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Altering Flow Control Messages

ELM327

ISO 15765-4 (CAN) provides for only eight data

bytes per frame of data. Of course, there are many

cases where the data which needs to be sent is longer

that 8 bytes and CAN has made provision for this,

allowing data to be separated into segments, then

recombined at the receiver.

To send one of these multi-line messages, the

transmitter in a CAN system will send a ‘First Frame’

message, and then wait for a reply from the receiver.

This reply, called a ‘Flow Control’ message contains

information concerning acceptable message timing,

etc., and is required to be sent before the transmitter

will send any more data. For ISO 15765-4, the type of

response is well defined, and never changes. The

ELM327 will automatically send this ISO 15765-4 Flow

Control response for you as long as the CAN Flow

Control option is enabled (CFC1), which it is by

default.

Several users have requested that we provide

more flexibility over the data sent in the Flow Control

message, and as of v1.1, we have provided a means

to do this. In order to change how the ELM327

responds when it needs to send a Flow Control

message, you need to change Flow Control ‘modes’.

The default Flow Control mode is number ‘0’. At

any time while you are experimenting, if you should

wish to restore the automatic Flow Control responses

(for ISO 15765-4), simply set the mode to 0:

>AT FC SM 0

This will immediately restore the responses to their

default settings.

Mode 1 has been provided for those that need

complete control over their Flow Control messages. To

use it, simply define the CAN ID (header) and data

bytes that you require be sent in response to a First

Frame message. If you try to set the mode before

defining these values, you will get an error:

>AT FC SM 1

You must set the headers and data first:

>AT FC SH 7E8

>AT FC SD 00 11 22

then you can set the mode:

>AT FC SM 1

From this point on, every First Frame message

received will be responded to with your custom

message.

The final mode currently supported allows the user

to set the data bytes which are to be sent, but not the

ID bits. The ID bits (header bytes) in mode 2 are set to

those which were received in the First Frame

message, without change. To use this mode, first

define your data bytes, then activate the mode:

>AT FC SD 00 11 22

>AT FC SM 2

For most people, there will be little need to

manipulate these ‘Flow Control’ messages, as the

defaults are designed to work with the CAN OBD

standards. If you wish to experiment, these special AT

commands offer that control for you.

Figure 7 summarizes the currently supported

modes:

Figure 7. Flow Control Mode Numbers

ELM327

Provides

Mode

ID Bits

Data Bytes

User

Provides

ID Bits

Data Bytes

no values

ID Bits

Data Bytes

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