Fail-Safe Cpu Technical Specifications; Fail-Safe Additions/Exceptions; Areas Of Application; Restrictions With "Read_Dbl" And "Writ_Dbl - Siemens SIMATIC S7 Functional Safety Manual

Technical specifications
A.2 Fail-Safe CPU technical specifications
A.2

Fail-Safe CPU technical specifications

A.2.1

Fail-Safe additions/exceptions

The following sections discuss exceptions and additions for the CPU 1214FC and
CPU1215FC that differentiates them from the standard CPU 1214C and CPU 1215C.
A.2.1.1

Areas of application

The S7-1200 fail-safe CPUs are intended for applications including Functional Safety
requirements. In addition to the safety program, you can also program standard applications.
A.2.1.2

Restrictions with "READ_DBL" and "WRIT_DBL"

If an F-DB is specified as the target address, a READ_DBL instruction execution fails when
reading data from load memory into work memory.
If an F-DB is specified as the target address, a WRIT_DBL instruction execution fails when
writing data from work memory to load memory.
A.2.1.3

Restrictions to configuring the retentive behavior of data blocks

The configuration of retentive data blocks is not supported for F-DBs.
This means that the current values of the F-DBs will not be retentive in the event of Power
OFF/ON and Restart (STOP-RUN) of the fail-safe CPU. The F-DBs are always set to the
initial values from the load memory.
In the F-DBs, for all tags the "Retain" check box is grayed out.
A.2.1.4

Probabilities of failure

Probability of failure values are estimated using standard data tables and calculation
methods according to international standards, specifically for the purpose of calculating PFD
and PFH values according to IEC 61508:2010 and related functional safety standards. The
calculations assume products operated within specifications and repair of diagnosed faults
within 100 hours:
S7-1200 fail-
safe CPUs
144
Operation in Low Demand
Mode,
Average probability of a dan-
gerous failure on demand
(PFD_avg)
< 2.00E-05
Operation in High Demand or
Continuous Mode,
Average frequency of a dan-
gerous failure per hour (PFH)
< 1.00E-09 1/h
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
Proof test inter-
val
(Mission time or
Useful lifetime)
20 years