ELM329
Communicating with the ELM329 (continued)
(syntax errors) will always be signalled by a single
question mark. These include incomplete messages,
incorrect AT commands, or invalid hexadecimal digit
strings, but are not an indication of whether or not the
message was understood by the vehicle. One must
keep in mind that the ELM329 is a protocol interpreter
that makes no attempt to assess the OBD messages
for validity – it only ensures that hexadecimal digits
were received, combined into bytes, then sent out the
OBD port, and it does not know if a message sent to
the vehicle was in error.
While processing OBD commands, the ELM329
will continually monitor for either an active RTS input
(if enabled), or an RS232 character received. Either
one can interrupt the IC, quickly returning control to
the user, while possibly aborting any initiation, etc. that
was in progress. After generating a signal to interrupt
the ELM329, software should always wait for either the
prompt character (‘>’ or hex 3E), or a low level on the
Busy output before beginning to send the next
command.
‘AtZ’ are all exactly the same to the ELM329. All
commands may be entered as you prefer, as no one
method is faster or better. The ELM329 also ignores
space characters and all control characters (tab, etc.),
so they can be inserted anywhere in the input if that
improves readability.
One other feature of the ELM329 is the ability to
repeat the last command (AT or OBD) when only a
single carriage return character is received. If you
have sent a command (for example, 01 0C to obtain
the rpm), you do not have to resend the entire
command in order to obtain an update from the vehicle
- simply send a carriage return character, and the
ELM329 will repeat the command for you. The
memory buffer only remembers the previous command
- there is no provision in the current ELM329 to
provide storage for any more.
Finally, it should be noted that the ELM329 is not
case-sensitive, so the commands ‘ATZ’, ‘atz’, and
Please Note:
There is a very small chance that NULL characters (byte value 00) may occasionally
be inserted into the RS232 data that is transmitted by the ELM329.
Microchip Technology has reported that some ICs which use the same EUSART as
in the ELM329 may, under very specific (and rare) conditions, insert an extra byte
(always of value 00) into the transmitted data. If you are using a terminal program to view
the data, you should select the ‘hide control characters’ option if it is available, and if you
are writing software for the ELM329, then monitor incoming bytes, and ignore any that
are of value 00 (ie. remove NULLs).
ELM329DSB
Elm Electronics – Circuits for the Hobbyist
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