vehicles,

and can be accessed as follows…

04 : OBD I

05 : not intended to meet any OBD requirements

06 : EOBD (Europe)

Ensure that the ELM320 is properly connected to

your vehicle, and powered. Most vehicles will not

respond without the ignition key in the ON position, so

turn the ignition on, but do not start the vehicle. At the

prompt, issue the mode 01 PID 00 command:

Some modes may provide multi-line responses

(09, if supported, can display the vehicle’s serial

number). The ELM320 will attempt to display all

responses in these cases, but only if it is allowed

sufficient time to process each. There may be

occasions when the vehicle responds too quickly to

allow time for reprocessing, and lines could be lost.

Hopefully this has shown how typical requests

proceed. It has not been meant to be a definitive

source on modes and PIDs – this information can be

obtained from the SAE (http://www.sae.org/), from the

manufacturer of your vehicle, ISO (http://iso.org/), or

from various other sources on the web.

>01 00

A typical response could be as follows:

41 00 BE 1F B8 10

The 41 00 signifies a response (4) from a mode 1

request from PID 00 (a mode 2, PID 00 request is

answered with a 42 00, etc.). The next four bytes (BE,

1F, B8, and 10) represent the requested data, in this

case a bit pattern showing which of PIDs 1 through 32

are supported by this mode (1=supported, 0=not).

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

9 of 16

ELM320

Interpreting Trouble Codes

Likely the most common use that the ELM320 will

be put to is in obtaining the current Diagnostic Trouble

Codes or DTCs. Minimally, this requires that a mode

03 request be made, but first one should determine

how many trouble codes are presently stored. This is

done with a mode 01 PID 01 request as follows:

is only one trouble code here. The response has been

padded with 00’s as is required by the standard, and

the extra 0000’s do not represent actual trouble codes.

As was the case when requesting the number of

stored codes, the most significant bits of each trouble

code also contain additional information. It is easiest to

use the following table to interpret the first digit of

trouble codes as follows:

>01 01

To which a typical response might be:

41 01 81 07 65 04

If the first hex digit received is this,

Replace it with these two characters

The 41 01 signifies a response to our request, and

the first data byte (81) is the result that we are looking

for. Clearly there would not be 81(hex) or 129(decimal)

trouble codes if the vehicle is operational. In fact, this

byte does double duty, with the most significant bit

being used to indicate that the malfunction indicator

lamp (MIL, or ‘Check Engine’) has been turned on by

one of this module’s codes (if there are more than

one), while the other 7 bits provide the actual number

of stored codes. To determine the number of stored

codes, then, one needs to subtract 128 (or 80 hex)

from the number if it is greater than 128, and otherwise

simply read the number of stored codes directly.

The above response then indicates that there is

one stored code, and it was the one that set the MIL or

‘Check Engine’ lamp on. The remaining bytes in the

response provide information on the types of tests

supported by that particular module (see SAE

document J1979 for further information).

In this instance, there was only one line to the

response, but if there were codes stored in other

modules, they each could have provided a line of

response. To determine which module is reporting the

trouble code, one would have to turn the headers on

(ATH1) and then look at the third byte of the three byte

header for the address of the module that sent the

information.

P0

P1

P2

P3

C0

C1

C2

C3

B0

B1

B2

B3

U0

U1

U2

U3

Powertrain Codes - SAE defined

0

1

2

3

4

5

6

7

8

9

A

B

C

D

E

F

“

“ - manufacturer defined

“ - SAE defined

“ - jointly defined

Chassis Codes - SAE defined

“

“ - manufacturer defined

“ - reserved for future

Body Codes - SAE defined

“

“ - manufacturer defined

“ - reserved for future

Network Codes - SAE defined

“

“ - manufacturer defined

“ - reserved for future

Taking the example trouble code (0133), the first

digit (0) would then be replaced with P0, and the 0133

reported would become P0133 (which is the code for

an ‘oxygen sensor circuit slow response’). As for

further examples, if the response had been D016, the

code would be interpreted as U1016, while 1131 would

be P1131.

Had there been codes stored by more than one

module, or more than three codes stored in the same

module, the above response would have consisted of

multiple lines. To determine which module is reporting

each trouble would then require turning the headers on

with an ATH1 command.

Having determined the number of codes stored,

the next step is to request the actual trouble codes

with a mode 03 request:

>03

A response to this could be:

43 01 33 00 00 00 00

The ‘43’ in the above response simply indicates

that this is a response to a mode 03 request. The other

6 bytes in the response have to be read in pairs to

show the trouble codes (the above would be

interpreted as 0133, 0000, and 0000). Note that there

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

10 of 16

ELM320

Resetting Trouble Codes

The ELM320 is quite capable of resetting

diagnostic trouble codes, as this only requires issuing

a mode 04 command. The consequences should

always be considered before sending it, however, as

more than the MIL (or ‘Check Engine’ lamp) will be

reset. In fact, issuing a mode 04 will:

the SAE specifies that scan tools must verify that a

mode 04 is intended (“Are you sure?”) before actually

sending it to the vehicle, as all trouble code

information is immediately lost when the mode is sent.

Recall, though, that the ELM320 does not monitor the

content of messages, so it will not know to ask for

confirmation of the mode request - this would have to

be the duty of a software interface if one is written.

As stated, to actually erase diagnostic trouble

codes, one need only issue a mode 04 command. A

response of 44 from the vehicle indicates that the

mode request has been carried out, the information

erased, and the MIL turned off. Some vehicles may

require a special condition to occur (the ignition on but

the engine not running, etc.) before it will respond to a

mode 04 command.

- reset the number of trouble codes

- erase any diagnostic trouble codes

- erase any stored freeze frame data

- erase the DTC that initiated the freeze frame

- erase all oxygen sensor test data

- erase mode 06 and 07 test results

Clearing of all of this information is not unique to

the ELM320, as it occurs whenever a scan tool is used

to reset your codes. Understand that the loss of this

data could cause your car to run poorly for a short time

while the system recalibrates itself.

That is all there is to clearing the codes. Once

again, be very careful not to inadvertently issue an 04!

To avoid inadvertently erasing stored information,

Error Messages

When hardware or data problems are

encountered, the ELM320 will respond with one of the

following short messages. Here is a brief description of

each…

<DATA ERROR

The error check result (CRC byte) was not as

expected, indicating a data error in the line pointed

to (the ELM320 still shows you what it received).

There could have been a noise burst which

interfered, or a circuit problem. Try re-sending the

request.

BUS BUSY

The ELM320 tried to send the mode command or

request for about 0.5 seconds without success.

Messages are all assigned priorities, which allows

one message to take precedence over another.

More important things may have been going on, so

try re-issuing your request.

NO DATA

There was no response obtained from the vehicle

before a timeout occurred. The mode requested may

not be supported, so the vehicle ignored you, or the

timeout value was too short, or possibly the ignition

key needs to be turned to ‘on’. Try issuing a 01 00

command to be sure that the vehicle is ready to

receive commands, and if that works, try adjusting

the timeout to a longer value using the Set Timeout

AT command.

BUS ERROR

An attempt was made to send a message, and the

data bus voltage did not respond as expected. This

could be because of a circuit short or open, so check

all of your connections and try once more.

DATA ERROR

?

There was an incomplete message received, and it

was not enough to form a meaningful response. This

may have been caused by the key being turned off,

or a loose connection, for example. Any monitoring

that was in progress will have been aborted.

This is the standard response for a misunderstood

command received on the RS232 bus. Usually it is

due to a typing mistake.

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

11 of 16

ELM320

Monitoring the Bus

Some vehicles use the OBD bus for information

who is sending to whom, you will need to first turn

headers on (AT H1) before beginning to monitor (AT

MA). Either way, you may end up with an

overwhelming amount of information that you may

want to filter, showing only specific messages.

If, for example, you find that the engine controller’s

address seems to be 10, you may want to restrict the

data displayed to only messages from that ECU. To do

so, you would monitor only for messages transmitted

from address 10, by issuing AT MT 10 from your

terminal program. Only messages with 10 in the third

byte of the header will be displayed. Similarly, you may

wish to only see messages which are being sent to

address 3B. To monitor for these, send AT MR 3B and

only messages with 3B as the second header byte will

be shown.

There are a few limitations to the current

monitoring modes that you should be aware of. First,

there is no internal buffering of OBD messages as

data is being sent on the RS232 connection, so

information may be missed while the IC is busy. If

under computer control, you may want to consider the

‘packed data’ mode to reduce the chance of this. The

second limitation is that the data being printed only

extends up to the End Of Data symbol, and does not

include any In-Frame Response bytes that may be

present. However, for most users this will not be of

consequence.

transfer during normal vehicle operation, passing a

great deal of information over it. A lot can be learned if

you have the good fortune to connect to one of these

vehicles, and are able to decipher the contents of the

messages. By the same token, you can do a lot of

harm if you are careless, so be very careful.

To see how your vehicle uses the OBD bus, you

will have to enter one of the ELM320’s monitoring

modes. The simplest is the “Monitor All” mode which is

entered into by simply sending the command AT MA

from your terminal program. Once received, the IC will

continually display any information it sees on the OBD

bus, regardless of transmitter or receiver addresses.

Monitoring modes can only be stopped by sending

something over the RS232 connection to the ELM320.

It is not critical what you send - any single character

will interrupt the processor, and return it to the

command mode waiting for an input. Note that the

character you send is discarded and has no effect on

any subsequent commands. The IC will always finish a

task in progress (printing a line, for example) before

returning to wait for input, so always wait for the

prompt character (‘>’) before continuing to issue other

commands.

If the headers are not currently displayed, simply

typing ATMA shows only the contents of messages,

not the transmitter and receiver addresses. To show

Computer Control – Using Packed Data

If a person is simply asking a vehicle for the

current Diagnostic Trouble Codes, speed is normally

not an issue, as data is displayed (essentially) as

quickly as it can be read. If interfaced to a computer,

however, speed may be important.

when in this mode – if the headers are to be displayed,

they are sent, if in monitoring mode, data is continually

sent, etc. The only difference is in the format in which

the OBD responses are returned to the controlling

computer.

The packed data mode is a convenient means to

effectively triple the ELM320’s data transfer rate while

maintaining the connection at 9600 baud. Once

entered (with AT PD), all OBD messages will be

returned as a single length byte followed by the actual

data bytes. There are no space characters sent

between bytes, no carriage returns or linefeeds – the

data is retransmitted exactly as received from the

vehicle (except for the change to 9600 baud). While no

Often there is no response from the vehicle for a

particular request. When in the default (formatted data)

mode, this is shown with ‘NO DATA’ being printed, but

while in the packed data mode you will only receive a

single length byte of value 0 (zero).

While rare, errors may occasionally be detected in

the vehicle’s data. Normally, a ‘<DATA ERROR’ would

be printed for this, but in the Packed Data mode, the

checksum (CRC) errors are identified by setting the

most significant bit of the length byte. Because of this,

one should always check the length byte for a value of

128 or greater before processing the remainder of the

message.

longer readable on

a

terminal, computers will

understand the information just the same, and will gain

speed through both reduced transfer and conversion

times. The ELM320 does not function any differently

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

12 of 16

ELM320

Advanced Data Retrieval – Setting the Headers

Prior to v2.0, the ELM320 used a fixed format for

the message headers, allowing only for the retrieval of

the mandated diagnostic codes, not allowing the user

to change them. The IC is now fully programmable,

however, allowing the headers to be changed and a

great deal more information to be obtained, if your

vehicle supports it. Note that only the OBDII diagnostic

codes have been mandated, so there is no

requirement for all vehicles to support these extra

capabilities.

receive address selected stays in effect until changed

by another AT SR, or reinstatement of the automatic

mode.

Having set the headers, all one needs to do is

issue the secondary ID for fluid temperature (10) at the

prompt. If the display of headers is set to on, the

conversation would typically look like this:

>10

81 49 10 10 2E 41

The diagnostic trouble codes that most people are

familiar with are described by SAE standard J1979

(ISO15031-5). This is really a specific instance of the

many modes allowed by the J2178-4 standard, which

allows for information transfer through what is known

as ‘functional addressing’. For the OBDII mandated

diagnostics, requests are actually made to the

functional address 6A, with whatever processor is

responsible for this function answering the request.

Theoretically many different processors can respond

to a single functional request, each contributing their

insight as to the information requested.

To retrieve some of this extra information, the

function being addressed needs to be known. For

example, consider that you have studied the J2178

standards and want to request that the processor

responsible for Engine Coolant provide the current

Fluid Temperature. You determine that Engine Coolant

is functional address 48, you know that your address

as a scan tool is normally F1, and that since the

ELM320 only supports single IFR responses (type 1),

you choose A1 as the initial priority/type byte.

Ignoring the first three (header) bytes, and the final

check digit, one can see that the response to ID 10 is

the byte 2E. You may find that some requests, being

of a low priority, are not answered immediately,

possibly causing a NO DATA result. In these cases,

you may want to adjust the timeout value, perhaps first

trying the maximum (with AT ST FF).

Using the physical addressing modes described

by the J2190 standard involves an almost identical

process. The main difference is that you must know

the physical address of the device which you want to

speak to. This is always the third byte of a message

sent by any device, so can be determined by

monitoring the headers (for the above response, the

sender’s address is 10). Knowing that you wish to talk

to address 10, that your physical address is F1, and

that for type 1 IFR with physical addressing E4 may be

appropriate for the first byte, you would change the

header bytes using AT SH E4 10 F1. If Auto Receive

is enabled, the receive address will automatically be

set to F1, your physical address (the ELM320 knows

to do this from the first byte). As before, this header

will remain in effect for every message sent until

changed to something else.

One caution to note with physical addressing.

There are modes which initiate the constant sending of

data, and if the ELM320’s timeout is set longer than

the duration between responses, the ELM320 may

return messages forever. In these cases, just like in

the monitoring modes, a single character will have to

be sent to interrupt the process.

Finally, please note that while we have provided

some information on the SAE standards for the

examples, Elm Electronics will only reply to requests

for clarification on our product’s operation, and not on

the standards. It is the customer’s responsibility to

obtain their own information on the relevant standards,

and on their vehicle. Requests to Elm Electronics for

this information will go unanswered.

Combining the above then, it is desirable to set the

three header bytes to A1 48 F1. This is done with the

Set Header command, which would be issued at the

prompt as follows:

>AT SH A1 48 F1

The three header bytes assigned in this manner

will stay in effect until changed with another AT SH

command, a reset, etc. If the default Auto Receive

mode has been selected, the receive address will

automatically be set to 49 (the second byte plus one).

This is consistent with the functional pairs assigned by

J2178-4. If you decide that this is not appropriate for

your case, you can always set the receive address to

what you wish using the AT SR command. For

example, if you wanted to obtain a response that is

being sent to address E2 instead, you would use AT

SR E2 to override the automatic receive mode. Any

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

13 of 16

ELM320

Quick Guide for Reading Trouble Codes

If you don’t use your ELM320 for some time, this

data sheet may seem like quite a bit to review when

your ‘Check Engine’ light does eventually come on.

The following provides a quick procedure which may

prove helpful in that case:

Connect using HyperTerminal, ZTerm, etc.,

9600 8N1, and no handshaking

Key on, but vehicle not running

>ATZ

to be sure the IC is reset and responding

>0100

to be sure the car is responding

>0101

to see how many codes are present

Look at the second digit of the 3rd byte.

>03

to see the codes

Ignore the first byte and read the others in

pairs. The table on page 10 helps.

FIX THE VEHICLE!

>04

to reset the codes

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

14 of 16

< http://www.elmelectronics.com/ >

ELM320

Example Application

The SAE J1962 standard dictates that all OBD

compliant vehicles must provide a standard connector

near the driver’s seat, the shape and pinout of which is

shown in Figure 3 below. The circuitry described here

will be used to connect to this plug without modification

to your vehicle.

The male J1962 connector required to mate with a

vehicle’s connector may be difficult to obtain in some

locations, and you could be tempted to improvise by

making your own connections to the back of your

vehicle’s connector. If doing so, we recommend that

you do nothing which would compromise the integrity

of your vehicle’s OBD network. The use of any

connector which could easily short pins (such as an

RJ11 type telephone connector) would definitely not

be recommended.

The circuit of Figure 4 on the next page shows

how the ELM320 would typically be used. Circuit

power has been obtained from the vehicle (via OBD

pins 16 and 5) and, after some minor filtering, is

presented to a low power (100 mA) 5 volt regulator.

The output of this regulator powers several points in

the circuit as well as an LED (for visual confirmation

that power is present).

The remaining two connections to the vehicle

(OBD pins 2 and 10) are for the differential data

system specified by the J1850 PWM standard. When

no data is being transmitted, both wires are idle with

the transistor drivers off, and the resistive pullup and

pulldown allow voltage levels to float to the supply

levels. Note that the PNP driver transistor and the

2.7KW pullup resistor both have series protection

diodes to prevent backfeeds into the ELM320 circuitry.

The ELM320 has only one OBD data output line

(pin 7). It is an active low signal, so must be used to

drive the open-collector ‘Bus +’ signal via the PNP

transistor as shown. By using a portion of this same

signal to drive the NPN transistor for the ‘Bus -’ signal,

one obtains open collector differential drive.

negative supply. The RS232 pin connections shown

are for a 25 pin connector. If you are using a 9 pin, the

connections would be 2(RxD), 5(SG) and 3(TxD).

RS232 data from the computer is directly

connected to pin 5 of the IC through only a 47KW

current limiting resistor. This resistor allows for voltage

swings in excess of the supply levels while preventing

damage to the ELM320. A single 100KW resistor is

also shown in this circuit so that pin 5 is not left floating

if the computer is disconnected.

Transmission of RS232 data is via the single PNP

transistor connected to pin 6. This transistor allows the

output voltage to swing between +5V and the negative

voltage stored on the 0.1µF capacitor (which is

charged by the computer’s TxD line). Although it is a

simple connection, it is quite effective for this type of

application.

Finally, the crystal shown connected between pins

2 and 3 is a common TV type that can be easily and

inexpensively obtained. The 27pF crystal loading

capacitors shown are only typical, so you may have to

select other values depending on what is specified for

the crystal you obtain.

This completes the description of the circuit. While

it is the minimum required to talk to an OBD equipped

vehicle (it relies on such techniques as using the

internal current limiting of the 78L05 for circuit

protection, for example), it is a fully functional circuit.

As an experimenter, you may want to expand on it,

though, providing more protection from faults and

electrostatic discharge, or providing

a

different

interface for the RS232 connection to the computer.

Then perhaps a Basic program to make it easier to talk

to the vehicle, and a method to log your findings,

and…

1

9

8

Data is received from the OBD bus, then conerted

and level-shifted by the NPN/PNP transistor pair that

are shown connected to pin 4 of the ELM320. The

NPN transistor detects the differential data signal while

allowing for the presence of common mode voltages,

and the PNP transistor provides the 0 to 5 volt levels

required by OBDIn.

16

Figure 3. Vehicle Connector

A very basic RS232 interface is shown connected

to pins 5 and 6 of the ELM320. This circuit ‘steals’

power from the host computer in order to provide a full

swing of the RS232 voltages without the need for a

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

15 of 16

ELM320

OBD

Interface

Notes: -

NPN transistors are

2N3904 or similar

78L05

16

+5V

750W

(Battery

Positive)

‘Power On’

LED

-

PNP transistors are

2N3906 or similar

0.47µF

5

Diodes are 1N4148,

1N4001, etc.

(Signal

Ground)

+5V

4.7KW

2

(Bus +)

4.7KW

10KW

10

(Bus -)

+5V

0.01µF

2.7KW

+5V

1

2

3

4

8

7

6

5

27pF

3.58MHz

RS232

Interface

10KW

27pF

3 (RxD)

+5V

4.7KW

0.1µF

10KW

7 (SG)

47KW

4.7KW

2 (TxD)

4.7KW

100KW

Figure 4. Typical OBD to RS232 Interface

ELM320DSC

Elm Electronics – Circuits for the Hobbyist

16 of 16

< http://www.elmelectronics.com/ >


型号：	ELM320DSC
厂家：	ELM ELECTRONICS
描述：	OBD (PWM) TO RS232 INTERPRETER OBD （ PWM），以RS232翻译
文件：	总16页 (文件大小：100K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

ELM320DSC [ELM]

相关型号：

ELM320P

ELM320SM

ELM320_09

ELM322

ELM322DSE

ELM322P

ELM322SM

ELM322_09

ELM323

ELM323P

ELM323SM

ELM323_09