Operation and Maintenance
Fail-Safe Systems
6-2 A5E00085588-03
Fiber-Optic Cables Between the Synchronization Modules in the S7-400 FH
!
Safety Note – Duplicate Masters must be avoided
In a fail-safe and fault-tolerant S7 FH System, you must prevent both CPUs from
being master at the same time, since this may result in hazardous faults.
Such a state (the two CPUs are both masters at the same time) can occur if the
two fiber-optic cables used to connect the CPUs are removed or interrupted
simultaneously when the S7-400 FH is in a redundant configuration. This must be
prevented by laying separate fiber-optic cables.
This state (two CPUs both masters at the same time) can also occur after a CPU is
repaired if the CPUs have not been connected via both fiber-optic cables before
the power supply is switched back on.
Take organizational steps to ensure that, after a CPU has been replaced, both
fiber-optic cable connections are established before the power supply is switched
on.
You can find information on replacing components in fault-tolerant systems in
manual /4/. Please refer to the references in Appendix B.)
6.3 Working with the Safety Program
You must take into account the following when working with the Safety Program:
• You must not operate Safety Programs directly when safety mode is activated!
You can enter safety parameters:
- by means of fail-safe conversion blocks.
- in CFC test mode.
• Access to the CPU must be protected with a password.
• The offline project in the programming device/ES must always be kept
consistent with the CPU. In other words, no old programs, charts or blocks
should be copied to a project.
!
Safety Note – Safety measures must be followed
If you don’t follow the above safety measures, this may result in errors in the
execution of the safety program and in the Safety Program Shutdown.
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