ELM327P datasheet(12/76 Pages) ELM

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ELM327

ELM327DSH

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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

Higher RS232 Baud Rates’ section, on pages 59 and

60.

If successful, the actual baud rate (in kbps) will be

4000 divided by the divisor (hh).

BRT hh

[ set Baud Rate Timeout to hh ]

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.

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 ]

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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