Allen-Bradley ProcessLogix 1756-IA16, Reference Manual, Page: 75 / 120

Publication 1757-RM810A-EN-P - May 2002
Functional Blocks 3-35
OVRDSEL(Override Selector)
Description Provides override feedback data to every block in an upstream cascade control strategy. Also provides bypass
processing, control initialization, and override feedback propagation.
Function Accepts up to four inputs (primaries) and selects the one with the highest or lowest value.
Timeout
Monitoring In cascade mode, performs timeout monitoring on all inputs X [1] through X [4] that are not bypassed. If an input value
is not updated within a predefined time, this block invokes the following timeout processing.
1. Sets the “input timeout” flag (TMOUTFL).
2. Sets the input value to Bad (NaN).
3. Requests the input’s primary to initialize.
This block does not support mode shedding on timeout.
Inputs Accepts up to 4 inputs -- X [1] through X [4].
At least 2 inputs (X1 and X2) are required, others are optional.
You can configure a 15-character description for each input.
The inputs must come from other function blocks; an operator cannot store to them.
Input Ranges XEUHI and XEULO define the full range of inputs.
XEUHI represents the 100% of full scale value.
XEULO represents the 0% of full scale value.
Outputs Block has following initializable outputs:
•
OP = Calculated output in percent.
•
OPEU = Calculated output in engineering units
Note that the default OP connection pin is exposed on the blocks and the implicit/hidden connection function
automatically makes the appropriate value/status parameter (OPX/OPEUX) connection when required.
Output Ranges CVEUHI and CVEULO define the full range of CV in engineering units. If this block has a secondary, it brings the
secondary’s input range through the BACKCALC and sets its CV range to that. If it has no secondary, CVEUHI and
CVEULO track its own input range (XEUHI and XEULO).
•
OPHILM and OPLOLM (user-specified values) define normal high and low limits for OP as a percent of the CV
range.
•
OP clamps to these limits if algorithm's calculated result (CV) exceeds them or another block or user program
attempts to store an OP value exceeding them.
•
Operator may store an OP value outside these limits.
•
OPEXHILM and OPEXLOLM (user-specified values) define the extended high and low limits for OP as a percent of
the CV range.
•
Operator is prevented from storing an OP value that exceeds these limits.
Parameters on following page
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