Siemens SIMATIC S7 F, Manual

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Fail-Safe Blocks
Fail-Safe Systems
8-102
A5E00085588-03
8.9.2 F_SR_FF
Function
The block executes the function of an SR flipflop (setting dominant).
The SR flipflop is set if the signal state at the input R = 0 and at the input S = 1.
The flipflop is reset if the input R = 1 and the input S = 0. If the result of the logic
operation is 1 at both inputs, the flipflop is set.
Truth Table
R S QN QNn
0 0 Qn-1 QNn-1
0 1 1 0
1 0 0 1
1 1 1 0
I/Os
Name Data Type Explanation Default
Inputs: R F_BOOL Reset 0
S F_BOOL Set 0
Outputs: Q F_BOOL Output 0
QN F_BOOL Negating output 1
Note
If, when you create the program, you preset the Q output in CFC with the initial
value 1, it will remain set after startup (cold restart or warm restart) until the signal
state at the R input changes to 1 (at input S = 0).
Note that the initial values of output parameters do not appear in the printout of the
CFC chart. They must be checked in the printout of the safety program.
Error Handling
In the event of an error that is critical to safety, the system function SFC F_CTRL
is called. This records the event in the Diagnostic Buffer and requests a switch to
the reserve CPU if the error occurred only on the master CPU. For non-redundant
systems or a common-cause error occurring in both CPUs, the shutdown logic can
be configured to either disable the erred F-run-time group or the entire Safety
Program .