ELM327P datasheet(8/51 Pages) ELM

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ELM327

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AT Commands (continued)

BRT hh

[ set Baud Rate Timeout to hh ]

This command allows the timeout used for the

Baud Rate handshake (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 75msec. Note that a value of 00 does not

result in 0 msec - it provides the maximum time of 256

x 5.0 msec.

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

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 these extra bytes that the CAN

diagnostics systems require. With formatting on, the

trailing (unused) data bytes that are received in a

frame will be removed as well, and only the relevant

ones will be shown. Beginning with v1.2 of the

ELM327, lines with invalid PCI bytes are now ignored,

rather than showing them as ‘<DATA ERROR’s.

Occasionally, long (multi-frame) responses are

returned by the vehicle. To help you analyze these, the

Auto Formatting mode will extract the total data length

and print it on one line. Following this will be each

segment of the message, with the segment number (a

single hexadecimal digit) shown at the beginning of the

line with a colon (':') as a separator.

You may also see the characters 'FC: ' at the

beginning of a line (if you are experimenting). This

represents a Flow Control message that is sent in

response to a multi-line message. 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 whether you have

selected the CAF1 or the CAF0 modes).

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.

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

discarded. This three nibble version of the CAN Filter

command makes it a little easier to set filters with 11

bit ID CAN systems. Only the rightmost 11 bits of the

provided nibbles are used, and the most significant bit

is ignored. The data is actually stored as four bytes

internally however, with this command adding leading

zeros for the other bytes. See the CM command(s) for

more details.

CF hh hh hh hh [ set the CAN ID Filter to hhhhhhhh ]

This command allows all four bytes (actually 29

bits) of the CAN Filter to be set at once. The 3 most

significant bits will always be ignored, and can be

given any value. Note that this command can be used

to enter 11 bit ID filters as well, since they are stored in

the same locations internally (entering AT CF 00 00 0h

hh is exactly the same as entering the shorter AT CF

hhh command).

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