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1 Features and System Configuration
1-14
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
These requests are handled in the order of the importance of the messages for the system as a
whole. This proves especially advantageous in overload situations. Since bus access is prioritized
on the basis of the messages, it is possible to guarantee low individual latency times in real-time
systems.
Identifier
The CAN protocol supports two message frame formats, the only essential difference being in the
length of the identifier (ID). In the standard format the length of the ID is 11 bits and in the extended
format the length is 29 bits. The message frame for transmitting messages on the bus comprises
seven main fields.
Arbitration Field
A message in the standard format begins with the start bit (Start Of Frame). This is followed by the
arbitration field, which contains the identifier and the RTR (Remote Transmission Request) bit,
which indicates whether it is a data frame or a request frame without any data bytes (remote frame).
Control Field
The control field contains the IDE (Identifier Extension) bit, which indicates either standard format or
extended format, a bit reserved for future extensions and, in the last 4 bits, a count of the data bytes
in the data field. The number of bytes in the data field is indicated by the Data Length Code (DLC),
the data length code is 4 bits wide.
Data Field
The data field ranges from 0 to 8 bytes in length and is followed by the CRC field, which is used as a
frame security check for detecting bit errors.
1-2-6
Message Frame Formats
Arbitration field
Control
Field
Data Field
CRC Field Ack
Field
End of
Frame
Int
Bus idle
S
O
F
11 bit IDENTIFIER
R
T
R
I
D
E
r
0 DLC 0 - 8 bytes
15 bit CRC
Number of Data
Bytes
Data Length Code
DLC3
DLC2
DLC1
DLC0
0
Dominant
Dominant
Dominant
Dominant
1
Dominant
Dominant
Dominant
Recessive
2
Dominant
Dominant
Dominant
Recessive
3
Dominant
Dominant
Recessive
Dominant
4
Dominant
Recessive
Recessive
Recessive
5
Dominant
Recessive
Dominant
Dominant
6
Dominant
Recessive
Recessive
Dominant
7
Dominant
Recessive
Recessive
Recessive
8
Recessive
Dominant
Dominant
Dominant
Summary of Contents for CJ1W-CORT21
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