National CP3BT26 manual Can Frame Types, Can Frame Fields, Start of Frame SOF, Arbitration Field

The CAN protocol allows several transmitting modules to	written by a message with a higher priority. As soon as a
start a transmission at the same time as soon as they detect	transmitting module detects another module with a higher
the bus is idle. During the start of transmission, every node	priority accessing the bus, it stops transmitting its own frame
monitors the bus line to detect whether its message is over-	and switches to receive mode, as shown in Figure 35.

TxPIN

MODULE A

RxPIN

TxPIN

MODULE B

RxPIN

RECESSIVE

BUS LINE

DOMINANT

MODULE A SUSPENDS TRANSMISSION

DS019

Figure 35. CAN Message Arbitration

CP3BT26

If a data or remote frame loses arbitration on the bus due to a higher-prioritized data or remote frame, or if it is destroyed by an error frame, the transmitting module will automatically retransmit it until the transmission is successful or software has canceled the transmit request.

If a transmitted message loses arbitration, the CAN module will restart transmission at the next possible time with the message which has the highest internal transmit priority.

19.2.1CAN Frame Types

Communication via the CAN bus is basically established by means of four different frame types:

„Data Frame

„Remote Frame

„Error Frame

„Overload Frame

Data and remote frames can be used in both standard and extended frame format. If no message is being transmitted, i.e., the bus is idle, the bus is kept at the “recessive” level.

Remote and data frames are non-return to zero (NRZ) cod- ed with bit-stuffing in every bit field, which holds computable information for the interface, i.e., start of frame, arbitration field, control field, data field (if present), and CRC field.

Error and overload frames are also NRZ coded, but without bit-stuffing.

After five consecutive bits of the same value (including in- serted stuff bits), a stuff bit of the inverted value is inserted into the bit stream by the transmitter and deleted by the re- ceiver. The following shows the stuffed and destuffed bit stream for consecutive ones and zeros.

Original or	10000011111 . . .	01111100000 . . .
unstuffed bit stream
Stuffed bit stream	1000001111101 . . .	0111110000010 . . .
(stuff bits in bold)

19.2.2CAN Frame Fields

Data and remote frames consist of the following bit fields:

„Start of Frame (SOF)

„Arbitration Field

„Control Field

„Data Field

„CRC Field

„ACK Field

„EOF Field

Start of Frame (SOF)

The Start of Frame (SOF) indicates the beginning of data and remote frames. It consists of a single “dominant” bit. A node is only allowed to start transmission when the bus is idle. All nodes have to synchronize to the leading edge (first edge after the bus was idle) caused by the SOF of the node which starts transmission first.

Arbitration Field

The Arbitration field consists of the identifier field and the RTR (Remote Transmission Request) bit. For extended frames there is also a SRR (Substitute Remote Request) and a IDE (ID Extension) bit inserted between ID18 and ID17 of the identifier field. The value of the RTR bit is “dom- inant” in a data frame and “recessive” in a remote frame.

Control Field

The Control field consists of six bits. For standard frames it starts with the ID Extension bit (IDE) and a reserved bit (RB0). For extended frames, the control field starts with two reserved bits (RB1, RB0). These bits are followed by the 4- bit Data Length Code (DLC).

The CAN receiver accepts all possible combinations of the reserved bits (RB1, RB0). The transmitter must be config- ured to send only zeros.

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