Principles Of Data Exchange; Principles Of Non-Destructive Bitwise Arbitration - Omron CJ1W-CORT21 Operation Manual

1-2-3

Principles of Data Exchange

 Unit Addressing
When data is transmitted by CAN, no units are addressed, but instead, the content of the message
(e.g. rpm or engine temperature) is designated by an identifier that is unique throughout the net-
work. The identifier defines not only the content but also the priority of the message. This is impor-
tant for bus allocation when several units are competing for bus access.
 Sending Messages
If the CPU of a given unit wishes to send a message to one or more units, it passes the data to be
transmitted and their identifiers to the assigned CAN chip. The CPU only has to initiate data
exchange. The message is constructed and transmitted by the CAN chip. As soon as the CAN chip
receives the bus allocation, all other units on the CAN network become receivers of this message.
Each unit in the CAN network, having received the message correctly, performs an acceptance test
to determine whether the data received is relevant for that unit. If the data are of significance for the
unit concerned they are processed, otherwise they are ignored.
 Addressing scheme
A high degree of system and configuration flexibility is achieved as a result of the content-oriented
addressing scheme. It is very easy to add units to the existing CAN network without making any
hardware or software modifications to the existing units, provided that the new units are purely
receivers. Because the data transmission protocol does not require physical destination addresses
for the individual components, it supports the concept of modular electronics and also permits multi-
ple reception (broadcast, multicast) and the synchronization of distributed processes. Measure-
ments needed as information by several controllers can be transmitted via the network, in such a
way that it is unnecessary for each controller to have its own sensor.
1-2-4

Principles of Non-Destructive Bitwise Arbitration

For the data to be processed in real time they must be transmitted rapidly. This not only requires a
physical data transfer path with up to 1 Mbps but also calls for rapid bus allocation when several units
wish to send messages simultaneously.
 Priority
In real-time processing the urgency of messages to be exchanged over the network can differ
greatly. A rapidly changing dimension (e.g. engine load) has to be transmitted more frequently and
therefore with less delays than other dimensions (e.g. engine temperature) which change relatively
slowly.
1
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
Bus line
2 3
1 Features and System Configuration
recessive
dominant
1 loses 3 loses
1
1-11