ELM327
AT Command Descriptions (continued)
BRD hh
[ try Baud Rate Divisor hh ]
This command is used to change the RS232 baud
rate divisor to the hex value provided by hh, while
under computer control. It is not intended for casual
experimenting - if you wish to change the baud rate
from a terminal program, you should use PP 0C.
Since some interface circuits are not able to
operate at high data rates, the BRD command uses a
sequence of sends and receives to test the interface,
with any failure resulting in a fallback to the previous
baud rate. This allows several baud rates to be tested
and a reliable one chosen for the communications.
The entire process is described in detail in the ‘Using
If successful, the actual baud rate (in kbps) will be
4000 divided by the divisor (hh).
BRT hh
[ set Baud Rate Timeout to hh ]
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.
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
allows operation in systems that do not use PCI bytes
as ISO 15765-4 does.
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.
CEA
[ turn off the CAN Extended Address ]
This command allows the timeout used for the
Baud Rate handshake (ie. AT BRD) to be varied. The
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.
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 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.
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
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ELM327DSH
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