Allen-Bradley ProcessLogix 1756-IA16, Reference Manual, Page: 97 / 120

Publication 1757-RM810A-EN-P - May 2002
Functional Blocks 3-57
SWITCH
Description Accepts up to 8 initializable inputs (that is, primaries) and operates as a single-pole, 8-position rotary switch.
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An Operator, user program or another block may change switch position.
Function Typically used to assign different primary to a secondary; allows user to select one from as many as 8 inputs and
outputs the selected value.
Timeout
Monitoring In cascade mode, performs timeout monitoring on all inputs X [1] through X [8]. If an input value is not updated within
a predefined time, this block invokes the following timeout processing.
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Sets the “input timeout” flag (TMOUTFL).
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Sets the input value to Bad (NaN).
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Requests the input’s primary to initialize.
This block does not support mode shedding on timeout.
Control
Initialization Block brings initialization requests from its secondary through BACKCALC. In addition, the secondary may propagate
oneshot initialization requests to this block. However, SECINITOPT [1..8] may be used to ignore initialization requests
from this secondary.
If the secondary is requesting initialization, block:
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Initializes its output:
– CV = initialization value from the secondary
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Builds an initialization request for selected primary as:
– INITREQ(s) = ON
– INITVAL(s) = CV – OPBIAS.FIX
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If TRACKING is ON, block also builds an initialization request for the non-selected primaries as:
– INITREQ(n) = ON
– INITVAL(n) = CV – OPBIAS.FIX
Override
Feedback
Processing If this block is in a cascade strategy with a downstream Override Selector block, it receives override feedback data.
The data consists of an override status, override feedback value and an override offset flag. The status indicates if this
block is in the selected or unselected strategy. The offset flag only applies to PID type function blocks. However,
SECINITOPT [1..8] may be used to ignore override requests from the secondary.
When override status changes from selected to unselected, this block:
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Computes a feedback value for the selected primary.
– The selected primary feedback value = BACKCALCOUT.ORFBVAL - OPBIAS.FIX - OPBIAS.FLOAT
– The non-selected primaries are propagated with “not selected” status.
Inputs Accepts up to 8 initializable inputs -- X [1] through X [8].
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X [1] and X [2] are required; X [3] through X [8] are optional.
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Inputs must be pulled from other blocks (cannot be stored).
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You can configure a description of up to 15 characters for each input.
Input Ranges
and Limits User must specify an X-input engineering unit range, XEUHI and XEULO, which defines the full range of inputs (for all
X-inputs).
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XEUHI represents the 100% of full scale value.
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XEULO represents the 0% of full scale value.
Block provides its input range (XEUHI/XEULO) to the primaries through BACKCALC. The primaries use this for their
output range (CVEUHI/CVEULO).
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