National CP3BT26 manual Data Length Code DLC, Data Field, Cyclic Redundancy Check CRC, ACK Field

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:

x15 + x14 + x10 + x8 + x7 + x4 + x3 + 1

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.

19.2.3CAN 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.

STANDARD DATA FRAME (number of bits = 44 + 8N)

					8N (0 < N < 8)	16
		Arbitration Field	Control Field		Data Field	CRC Field	END OF
							FRAME
	STARTFRAMEOF	11	4	8	8	15	ACKNOWLEDGEMENT ACK DEL
						CRC
		ID 10	ID0 RTR IDE RB0 DLC3	DLC		CRCDEL
	d		d d d			r	r r r r r r r r
		IDENTIFIER	DATA
		10 ... 0
			LENGTH CODE
					Bit Stuffing
Note:							DS020
d = dominant
r	= recessive

Figure 36. Standard Data Frame

EXTENDED DATA FRAME (number of bits = 64 + 8N)

FRAME								8N (0 < N < 0)		16
	Arbitration Field					Control Field		Data Field		CRC Field	END OF
											FRAME
	11			18			4	8	8	15
START OF											SCK ACK DEL
										CRC
	ID 28	ID18	SRR IDE	ID17	ID0	RTR RB1 RB0 DLC3	DLC			CRCDEL
d			r r			d d d				r	r r r r r r r r
	IDENTIFIER			IDENTIFIER			DATA
	28 ... 18
				17 ... 0		LENGTH CODE
							Bit Stuffing
Note:
d = dominant											DS021
r = recessive

Figure 37. Extended Data Frame

www.national.com 112