ELM634
Communicating with the ELM634
The ELM634 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 ELM634 will send the message:
ELM634 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
ELM634’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 ELM634 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 ELM634 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 ELM634
(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 ELM634 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 ELM634 is ready to
receive commands.
For convenience, the ELM634 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 ELM634 is not
case-sensitive, so ‘ATZ’ is equivalent to ‘atz’ and to
‘AtZ’, which may be helpful for some people.
AT Commands
The ELM634 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 ELM634 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
ELM634DSB
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 ELM634.
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Elm Electronics – Circuits for the Hobbyist
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