Manual Elm 327 (page 13 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

AT Command Descriptions (continued)

OBD buffer or not. This may be useful when viewing

long data streams (with AT AL), as it represents the

actual number of bytes received, mod 256. Note that

only the first twelve bytes received are stored in the

buffer.

BI [ Bypass the Initialization sequence ]

This command should be used with caution. It

allows an OBD protocol to be made active without

requiring any sort of initiation or handshaking to occur.

The initiation process is normally used to validate the

protocol, and without it, results may be difficult to

predict. It should not be used for routine OBD use, and

has only been provided to allow the construction of

ECU simulators and training demonstrators.

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 50 and

51.

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

4000 divided by the divisor (hh). The value 00 is not

accepted by the BRD command.

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

AT0, AT1 and AT2 [ Adaptive Timing control ]

When receiving responses from a vehicle, the

ELM327 has traditionally waited the time set by the

AT ST hh setting for a response. To ensure that the IC

would work with a wide variety of vehicles, the default

value was set to a conservative (slow) value. Although

it was adjustable, many people did not have the

equipment or experience to determine a better value.

The Adaptive Timing feature automatically sets the

timeout value for you, to a value that is based on the

actual response times that your vehicle is responding

in. As conditions such as bus loading, etc. change, the

algorithm learns from them, and makes appropriate

adjustments. Note that it always uses your AT ST hh

setting as the maximum setting, and will never choose

one which is longer.

There are three adaptive timing settings that are

available for use. By default, Adaptive Timing option 1

(AT1) is enabled, and is the recommended setting.

AT0 is used to disable Adaptive Timing (so the timeout

is always as set by AT ST), while AT2 is a more

aggressive version of AT1 (the effect is more

noticeable for very slow connections – you may not

see much difference with faster OBD systems). The

J1939 protocol does not support Adaptive Timing – it

uses fixed timeouts as set in the standard.

BD [ perform an OBD Buffer Dump ]

All messages sent and received by the ELM327

are stored temporarily in a set of twelve memory

storage locations called the OBD Buffer. Occasionally,

it may be of use to view the contents of this buffer,

perhaps to see why an initiation failed, to see the

header bytes in the last message, or just to learn more

of the structure of OBD messages. You can ask at any

time for the contents of this buffer to be ‘dumped’

(ie printed) – when you do, the ELM327 sends a length

byte (representing the length of the message in the

buffer) followed by the contents of all twelve OBD

buffer locations. For example, here’s one ‘dump’:

>AT BD

05 C1 33 F1 3E 23 C4 00 00 10 F8 00 00

The 05 is the length byte - it tells us that only the

first 5 bytes (ie C1 33 F1 3E and 23) are valid. The

remaining bytes are likely left over from a previous

operation.

The length byte always represents the actual

number of bytes received, whether they fit into the

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