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ELM322P

OBD (VPW) to RS232 Interpreter

Elm Electronics 

ELM322

Overview

The following describes how to use the ELM322 to

obtain a great deal of information from your vehicle. To

some, the quantity of information will be overwhelming,

and for others it will not be 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

features of this product as well.

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 ELM322

The ELM322 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.

Properly connected and powered, the ELM322 will

initially display the message:

ELM322 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 the

OBD connection is still unknown.

The ‘>’ character displayed above is the ELM322’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 ELM322’s internal use, or for

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

Commands for the ELM322 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 can contain only the

ASCII characters for hexadecimal digits (0 to 9 and A

to F). This allows the ELM322 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 ELM322

must be terminated with a carriage return character

(hex ‘0D’) before they will be acted upon. If an

incomplete string is sent and no carriage return

appears, an internal timer will automatically abort the

incomplete message after about 10 seconds. Should

this happen, the ELM322 will print a single question

mark to show that the input was not understood (and

was ignored).

Messages that are misunderstood by the ELM322

(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 ELM322 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 ELM322 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

ELM322 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).

ELM322DSF

Elm Electronics – Circuits for the Hobbyist

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

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