wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

WorldFip 34-5
TABLE 34.1
Variable
Name
List of Variables to Be Scheduled
Refresh Period
(ms)
Data Type
Transaction
Time (µs)
A 5 INT_8 170
B 10 INT_8 170
C 15 INT_16 178
D 15 SFPOINT 194
E 20 INT_16 178
F 30 UNS_32 194
The BA must first analyze the single variables and achieve their refresh period (which will be
expressed in milliseconds) and the data type (e.g., 8 bit integer number and short floating point number)
in order to compute the time required by each variable. This time is made up of three components:
the time required by the BA to transmit the ID frame that polls a producer, the turnaround
time (i.e., the time between the transmission of an ID frame by the BA and the reply by an information
producer), and the time required by the producer to send the data frame. The values reported in
Table 34.1 are based on a transmission bit rate of 1.Mbps and a turnaround time of 20.ms (the range
allowed by WorldFIP with a bit rate 1.Mbps is from 10 to 70.ms).
From the values reported in Table 34.1, we can infer that transmission of single variables is not very
efficient in WorldFip. In fact, we have a heavy transmission overhead since a single variable requires two
frames and some turnaround time. Of course, the situation becomes better when the dimension of the
data increases as the rate (data field)/(transmission overhead) is improved.
Figure 34.5 shows a possible schedule of the variables listed in Table 34.1 to be stored into the bus
arbitrator table (BAT). Time on X-axis is divided into elementary cycles that correspond to the time
required to transmit variables with the shortest refresh period. This organization of the data scan simplifies
the construction of the scheduling table as we divide the time in fixed slots and then try to allocate
all variables into the various time slots without violating their time constraints. Time on Y-axis is the
sum of the time required to transmit all the variables that are scheduled inside a time slot.
In Figure 34.5, we can see that in the first elementary cycle, all the variables are transmitted. In the
subsequent cycles, the variables are transmitted according to a distribution based on their time constraints.
We observe that no elementary cycle overcomes the value of 5.ms, so that the schedule does not
violate any time requirement.
5 ms Time available for acyclic traffic
A
A
B
A
B
C
B
C
D
A A C A A D
E A C B D B C A E
F A B D E A F A E D B A F A Time
FIGURE 34.5 Possible schedule of the variables listed into Table 34.1.
© 2011 by Taylor and Francis Group, LLC