Bedienungsanleitung Elm 327 (Seite 40 von 94) (Englisch)

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Setting the Headers (continued)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

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causing a ‘NO DATA’ result. In these cases, you may

want to adjust the timeout value, perhaps first trying

the maximum (ie use AT ST FF). Many vehicles will

simply not support these extra addressing modes.

The other, and more common method of obtaining

information is by physical addressing, in which you

direct your request to a specific device, not to a

functional group. To do this, you again need to

construct a set of header bytes that direct your query

to the physical address of the processor, or ECU. If

you do not know the address, recall that the sender of

information is usually shown in the third byte of the

header. By monitoring your system for a time with the

headers turned on (AT H1), you can quickly learn the

main addresses of the senders. The SAE document

J2178 assigns address ranges to these devices if you

are unsure of which might be most appropriate.

When you know the address that you wish to

‘speak to,’ simply use it for the second byte in the

header (assume an address of 10 for this example).

Combine that with your knowledge of SAE J2178 to

choose a priority/type byte (assume a value of E4 for

this example, as if the vehicle is J1850 PWM). Finally,

you need to identify yourself to the target, so that

responses can be returned to you. As is customary for

diagnostic tools, we’ll use an address of F1. As before,

these three bytes are then assigned to the header with

the set header command:

>AT SH E4 10 F1

From this point on, all messages that the ELM327

sends will use these three bytes for the header. All that

needs to be done now is to request data from the

vehicle. For physical addressing, this is often done

using mode 22:

>22 11 6B

62 11 6B 00 00

The response to this command is of the same

format to those seen for ‘standard’ OBD requests. The

request has been repeated (with 40 added to the

mode value in order to show that this is a response),

and this is followed by the actual data (00 00 in this

case). The PIDs used with mode 22 are usually

proprietary to each manufacturer and are generally not

published widely, so you may have difficulty in

determining the ones to use with your vehicle. Elm

Electronics does not maintain lists of this information,

and cannot provide any further details for you. Mode

22 and others are described in more detail in the SAE

standards document J2190, ‘Enhanced E/E Diagnostic

Test Modes’.

The ISO14230-4 standard defines its header bytes

a little differently. Advanced experimenters will be

aware that for ISO 14230-4, the first header byte must

always include the length of the data field, which

varies from message to message. From that, one

might assume that the you would need to redefine the

header for every message that is to be sent – not so!

The ELM327 always determines the number of bytes

that you are sending, and inserts that length for you, in

the proper place for the header that you are using. If

you are using the standard ISO 14230-4 header, the

length will be put into the first header byte, and you

need only provide the two (most significant) bits of this

byte when defining the header. What you place in the

rest of the byte will be ignored by the ELM327 unless

you set it to 0. If it is 0, it is assumed that you are

experimenting with KWP four byte headers, and the

ELM327 then creates the fourth header byte for you.

Again, you do not need to provide any length to be put

into this byte – it is done for you.

Addressing within the CAN (ISO 15765-4)

protocols is quite similar in many ways. First, consider

the 29 bit standard. The ELM327 splits the 29 bits into

a CAN Priority byte and the three header bytes that we

are now familiar with. This is how they are combined

for use by the ELM327:

>AT SH xx yy zz>AT CP vv

vv xx yy zz

5 bits only

xx yy zzvv

29 bit ID

The CAN standard states that for diagnostics, the

priority byte (‘vv’ in the diagram) will always be 18 (it is

the default value used by the ELM327). Since it is

rarely changed, it can be assigned separately from the

other header bytes, using the CP command.

The next byte (‘xx’) describes the type of message

that this is, and is set to hex DB for functional

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