ELM327P datasheet(33/51 Pages) ELM

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ELM327

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Multiline Responses (Cont’d)

ask the vehicle for the Calibration ID (09 04) and are

presented with the following response:

>09 04

013

0: 49 04 01 35 36 30

1: 32 38 39 34 39 41 43

013

0: 49 04 01 35 36 30

2: 00 00 00 00 00 00 31

1: 32 38 39 35 34 41 43

2: 00 00 00 00 00 00 00

which is quite confusing. The first group (the 013, 0:, 1:

group) seems to make some sense, but the following

group is very confusing. Why are there two segment

twos? Which ECU do these belong to? The only way

to tell is to turn on the headers, and repeat your

request:

>AT H1

>09 04

7E8 10 13 49 04 01 35 36 30

7E8 21 32 38 39 34 39 41 43

7E9 10 13 49 04 01 35 36 30

7E8 22 00 00 00 00 00 00 31

7E9 21 32 38 39 35 34 41 43

7E9 22 00 00 00 00 00 00 00

This time, the order appears to be the same, but

be aware that it may not be – that is why the standard

requires that sequence codes be transmitted with

multiline responses.

Looking at the first digits of these responses, you

can see that some begin with 7E8, and some begin

with 7E9.

This is the special CAN IDs representing

ECU#1

and

ECU#2,

respectively.

Grouping

the

responses from the two ECUs gives:

7E8 10 13 49 04 01 35 36 30

7E8 21 32 38 39 34 39 41 43

7E8 22 00 00 00 00 00 00 31

and

7E9 10 13 49 04 01 35 36 30

7E9 21 32 38 39 35 34 41 43

7E9 22 00 00 00 00 00 00 00

From these, the messages can be assembled in

their proper order. To do this, look at the byte following

the CAN ID - it is what is known as the PCI byte, and

is used to tell what type of data follows. In this case,

the PCI byte begins with either a 1 (for a ‘First Frame’

message), or a 2 (for the ‘Consecutive Frames’). The

second half of the PCI byte shows the order in which

the information is to be assembled (ie. the segment

number). In this case, the segment numbers are

already in order, but if they had not been, it would

have been necessary to rearrange the messages to

place them in order.

Each OBD standard has some minor peculiarities.

Hopefully this has helped you with some of the more

difficult ones. If you are still having trouble, we urge

you to purchase the relevant standard, and study it.

ELM327P Datasheet(PDF) 33 Page - ELM Electronics

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