Image Processing Reference
In-Depth Information
Cycle time
Reserve
Master
Slave 1
Slave 2
Slave 3
Slave 4
Slave 5
Synchronization, start of frame
High-priority, periodic traffic
Low-priority, aperiodic traffic
FIGURE .
Bus cycle with cyclic polling and room for aperiodic traffic (PROFIBUS-DP V).
A distinction between different flavors of polling can be made depending on the way data are
addressed. he classical master-slave polling in the examples listed above uses explicit node address-
ing, i.e., the communication approach is device-centric. Another possibility is to identify the desired
data by name, meaning that the request from the master concerns not a device, but a certain process
variable. his is an approach employed by the producer/consumer model in WorldFIP, where the Bus
Arbitrator polls a variable and the node generating (“producing”) it then sends it onto the network,
or in LIN, where the master retrieves data from the slaves by polling the message identiiers. he link
active scheduler in Foundation Fieldbus performs a similar function by prompting devices to send
their scheduled data to the consumers. his polling variant is sometimes called central polling. From
the network traffic point of view, the two approaches are equal, each consists of a request-response
pair of messages per variable exchange.
A peculiarity of the bus arbitration in WorldFIP, however, is that the polling mechanism accounts
for different periodicity requirements of the individual variables. To this end, the polling cycle
(here called macrocycle) is compiled at configuration time from several elementary cycles or micro-
cycles in a static schedule to ensure that each variable will be sampled as often as needed under the
constraint that the total data rate originating from this scheduled traffic is at any given time lower
than the maximum data rate of the bus. he number of elementary cycles a macrocycle must have is
essentially defined by the least common multiple of the periodicities divided by the highest common
denominator of the periodicities (Figure .).
Bus arbitrator poll message
Producer broadcast
Variable 1
Variable 2
Variable 3
Variable 4
Microcycle
t
Macrocycle
FIGURE .
Bus cycle in WorldFIP.
 
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