National CP3BT26 - Manual (Page 112)

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CP3BT26
Data Length Code (DLC)
The DLC field indicates the number of bytes in the data field.
It consists of four bits. The data field can be of length zero.
The admissible number of data bytes for a data frame rang-
es from 0 to 8.
Data Field
The Data field consists of the data to be transferred within a
data frame. It can contain 0 to 8 bytes. A remote frame has
no data field.
Cyclic Redundancy Check (CRC)
The CRC field consists of the CRC sequence followed by
the CRC delimiter. The CRC sequence is derived by the
transmitter from the modulo 2 division of the preceding bit
fields, starting with the SOF up to the end of the data field,
excluding stuff-bits, by the generator polynomial:
The remainder of this division is the CRC sequence trans-
mitted over the bus. On the receiver side, the module di-
vides all bit fields up to the CRC delimiter excluding stuff
bits, and checks if the result is zero. This will then be inter-
preted as a valid CRC. After the CRC sequence a single “re-
cessive” bit is transmitted as the CRC delimiter.
ACK Field
The ACK field is two bits long and contains the ACK slot and
the ACK delimiter. The ACK slot is filled with a “recessive”
bit by the transmitter. This bit is overwritten with a “domi-
nant” bit by every receiver that has received a correct CRC
sequence. The second bit of the ACK field is a “recessive”
bit called the acknowledge delimiter.
The End of Frame field closes a data and a remote frame. It
consists of seven “recessive” bits.
19.2.3 CAN Frame Formats
Data Frame
The structure of a standard data frame is shown in
Figure 36. The structure of an extended data frame is
shown in Figure 37.
Figure 36. Standard Data Frame
Figure 37. Extended Data Frame
x
15
+ x
14
+ x
10
+ x
8
+ x
7
+ x
4
+ x
3
+ 1
STANDARD DATA FRAME (number of bits = 44 + 8N)
Control Field Data Field END OF
FRAME
CRC Field
CRC
Arbitration Field
IDENTIFIER
10 ... 0
11 4
DATA
LENGTH CODE
ST
AR
T OF FRAME
ID 10 ID0 RT
R
IDE RB0 DLC3 DLC
d d d d r r r r r r r r r
CRC DEL A
CK DEL
A
CKNO
WLEDGEMENT
Bit Stuffing
8 8 15
16 8N (0 < N < 8)
Note:
d = dominant
r = recessive DS020
EXTENDED DATA FRAME (number of bits = 64 + 8N)
Control Field Data Field END OF
FRAME
CRC Field
CRC
Arbitration Field
IDENTIFIER
28 ... 18
11 4
DATA
LENGTH CODE
ST
AR
T OF FRAME
ID 28 ID18 ID17 ID0 SRR IDE RT
R
RB1 RB0 DLC3 DLC
d r r d d d r r r r r r r r r
CRC DEL A
CK DEL
SCK
Bit Stuffing
18 8 15
16 8N (0 < N < 0)
8
IDENTIFIER
17 ... 0
Note:
d = dominant
r = recessive
DS021