ELM327
OBD Commands
If the bytes that you send to the ELM327 do not
begin with the letters ‘A’ and ‘T’, they are assumed to
be OBD commands for the vehicle. Each pair of ASCII
bytes will be tested to ensure that they are valid
hexadecimal digits, and will then be combined into
data bytes for transmitting to the vehicle.
OBD commands are actually sent to the vehicle
embedded in a data packet. Most standards require
that three header bytes and an error checksum byte
be included with every OBD message, and the
ELM327 adds these extra bytes to your command
bytes for you. The initial (default) values for these
extra bytes are usually appropriate for most requests,
but if you wish to change them, there is a mechanism
to do so (see the ‘Setting the Headers’ section).
Most OBD commands are only one or two bytes in
length, but some can be longer. The ELM327 will limit
the number of bytes that can be sent to the maximum
number allowed by the standards (usually seven bytes
or 14 hexadecimal digits). Attempts to send more
bytes will result in an error – the entire command is
then ignored and a single question mark printed.
Hexadecimal digits are used for all of the data
exchange with the ELM327 because it is the data
format used most often in the OBD standards. Most
mode request listings use hexadecimal notation, and it
is the format most frequently used when results are
shown. With a little practice, it should not be very
difficult to deal in hex numbers, but some people may
want to use a table such as Figure 1, or keep a
calculator nearby. Dealing with the hex digits can not
be avoided - eventually all users need to manipulate
the results in some way (combining bytes and dividing
by 4 to obtain rpm, dividing by 2 to obtain degrees of
advance, converting temperatures, etc.).
As an example of sending a command to the
vehicle, assume that A6 (or decimal 166) is the
command that is required to be sent. In this case, the
user would type the letter A, then the number 6, then
would press the return key. These three characters
would be sent to the ELM327 by way of the RS232
port. The ELM327 would store the characters as they
are received, and when the third character (the
carriage return) was received, would begin to assess
the other two. It would see that they are both valid hex
digits, and would convert them to a one byte value (the
decimal value is 166). The header bytes and a
checksum byte would then be added, and a total of
five bytes would typically be sent to the vehicle. Note
that the carriage return character is only a signal to the
ELM327DSH
ELM327, and is not sent to the vehicle.
After sending the command, the ELM327 listens
on the OBD bus for replies, looking for ones that are
directed to it. If a message address matches, the
received bytes will be sent on the RS232 port to the
user, while messages received that do not have
matching addresses will be ignored (but are often still
available for viewing with the AT BD command).
The ELM327 will continue to wait for messages
addressed to it until there are none found in the time
that was set by the AT ST command. As long as
messages continue to be received, the ELM327 will
continue to reset this timer, and look for more. Note
that the IC will always respond to a request with some
reply, even if it is to say ‘NO DATA’ (meaning that
there were no messages found, or that some were
found but they did not match the receive criteria).
Hexadecimal
Number
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
Decimal
Equivalent
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Figure 1. Hex to Decimal Conversion
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