ELM327P datasheet(13/76 Pages) ELM

13 / 76 page

13 of 76

ELM327

ELM327DSH

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

ones that have the Tester Address in the first 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

lengthy

discussion of extended addressing in the ‘Using CAN

Extended Addresses’ section on page 47.

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 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 may be

given any value. This command may 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).

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. The default setting is CFC1 - Flow Controls

on.

Note that during monitoring (AT MA, MR, or MT),

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 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.

AT Command Descriptions (continued)

ELM327P 数据表(PDF) 13 Page - ELM Electronics

ELM327P 数据表(HTML) 13 Page - ELM Electronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76

数据表下载