Siemens SIMATIC S7-400H Manual page 60

Link-up and Update
Minimum I/O hold time:
This is the period of time between copying of the outputs from the master
CPU to the standby CPU and the time of transition to the Redundant system
mode or master/standby switch-over (time at which the former master CPU
switches to STOP mode and the new master CPU switches to RUN mode).
During this time the outputs of both CPUs are activated. This prevents
ramping of the I/O even in the event of an update with master/standby
switch-over.
The min. I/O hold time is of particular significance in an update with
master/standby switch-over.
The start times of the monitoring timers are shown in Figure 5-2 (underlaid boxes).
In each case the times end when the Redundant system mode occurs or on
master/standby switch-over (i.e. when the new master switches to RUN mode) at
the end of the update.
The times relevant to the update are summarized in the figure below.
Update
t t
1 2
t : end of current OBs up to priority class 15
1
t : stop of all communication functions
2
t : end of watchdog interrupt OB with special treatment
3
t : end of copying of outputs to standby CPU
4
t : Redundant system mode or master/standby switch-over
5
Figure 5-4 Significance of the times relevant during the update
Reaction to time-out
If one of the times monitored exceeds the maximum value configured then the
following process is started:
5-16
t t
3 4
blocking time for priority classes > 15
communication delay
scan-cycle time extension
S7-400H Programmable Controller Fault-Tolerant Systems
t
5
min. I/O hold time
A5E00068197-04
t