Message Frame Formats - Omron CJ1W-CORT21 Operation Manual

1 Features and System Configuration
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.
1-2-6

Message Frame Formats

 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.
Number of Data
Bytes
0
1
2
3
4
5
6
7
8
 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.
1-14
Arbitration field
S R I
O 11 bit IDENTIFIER T D
F R E
DLC3 DLC2
Dominant Dominant
Dominant Dominant
Dominant Dominant
Dominant Dominant
Dominant Recessive
Dominant Recessive
Dominant Recessive
Dominant Recessive
Recessive Dominant
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
Control Data Field CRC Field
Field
r
0 DLC 0 - 8 bytes 15 bit CRC
Data Length Code
DLC1
Dominant
Dominant
Dominant
Recessive
Recessive
Dominant
Recessive
Recessive
Dominant
Ack End of Int Bus idle
Field Frame
DLC0
Dominant
Recessive
Recessive
Dominant
Recessive
Dominant
Dominant
Recessive
Dominant