Fail-Safe Blocks
Fail-Safe Systems
8-92 A5E00085588-03
8.8 Comparison Blocks for Two Input Values of the Same
Type
Block Description
F_LIM_HL Monitoring for upper limit violation of a REAL value
F_LIM_LL Monitoring for lower limit violation of a REAL value
F_2oo3_R Selects median of 3 REAL values
F_1oo2_R Selects between 2 REAL values based on diagnostics
8.8.1 F_LIM_HL
Function
This block monitors the input variable U for limit violation (U_HL). A hysteresis can
also be specified to avoid fluttering of the output QH in the event of fluctuations of
the input value.
• U ≥ U_HL: In the event of violation of the upper limit, the output QH = 1.
• (U_HL – HYS) ≤ U < U_HL: QH remains unchanged in this range.
• U < (U_HL – HYS): In the event of violation of the lower limit (hysteresis), the
output QH = 0.
The limit and hysteresis are also available as non-fail-safe data at the outputs
U_HL_O and HYS_O for further processing in the standard program. The
hysteresis can be used to avoid fluttering of QH if the input value U fluctuates by
the limit value U_HL.
If either input variable U, U-HL or HYS contains an invalid REAL number, the
Substitute Input (SUBS_IN) will be passed directly to the output (QH).
If an invalid REAL number is generated during the calculations involving U, U-HL
and HYS, the output QH=1.
The output QHN corresponds to the negating output QH.
Note
The non-fail-safe outputs can be made available to the standard program without a
conversion block.
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