Image Processing Reference
In-Depth Information
Maximum: target token rotation time
Master 1
High-priority
Low-priority
Master 2
High-priority
Low-priority
Master 3
t
Token arrival
Token passing time
FIGURE .
Token passing in PROFIBUS.
2-7 bit periods
11-30 bit periods
Bus data
Master req.
Slave resp.
Token unused
Access counter
(implicit token)
3
4
1
2
3
4
1
Bus idle counter
30
20
0
10
40
50
60
70
80
FIGURE .
P-NET token passing and polling mechanism.
of individual frames are necessary. Periodicity of individual process variables is therefore less exact
as with polling, and jitter may be larger. Still, the total time of a token rotation cycle can be upper-
bounded to support real-time requirements. In any case, careful selection of the T TR value must be
done based on an a priori analysis of the expected network traffic and is essential to ensure optimal
use of the available communication bandwidth.
The implicit form of token passing is used by P-NET. his fieldbus uses a hybrid approach together
with polling. To keep the overhead of the volume of information that needs to be transferred on the
bus to the absolute minimum, the token is simulated by two counters, included in every master,
andisnotactuallypassedaroundthebus(Figure.).he“idlebusbitperiodcounter”(IC)is
incremented every bit period as long as the bus is free, and is reset as soon as some node sends
data. he access counter (AC) is incremented whenever the IC reaches the values , , , . . ., .
If the AC status matches the address of a master, this means that the master has the token and is
therefore able to access the bus for one single request-response data exchange with some other node.
To ensure that the token is not passed on, there are maximum values for the time a master can wait
before sending a request to a slave, and also for the time a slave may take until it starts its response.
If the master has no data to send, it remains silent, and after a further  increments, the token is
handed over. Once the AC reaches the number of masters contained within the bus system, up to a
maximum of , it is reset to the value . To avoid loss of synchronization due to the freely running
system clocks in the nodes during long idle periods, a master has to send at least a synchronization
 
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