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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
OK
>AT FC SD 00 11 22
OK
and then you can set the mode:
>AT FC SM 1
OK
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
OK
>AT FC SM 2
OK
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
OK
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
FC
Mode
0
1
2
ID Bits &
Data Bytes
User
Provides
ID Bits Data Bytes
no values
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.
60 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
ELM327
60


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