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ELM327

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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 must be assembled into one complete message.

One example of this is a request for the serial

number of the vehicle (mode 09, PID 02). This is often

a multiline reply that needs to be joined. In these

situations, you must take care to ensure that all of the

reply has been received and it is in the correct order

before assembling the message. The actual response

usually has a byte that shows the sequence of the

data, to help with this. Here is one example for a

typical SAE 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

Note that all OBD compliant vehicles do not

necessarily provide this information. Many older ones

do not, but as a rule the newer ones do. If your vehicle

does not support this parameter, you will only see a

“NO DATA” response.

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

above response do not show any vehicle information.

They only show that this is a response to an 09 02

request. The next byte on each line shows the order in

which the data is to be assembled. Assembling the

remainder of the data in that order, and ignoring the

first few 00’s gives:

31 44 34 47 50 30 30 52 35 35 42 31 32

33 34 35 36

Using an ASCII table to convert these hex digits

gives the following serial number 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

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, so that they clearly stand out (0:, 1:, etc.).

CAN systems add this hex digit (it goes from 0 to F

then repeats), to aid in reassembling the data, just as

the J1850 vehicle did.

The first line of this response says that there are

014 bytes of information to follow. That is 14 in

hexadecimal, or 20 in decimal terms, which agrees

with the 6 + 7 + 7 bytes shown on the three lines.

Serial numbers are generally 17 digits long however,

so how do we assemble the number from 20 digits?

The second line shown begins with the familiar 49

02, as this is a response to an 09 02 request. Clearly

they are not part of the serial number. CAN will

occasionally add a third byte to the response which we

see next (the ‘01’), which shows the number of data

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

VIN, so the response says there is only one data item).

That third byte can be ignored, so this 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.

The following shows an example of a different type

of multiline response that can occur when two or more

ECUs respond to one request. Here is a typical

response to an 01 00 request:

>01 00

41 00 BE 3E B8 11

41 00 80 10 80 00

This is difficult to decipher without knowing a little

more information. We need to turn the headers on to

actually see ‘who’ is doing the talking:

>AT H1

>01 00

48 6B 10 41 00 BE 3E B8 11 FA

48 6B 18 41 00 80 10 80 00 C0

Now, if you analyze the header, you can see that

the third byte shows ECU 10 (the engine controller)

and ECU 18 (the transmission) are both responding

with a reply that is valid for them. This type of

response occurs often, and you should be prepared for

it.

A final example shows how similar messages

might occasionally be ‘mixed up’ in a CAN system. We

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