Operation Manual

Communicating with the ELM327 (continued)
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 ELM327 is a protocol interpreter
that makes no attempt to assess the OBD messages
that you send 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 ELM327
will continually monitor for either an active RTS input,
or an RS232 character received. Either one will
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
ELM327, 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.
Finally, it should be noted that the ELM327 is not
case-sensitive, so the commands ‘ATZ’, ‘atz’, and
‘AtZ’ are all exactly the same to the ELM327. All
commands may be entered as you prefer, as no one
method is faster or better. The ELM327 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 ELM327 is the ability to
repeat any 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 resend the request to the vehicle - simply
send a carriage return character, and the ELM327 will
repeat the command for you. The memory buffer only
remembers one command though, and there is no
provision in the current ELM327 to provide storage for
any more.
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ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
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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 ELM327.
Microchip Technology has reported that some ICs which use the same EUSART as
in the ELM327 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 ELM327, then ignore incoming bytes that are of value 00 (ie.
remove NULLs).