AT Commands

The ELM633 is controlled with short commands

that all begin with the two characters ‘AT’ (which is

short for ATtention). These two characters serve no

purpose other than to help add validity to the

characters that follow. Modem manufacturers have

used this same technique for years, and it has become

customary to refer to commands that begin with these

characters as ‘AT Commands’.

The

ELM633

accepts

several

different

AT

commands, but only one at a time (it cannot process

multiple commands on one line, as most modems

can). Each is executed only upon receipt of the

terminating

carriage

return

character.

Several

commands do not have a visible response (AT D for

example), so completion of those commands will be

4 of 11

ELM633

ELM633DSB

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Communicating with the ELM633

The ELM633 relies on a standard RS232 serial

connection to communicate with the user. The data

rate is not adjustable and is set at 57600 baud, with 8

data bits, no parity bit, 1 stop bit, and no handshaking

(often referred to as 57600 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 this

connection (and the linefeed mode that you have

chosen). No special software is required to ‘talk’ to the

IC – a standard terminal program is all that is needed.

Once

it

has

been

properly

connected

and

powered, the ELM633 will send the message:

ELM633 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. The ‘>’ character displayed above is the

ELM633’s prompt character, which shows that the

device is in its idle state, ready to receive characters

on the RS232 port.

All messages that are sent to the ELM633 must

begin with the character ‘A’ followed by the character

‘T’, and must be terminated with a carriage return

character. No action is taken – commands are not

checked for errors, nor are they acted upon – until this

terminating carriage return is received. The one

exception is when a command is interrupted for some

reason, and no carriage return appears. In this case,

an internal timer will automatically abort the incomplete

message after about 15 seconds, and the ELM633 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 ELM633

(syntax errors) will always be signalled by this same

single question mark (‘?’). When this occurs, it is

usually due to a spelling mistake, so you often only

need to repeat the input, typing more carefully.

Occasionally, errors occur if the ELM633 is busy

processing LIN messages when an RS232 command

message begins. In these cases, the first character of

the RS232 command message will always be missed

by the IC, so the remaining characters will appear to

be

incorrect.

You

should

always

interrupt

the

monitoring process with a single character (it doesn’t

matter which one, as it will be ignored), then wait for

the prompt character (“>”) to appear before sending

any more. This ensures that the ELM633 is ready to

receive commands.

For convenience, the ELM633 has been designed

to ignore spaces and control characters in the input, so

if you prefer to add spaces, tabs, etc., to improve

readability, then go ahead. Also, the ELM633 is not

case-sensitive, so ‘ATZ’ is equivalent to ‘atz’ and to

‘AtZ’, which may be helpful for some people.

acknowledged by the printing of the characters ‘OK’.

Monitoring of the LIN bus can generally begin

without requiring the use of any AT commands, as the

factory default settings should be appropriate for most

situations. Occasionally, however, the user may wish

to customize settings, such as turning the character

echo off, and in these cases, AT commands must be

used.

To perform the desired AT command, simply send

the

characters

AT

followed

by

the

appropriate

characters from the following list. For example, to turn

character echoing off, simply send AT E0 followed by

a return character. To turn it back on, send AT E1.

The following is a summary of the commands that

are recognized by the current version of the ELM633.