Siemens SIMATIC S7-400H Manual page 64

Link-up and Update
Calculation of the min. I/O hold time (T
The following applies to the calculation of the min. I/O hold time:
with central I/O: T
with remote I/O: T
where T
With the use of central and remote I/O the resulting minimum I/O hold time is:
T = MAX (30 ms, 3
PH
Figure 5-5 shows the relationship between the min. I/O hold time and the max.
blocking time for priority classes > 15.
Figure 5-5
Note that
50 ms + min. I/O hold time
must apply. As a result, a large min. I/O hold time may determine the max.
blocking time for priority classes > 15.
Calculation of the max. blocking time for priority classes > 15 (T
The max. blocking time for priority classes > 15 is determined by 4 factors:
As shown in Figure 5-2, at the end of the update all the contents of data blocks
that have been modified since the last copy to the standby CPU are transferred
to the standby CPU again. The number and structure of the data blocks that
you describe in the high-priority priority classes determine the duration of this
process and thus the max. blocking time for priority classes > 15. A note is
given for the remedies specified below.
5-20
)
PH
= 30 ms
PH
= 3 T
PH TRmax
= maximum target rotation time
TRmax
of all DP master systems of the H station
T )
TRmax
master copies
outputs 50 ms
min. I/O
hold time
Relationship between the min. I/O hold time and the max. blocking time
for priority classes > 15.
(max. blocking time of priority classes > 15)
max. blocking time for
priority classes > 15
S7-400H Programmable Controller Fault-Tolerant Systems
)
P15
A5E00068197-04