ELM329LP datasheet(16/87 Pages) ELM

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ELM329L

ELM329L DSA

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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'.

This three digit variation of the CM command is used

to provide mask values for 11 bit ID systems (the most

significant bit is always ignored).

Note that a common storage location is used

internally for the 29 bit and 11 bit masks, so an 11 bit

mask could conceivably be assigned with the next

command (CM hh hh hh hh), should you wish to do the

extra typing.

CM hh hh hh hh [ set the CAN ID Mask to hhhhhhhh ]

This command is used to assign mask values for

29 bit ID systems. See the discussion under the

CM hhh command as it is essentially identical, except

for the length. Note that the three most significant bits

that you provide in the first digit will be ignored.

CP hh

[ set CAN Priority bits to hh ]

This command is used to modify the five most

significant bits of a 29 bit CAN ID for sending

messages (the other 24 bits are set with one of the AT

SH commands). Many systems use these bits to

assign a priority value to messages, and to determine

the protocol. Any bits provided in excess of the five

required are ignored, and not stored by the ELM329 (it

only uses the five least significant bits of this byte).

The default value for these priority bits is hex 18,

which can be restored at any time with the AT D

command.

CRA

[reset the CAN Rx Addr]

The AT CRA command is used to restore the CAN

receive filters to their default values. Note that it does

not have any arguments (ie no data).

CRA xyz

[set the CAN Rx Addr to xyz]

Setting the CAN masks and filters can be difficult

at times, so if you only want to receive information

from one address (ie. one CAN ID), then this

command may be very welcome. For example, if you

only want to see information from 7E8, simply send AT

CRA 7E8, and the ELM329 will make the necessary

adjustments to both the mask and the filter for you.

If you wish to allow the reception of a range of

values, you can use the letter X to signify a ‘don’t care’

condition. That is, AT CRA 7EX would allow all IDs

that start with 7E to pass (7E0, 7E1, etc.). For a more

specific range of IDs, you may need to assign a mask

and filter.

CRA wwxxyyzz [set the CAN Rx Addr to wwxxyyzz]

This command is identical to the previous one,

except that it is used to set 29 bit CAN IDs, instead of

11. Sending AT CRA will also reverse the changes

made by this command.

[ show the CAN Status counts ]

The CAN protocol requires that statistics be kept

regarding the number of transmit and receive errors

detected. If there should be a significant number of

errors (due to a hardware or software problem), the

device will go off-line in order to not affect other data

on the bus. The AT CS command lets you see both

the transmitter (Tx) and the receiver (Rx) error counts,

in hexadecimal. If the transmitter should be off (count

>FF), you will see ‘OFF’ rather than a specific count.

CSM0 and CSM1

[ CAN Silent Monitoring off or on ]

The ELM329 was designed to be completely silent

while monitoring a CAN bus so that it can report

exactly what it sees, without colouring the information

in any way. Occasionally (when bench testing, or when

connecting to a dedicated CAN port), it may be

preferred that the ELM329 does not operate silently (ie

you may want it to generate ACK bits, etc.), and this is

what the CSM command is for. CSM1 turns silent

monitoring on (no ACKs are sent), CSM0 turns it off.

AT Command Descriptions (continued)

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