Switch-Over Of The Active Channel On Link-Up And Update; No Pulses During Switch-Over Of The Active Channel; System Configuration And Design - Siemens SIMATIC S7-400H Manual

Switch-over of the active channel on link-up and update

On link-up and update with master/standby switch-over (see Section 6.2.1) the
active and slave channels are switched over in all the stations of the switched I/O.
OB 72 is invoked here.

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.
To ensure that total failures of a DP station occurring during failover are also
recognized, the failover process is monitored by both the individual DP stations
and by the DP master system.
When the minimum I/O retention time is set correctly (see Section 6.3), no
interrupts or records are lost during the failover. 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.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Using I/O on the S7-400H
7-9