ELM327P datasheet(42/76 Pages) ELM

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ELM327

ELM327DSH

Elm Electronics – Circuits for the Hobbyist

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Multiline Responses

There are occasions when a vehicle must respond

with more information than one ‘message’ is able to

show. In these cases, it responds with several lines

which the receiver must assemble into one complete

message.

One example of this is a request for the 17 digit

vehicle identification number, or VIN. This is available

from newer vehicles using a mode 09, PID 02 request

(but was not initially a requirement, so many older

vehicles do not support it). Here is one example of a

response that might be obtained from a J1850 vehicle:

>0902

49 02 01 00 00 00 31

49 02 02 44 34 47 50

49 02 03 30 30 52 35

49 02 04 35 42 31 32

49 02 05 33 34 35 36

The first two bytes (49 and 02) on each line of the

response are used to show that the information is in

reply to an 09 02 request. The next byte shows which

response it is, while the remaining four bytes are the

data bytes that are being sent. Assembling the data in

the order specified by the third byte, and ignoring the

first few 00’s (they are filler bytes - see J1979) gives:

31 44 34 47 50 30 30 52 35 35 42 31

32 33 34 35 36

The data values actually represent the ASCII

codes for the digits of the VIN. Using an ASCII table to

convert these into characters gives the following VIN

for the vehicle:

1 D 4 G P 0 0 R 5 5 B 1 2 3 4 5 6

CAN systems will display this information in a

somewhat different fashion. Here is a typical response

from a CAN vehicle:

>0902

014

0: 49 02 01 31 44 34

1: 47 50 30 30 52 35 35

2: 42 31 32 33 34 35 36

The CAN Formatting has been left on (the default),

making the reading of the data easier. With formatting

on, the sequence numbers are shown with a colon (‘:’)

after each. CAN systems add this single hex digit (it

goes from 0 to F then repeats), to aid in reassembling

the data, instead of the byte value that the J1850

vehicle did.

The first line of this response says that there are

014 bytes of information in total. That is 14 in hex, or

20 in decimal terms, which agrees with the 6 + 7 + 7

bytes shown on the three lines. The VIN numbers are

generally 17 digits long, however, so how do we

assemble the number from 20 digits?

This is done by discarding the first three bytes of

the message. The first two are the familiar 49 02, as

this is a response to an 09 02 request, so are not part

of the VIN. The third byte (the ‘01’), tells the number of

data items that are to follow (the vehicle can only have

one VIN, and this agrees with that). Ignoring the third

byte leaves 17 data bytes which are the serial number

(purposely chosen to be identical to the those of the

previous example). All that is needed is a conversion

to ASCII, in order to read them, exactly as before.

From these two examples, you can see that the

format of the data received may depend on the

CAN Messages and Filtering (continued)

12th bit, so you do not need to take special care with

it.

With a little practice, these commands are fairly

easy to master. Initially, try entering the filter and mask

values, then use a command such as AT MA to see

what the results are. The ELM327 knows that you are

trying to filter, and combines the effects of both

commands (it will do that for MR and MT as well). The

MA, MR and MT commands all have the extra benefit

that while they are in effect, the ELM327 will remain

quiet, not sending acknowledgement or error signals,

so anything you do while monitoring should not disrupt

other devices that are on the bus.

Note that if a filter has been set, it will be used for

all CAN messages, so setting filters and masks may

cause standard OBD requests to be ignored, and you

may begin seeing ‘NO DATA’ replies. If this happens,

and you are unsure of why, you may want to reset

everything to the default values (with AT D or possibly

AT WS) and start over.

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