Omron CJ1W-CORT21 Operation Manual page 43

 Data Frame
The data frame is composed of an Arbitration field, Control field, Data field, CRC field, and an ACK
field. The frame begins with a 'Start of frame' [SOF], and ends with an 'End of frame' [EOF] space.
The data field may be from 0 to 8 bytes. The frame check sequence is derived from a Cyclic Redun-
dancy Code (CRC); the coefficients are generated modulo-2: X15 + X14 + X10 + X8 + X7 + X4 + X3
+ 1. CAN implements five error detection mechanisms; 3 at the message level and 2 at the bit level
[Also incorporates error flags].
At the message level
• Cyclic Redundancy Checks (CRC)
• Frame Checks
• Acknowledgment Error Checks
At the bit level
• Bit Monitoring
• Bit Stuffing.
 Messages
Information on the bus is sent in fixed format messages of different but limited length. Whenever the
bus is free, any connected unit may start to transmit a new message. In CAN systems a CAN node
does not make use of any information about the system configuration (e.g. station addresses). This
has several important consequences.
System Flexibility
Nodes can be added to the CAN network without requiring any change in the software or hardware
of any node and application layer.
Message Routing
The content of a message is named by an Identifier. The Identifier does not indicate the destination
of the message, but describes the meaning of the data, so that all nodes in the network are able to
decide by Message Filtering whether the data is to be acted upon by then or not.
Multicast
As a consequence of the concept of Message Filtering, any number of nodes can receive and simul-
taneously act upon the same message.
Data consistency
Within a CAN network it is guaranteed that a message is simultaneously accepted either by all
nodes or by no node. Thus data consistency of a system is achieved by the concepts of multicast
and by error handling.
 Message Transfer
The (application) messages transmitted and received by the User Defined CAN Unit are called Data
Frames. There are two different formats for a data frame. The two formats differ in the length of the
message identifier. This identifier is a field in the frame and part of the arbitration field (see the sche-
matic picture of a data frame above). Data frames that have identifiers with a size of 11 bits are
denoted Standard Frames. The other format has frames containing a 29 bit identifierr. These frames
are referred to as Extended Frames. The maximum length of the data transmitted in one message is
8 bytes. The length of data in a message can be 8 bytes or less. If the data length is less then 8, the
frame always has 8 data bytes.
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
1 Features and System Configuration
1
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