ELM329LP datasheet(28/87 Pages) ELM

28 / 87 page

OBD Commands

If the bytes that you send to the ELM329 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.

Commands to the vehicle are actually sent

embedded in a data packet. The packet consists of

header bytes (ie CAN ID bits), as well as checksum

and other bits as defined by the ISO standards. The

ELM329 adds these extra bits and bytes to your

message as required - you do not normally have to

even consider them. If you do want to change the ID

bits or data lengths at some point, there is a

mechanism to do so (see the ‘Setting the Header / ID

Bits’ section).

Most OBD commands are only one or two bytes in

length, but some can be longer. The current version of

the ELM329 will accept up to eight data bytes to be

sent (there is no way to send any more bytes with this

version). Attempts to send more than eight 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 ELM329 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 ELM329 by way of the RS232

port. The ELM329 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/ID bytes would then

be added, and the complete message would then be

sent to the vehicle. Note that the carriage return

character is only a signal to the ELM329, and is never

sent to the vehicle.

After sending the command, the ELM329 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 converted to ascii characters

and sent on the RS232 port to the user, while

messages

received

that

not

have

matching

addresses will be ignored.

The ELM329 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 ELM329 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 87

ELM329L

ELM329L DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Figure 1. Hex to Decimal Conversion

Hexadecimal

Number

Decimal

Equivalent

ELM329LP Datasheet(PDF) 28 Page - ELM Electronics

ELM329LP Datasheet(HTML) 28 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 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87

Datasheet Download

Link URL