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Altering Flow Control Messages

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

than 8 bytes, and CAN has made provision for this by

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 response for you

as long as the CAN Flow Control option is enabled

(CFC1), which it is by default.

In order to provide complete control over the

sending of Flow Control messages and their content,

the ELM327 defines several AT FC commands.

The way in which the ELM327 responds to a First

Frame message is determined by the Flow Control

‘mode’, as set by the AT FC SM command. There are

currently three modes, as shown in the chart. The

default Flow Control mode is number ‘0’, meaning that

the ELM327 will do everything for you.

Flow Control mode 1 is provided for those users

that want complete control over their Flow Control

messages. To use it, you must define both the CAN ID

(header) and the data bytes that you require to be sent

in the response to a First Frame message. Note that 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

and then you can set the mode:

>AT FC SM 1

From this point on, every First Frame message

received will be responded to with the custom

message that you have defined (7E8 00 11 22 in this

example). Note that we show 11 bits for the ID in this

example, but you may also define 29 bit IDs.

The third mode currently supported allows setting

the data bytes which are to be sent, but not the ID bits.

The ELM327 sets the ID bits in mode 2 from those

which were received in the First Frame message – it

does not change them at all. To use this mode, first

define your data bytes, then activate the mode:

>AT FC SD 00 11 22

>AT FC SM 2

At any time while you are experimenting, if you

should wish to restore the automatic Flow Control

responses, simply set the mode to 0:

>AT FC SM 0

This will immediately restore the responses to their

(ISO 15765-4) default settings.

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.

The following chart summarizes the currently

supported flow control modes:

ELM327

Provides

Mode

ID Bits &

Data Bytes

User

Provides

ID Bits

Data Bytes

no values

ID Bits &

Data Bytes

Flow Control Modes

Note that the ELM327 will only send Flow Control

messages if the protocol’s data format is ISO15765-4.

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ELM327L DSA

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