Omron CJ1W-CORT21 Operation Manual page 47

 Non-destructive Bus Access
With methods of this type the bus is allocated to one and only one unit either immediately or within a
specified time following a single bus access (by one or more units). This ensures that each bus
access by one or more units leads to an unambiguous bus allocation. Examples of non-destructive
bus access are: token slot, token passing, round robin, bitwise arbitration.
 Destructive Bus Allocation
Simultaneous bus access by more than one unit causes all transmission attempts to be aborted and
therefore there is no successful bus allocation. More than one bus access may be necessary in
order to allocate the bus at all, the number of attempts before bus allocation is successful being a
purely statistical quantity. Examples of destructive bus access are: CSMA/CD, Ethernet.
 Unambiguous Bus Allocation
In order to process all transmission requests of a CAN network while complying with latency con-
straints at as low a data transfer rate as possible, the CAN protocol must implement a bus allocation
method that guarantees that there is always unambiguous bus allocation even when there are
simultaneous bus accesses from different units. The method of bitwise arbitration using the identifier
of the messages to be transmitted uniquely resolves any collision between a number of units want-
ing to transmit, and it does this at the latest within 13 (standard format) or 33 (extended format) bit
periods for any bus access period. Unlike the message-wise arbitration employed by the CSMA/CD
method this non-destructive method of conflict resolution ensures that no bus capacity is used with-
out transmitting useful information.
Even in situations where the bus is overloaded the linkage of the bus access priority to the content
of the message proves to be a beneficial system attribute compared with existing CSMA/CD or
token protocols: In spite of the insufficient bus transport capacity, all outstanding transmission
requests are processed in order of their importance to the overall system (as determined by the
message priority). The available transmission capacity is utilized efficiently for the transmission of
useful data since "gaps" in bus allocation are kept very small. The collapse of the whole transmis-
sion system due to overload, as can occur with the CSMA/CD protocol, is not possible with CAN.
Thus, CAN permits implementation of fast, traffic-dependent bus access which is non-destructive
because of bitwise arbitration based on the message priority employed.
 Bus Access Control
Non-destructive bus access can be further classified into two types depending on whether the con-
trol mechanisms are present in the system only once (centralized) or more than once (decentral-
ized).
• centralized bus access control
• decentralized bus access control
A communication system with a designated unit must provide a strategy to take effect in the event of
a failure of the master unit. This concept has the disadvantage that the strategy for failure manage-
ment is difficult and costly to implement and also that the takeover of the central unit by a redundant
unit can be very time-consuming. For these reasons and to circumvent the problem of the reliability
of the master unit (and thus of the whole communication system), the CAN protocol implements
decentralized bus control. All major communication mechanisms, including bus access control, are
implemented several times in the system, because this is the only way to fulfill the high requirements
for the availability of the communication system.
In summary it can be said that CAN implements a traffic-dependent bus allocation system that per-
mits, by means of a non-destructive bus access with decentralized bus access control, a high useful
data rate at the lowest possible bus data rate in terms of the bus busy rate for all units. The effi-
ciency of the bus arbitration procedure is increased by the fact that the bus is utilized only by those
units with pending transmission requests.
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
1 Features and System Configuration
1
1-13