Siemens SIMATIC S7 F, Manual

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Fail-Safe Blocks
Fail-Safe Systems
8-96
A5E00085588-03
8.8.3 F_2oo3_R
Function
This block selects the median value from three inputs and places the result at the
output. The QBAD output will be set if two or more of the three inputs present a
QBAD input.
Note
This function block is supplied as a block type. This adds one restriction to the
usage of this block: It may not be placed within another block type.
Note
The OUT output is always the median value of the inputs. Inputs with bad quality
(QBADx=TRUE) are not masked from the selection calculation. The OUT
calculation is NOT directly affected by QBAD.
I/Os
Name Data Type Explanation Default
Inputs:
IN1 F_REAL Input variable 1 0.0
IN2 F_REAL Input variable 2 0.0
IN3 F_REAL Input variable 3 0.0
QBAD1 F_BOOL IN1 invalid 0
QBAD2 F_BOOL IN2 invalid 0
QBAD3 F_BOOL IN3 invalid 0
DELTA REAL Allowable difference 0.0
Outputs: OUT F_REAL Median value 0.0
QBAD BOOL Invalid median value 0
DIS1 BOOL IN1 DELTA Discrepancy 0
DIS2 BOOL IN2 DELTA Discrepancy 0
DIS3 BOOL IN3 DELTA Discrepancy 0
The block employs a two-out-of-three selection scheme and is often used to detect
the failure of sensors and input processing subsystems. Typical use of this block
would have the V and QBAD outputs of three F_CH_AI blocks connected to the
F_2oo3_R’s respective IN and QBAD inputs.
At least two of the three inputs must have their QBAD input clear for QBAD output
to be clear.
The DIS outputs indicate a discrepancy between the respective input, the DELTA
input and the selected median (the difference between IN and OUT is greater than
DELTA).