ELM327P datasheet(19/51 Pages) ELM

19 / 51 page

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

single 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 automatically. The initial (default) values for

these extra bytes are usually appropriate for most

requests, but if you wish to change them, there is a

method 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 (seven bytes or 14

hexadecimal digits). Attempts to send more bytes, or

an odd number of hex digits, will result in a syntax

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 haxadecimal 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.

All users will be required to

manipulate the results in some way, though –

combining bytes and dividing by 4 to obtain rpm,

dividing by 2 to obtain degrees of advance, etc., and

may find a software front-end to be of help.

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 on to the vehicle.

After sending the command, the ELM327 listens

on the OBD bus for messages, looking for ones that

are directed to it. If a message address matches,

those 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 are received, the ELM327 will continue to

reset this timer. Note that the IC will always respond

with something, even if it is to say “NO DATA”

(meaning that there were no messages found that

were addressed to it).

19 of 51

ELM327

ELM327DSC

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Figure 1. Hex to Decimal Conversion

Hexadecimal

Number

Decimal

Equivalent

ELM327P Datasheet(PDF) 19 Page - ELM Electronics

ELM327P Datasheet(HTML) 19 Page - ELM Electronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

Datasheet Download

Link URL