ELM327LP datasheet(16/94 Pages) ELM

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ELM327L

ELM327L DSA

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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 ELM327 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.

As of firmware version 2.0, these commands also

enable or disable the sending of J1939 TP.CM_CTS

messages in response to TP.CM_RTS requests.

During monitoring (AT MA, MR, or MT), there are

never any Flow Controls sent no matter what the CFC

option is set to. The default setting is CFC1 - Flow

Controls on.

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 ELM327 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 ELM327 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. The values are right justified, so you

would need to provide five leading zeros followed by

the three mask bytes.

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 assign the five most

significant bits of the 29 bit CAN ID that is used for

sending messages (the other 24 bits are set with the

AT SH command). 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 ELM327 (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 hhh

[ set the CAN Rx Addr to hhh ]

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 ELM327 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.

To reverse the changes made by the CRA

command, simply send AT CRA or AT AR.

CRA hhhhhhhh [set the CAN Rx Addr to hhhhhhhh]

This command is identical to the previous one,

except that it is used with 29 bit CAN IDs. Sending

either AT CRA or AT AR will also reverse any 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

AT Command Descriptions (continued)

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