ELM327
AT Command Descriptions (continued)
time delay is given by hh x 5.0 msec, where hh is a
hexadecimal value. The default value for this setting is
0F, providing 75 msec. Note that a value of 00 does
not result in 0 msec - it provides the maximum time of
256 x 5.0 msec, or 1.28 seconds.
allows the ELM327 to be used with protocols that have
special formatting requirements.
Note that turning the display of headers on (with
AT H1) will override some of the CAF1 formatting of
the received data frames, so that the received bytes
will appear much like in the CAF0 mode (ie. as
received). It is only the printing of the received data
that will be affected when both CAF1 and H1 modes
are enabled, though; when sending data, the PCI byte
will still be created for you and padding bytes will still
be added. Auto Formatting on (CAF1) is the default
setting for the ELM327.
CAF0 and CAF1
[ CAN Auto Formatting off or on ]
These commands determine whether the ELM327
assists you with the formatting of the CAN data that is
sent and received. With CAN Automatic Formatting
enabled (CAF1), the IC will automatically generate the
formatting (PCI) bytes for you when sending, and will
remove them when receiving. This means that you can
continue to issue OBD requests (01 00, etc.) as usual,
without regard to the extra bytes that CAN diagnostics
systems require. Also, with formatting on, any extra
(unused) data bytes that are received in the frame will
be removed, and any messages with invalid PCI bytes
will be ignored. (When monitoring, however, messages
with invalid PCI bytes are shown, with a ‘<DATA
ERROR’ message beside them).
Multi-frame responses may be returned by the
vehicle with ISO 15765 and SAE J1939. To make
these more readable, the Auto Formatting mode will
extract the total data length and print it on one line,
then show each line of data with the segment number
followed by a colon (‘:’), and then the data bytes.
You may also see the characters 'FC:' on a line (if
you are experimenting). This identifies a Flow Control
message that has been sent as part of the multi-line
message signalling. Flow Control messages are
automatically generated by the ELM327 in response to
a ‘First Frame’ reply, as long as the CFC setting is on
(it does not matter if auto formatting is on or not).
Another type of message – the RTR (or ‘Remote
Transfer Request’) – will be automatically hidden for
you when in the CAF1 mode, since they contain no
data. When auto formatting is off (CAF0), you will see
the characters 'RTR' printed when a remote transfer
request frame has been received.
CEA
[ turn off the CAN Extended Address ]
The CEA command is used to turn off the special
features that are set with the CEA hh command.
CEA hh
[ set the CAN Extended Address to hh ]
Some CAN protocols extend the addressing fields
by using the first of the eight data bytes as a target or
receiver’s address. This type of formatting does not
comply with any OBD standard, but by adding it, we
allow for some experimentation.
Sending the CEA hh command causes the
ELM327 to insert the hh value as the first data byte of
all CAN messages that you send. It also adds one
more filtering step to received messages, only passing
ones that have the Tester Address in the first byte
position (in addition to requiring that ID bits match the
patterns set by AT CF and CM, or CRA). The AT CEA
hh command can be sent at any time, and changes
are effective immediately, allowing for changes of the
address ‘on-the-fly’. There is
a
more lengthy
discussion of extended addressing in the ‘Using CAN
Extended Addresses’ section on page 49.
The CEA mode of operation is off by default, and
once on, can be turned off at any time by sending AT
CEA, with no address. Note that the CEA setting has
no effect when J1939 formatting is on.
Turning the CAN Automatic Formatting off (CAF0),
will cause the ELM327 to print all of the received data
bytes. No bytes will be hidden from you, and none will
be inserted for you. Similarly, when sending a data
request with formatting off, you must provide all of the
required data bytes exactly as they are to be sent –
the ELM327 will not perform any formatting for you
other than to add some trailing 'padding' bytes to
ensure that the required eight data bytes are sent. This
CF hhh
[ set the CAN ID Filter to hhh ]
The CAN Filter works in conjunction with the CAN
Mask to determine what information is to be accepted
by the receiver. As each message is received, the
incoming CAN ID bits are compared to the CAN Filter
bits (when the mask bit is a ‘1’). If all of the relevant
bits match, the message will be accepted, and
processed by the ELM327, otherwise it will be
ELM327DSI
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