Siemens SIMATIC S7-400H Manual page 81

No pulses during switch-over of the active channel
To prevent temporary failure of the I/O or output of substitute values during
switch-over between the active and slave channel the DP stations of the switched
I/O maintain their outputs until switch-over is complete and the new active channel
has taken over processing.
In order that total failures of a DP station occurring during switch-over are also
recognized the switch-over process is monitored by both the individual DP stations
and by the DP master system.
No interrupts or data records are lost as a result of a switch-over. Automatic
repetition takes place if necessary.
System configuration and design
You should sort switched I/O with different transfer times into separate looms. This
simplifies calculation of the monitoring times, among other things.
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
Using I/O on the S7-400H
6-9