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1 Features and System Configuration
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CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
The priority at which a message is transmitted compared with another less urgent message is spec-
ified by the identifier of the message concerned. The priorities are defined during system design in
the form of corresponding binary values and cannot be changed dynamically. The identifier with the
lowest binary number has the highest priority.
Bitwise Arbitration
If the bus is free, any unit may start to transmit a message. Bus access conflicts are resolved by bit-
wise arbitration on the identifiers involved by each unit observing the bus level bit for bit. In accor-
dance with the "wired and" mechanism, by which the dominant state (logical 0) overwrites the
recessive state (logical 1), the competition for bus allocation is lost by all those units with recessive
transmission and dominant observation. During arbitration every transmitter compares the level of
the bit transmitted with the level that is motored on the bus. If these levels are equal the unit may
continue to send. All "losers" (levels are not equal) automatically become receivers of the message
with the highest priority and do not re-attempt transmission until the bus is available again.
The efficiency of the bus allocation system is determined mainly by the possible applications for a serial
bus system. In order to judge as simply as possibly which bus systems are suitable for which applica-
tions the literature includes a method of classifying bus allocation procedures. Generally we distinguish
between the following classes:
•
Bus allocation on a fixed time schedule.
•
Bus allocation on the basis of need.
Bus Allocation on a Fixed Time Schedule
Bus allocation done on a fixed time schedule, is made sequentially to each participant for a maxi-
mum duration regardless of whether this participant needs the bus at this moment or not. Examples
of this type of bus allocation are: token slot or token passing.
Bus Allocation on the Basis of Need
Allocation on the basis of need is defined as: the bus is allocated to one participant on the basis of
outstanding transmission requests, i.e. the allocation system only considers participants wishing to
transmit. Examples of this type of bus allocation are: CSMA, CSMA/CD, flying master, round robin
or bitwise arbitration.
CAN Bus Allocation
For CAN, bus allocation is negotiated purely among the messages waiting to be transmitted. This
means that the procedure specified by CAN is classified as allocation on the basis of need.
Another means of assessing the efficiency of bus arbitration systems is the bus access method:
•
Non-destructive bus access
•
Destructive bus allocation
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Efficiency of Bus Allocation
Bus Allocation
Bus Access
Summary of Contents for CJ1W-CORT21
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