ELM329 View Datasheet(PDF) - Elm Electronics

Part Name

Description

Manufacturer

ELM329

CAN Interpreter

Elm Electronics 

ELM329

AT Command Descriptions (continued)

the characters 'RTR' printed when one of these frames

has been received.

Turning the CAN Automatic Formatting off (CAF0),

will cause the ELM329 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 ELM329 will not perform any formatting for you

other than to add some trailing 'padding' bytes to

ensure that eight data bytes are sent, if required. This

allows operation in systems that do not use PCI bytes.

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 ELM329.

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

ELM329 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 data 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 this extended addressing in the ‘Using

CAN Extended Addresses’ section on page 55.

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.

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 ELM329, otherwise it will be

discarded. This version of the CAN Filter command is

used 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.

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

This command allows all four bytes (actually 29

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

significant bits will always be ignored, and may be

given any value. This command may be used to create

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).

CFC0 and CFC1

[ CAN Flow Control off or on ]

The ISO 15765-4 CAN protocol expects a ‘Flow

Control’ message to always be sent in response to a

‘First Frame’ message, and the ELM329 automatically

sends these without any intervention by the user. If

experimenting with a non-OBD system, it may be

desirable to turn this automatic response off, and the

AT CFC0 command has been provided for that

purpose. The default setting is CFC1 - Flow Controls

on.

Note that during monitoring (ie AT MA), there are

never any Flow Controls sent no matter what the CFC

option is set to.

CM hhh

[ set the CAN ID Mask to hhh ]

There can be a great many messages being

transmitted in a CAN system at any one time. In order

to limit what the ELM329 views, there needs to be a

system of filtering out the relevant ones from all the

others. This is accomplished by the filter, which works

in conjunction with the mask. A mask is a group of bits

that show the ELM329 which bits in the filter are

relevant, and which ones can be ignored. A ‘must

match’ condition is signalled by setting a mask bit to

'1', while a 'don't care' is signalled by setting a bit to '0'.

ELM329DSB

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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