Fail-Safe Blocks
Fail-Safe Systems
8-94 A5E00085588-03
8.8.2 F_LIM_LL
Function
This block monitors the input variable U for violation of the lower limit (U_LL). A
hysteresis can also be specified to avoid fluttering of the output QL in the event of
fluctuations in the input value.
• U ≥ U_LL: In the event of violation of the lower limit, the output QL = 1.
• U_LL < U ≤ (U_LL + HYS): QL remains unchanged in this range.
• U > (U_LL + HYS): In the event of upper limit violation + hysteresis, the output
QL = 0.
The limit and hysteresis are also available as non-fail-safe data at the outputs
U_LL_O and HYS_O for further processing in the standard program. The
hysteresis can be used to avoid fluttering of QL if the input value U fluctuates by
the limit value U_LL.
If either input variable U, U_LL or HYS contains an invalid REAL number, the
Substitute Input (SUBS_IN) will be passed directly to the output (QL).
If an invalid REAL number is generated during the calculations involving U, U-LL
and HYS, the output QL=1.
The output QLN corresponds to the negating output QL.
Note
The non-fail-safe outputs can be made available to the standard program without a
conversion block.
I/Os
Name Data Type Explanation Default
Inputs:
U F_REAL Input variable 0.0
U_LL F_REAL Lower limit 100.0
HYS F_REAL Hysteresis 0.0
SUBS_IN F_BOOL Substitute Input 0
Outputs:
QL F_BOOL 1: Lower limit violated 0
QLN F_BOOL Negating output QL 1
U_LL_O REAL Upper limit 100.0
HYS_O REAL Hysteresis 0
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