ELM327P datasheet(28/76 Pages) ELM

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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

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).

28 of 76

ELM327

ELM327DSH

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Figure 1. Hex to Decimal Conversion

Hexadecimal

Number

Decimal

Equivalent

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