F_2Oo3_R - Siemens SIMATIC S7 F Manual

Fail-Safe Blocks
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
Inputs: IN1
IN2
IN3
QBAD1
QBAD2
QBAD3
DELTA
Outputs: OUT
QBAD
DIS1
DIS2
DIS3
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).
8-96
Data Type Explanation
F_REAL Input variable 1
F_REAL Input variable 2
F_REAL Input variable 3
F_BOOL IN1 invalid
F_BOOL IN2 invalid
F_BOOL IN3 invalid
REAL Allowable difference
F_REAL Median value
BOOL Invalid median value
BOOL IN1 DELTA Discrepancy
BOOL IN2 DELTA Discrepancy
BOOL IN3 DELTA Discrepancy
Default
0.0
0.0
0.0
0
0
0
0.0
0.0
0
0
0
0
Fail-Safe Systems
A5E00085588-03