Allen-Bradley LOGIX 5000 Reference Manual page 83

Process Control Instructions
Instruction first run
Instruction first scan
Postscan
Structured Text
Condition/State
Prescan
Normal Execution
Postscan
When CVInitReq is set, or during instruction first scan, or on a set to cleared
transition of CVFault (bad to good), the instruction initializes the CVEU and CV
outputs to the value of CVInitValue. If the timing mode is not oversample and
EnableIn transitions from cleared to set, the instruction initializes the CVEU and
CV values. CVInitialization is cleared after the initialization and when CVInitReq
is cleared.
The CVInitValue normally comes from the analog output's readback value. The
CVInitReq value normally comes from the "In Hold" status bit on the analog
output controlled by CVEU. The initialization procedure is performed to avoid a
bump at startup in the output signal being sent to the field device.
When using cascaded PID loops, the primary PID loop can be initialized when the
secondary loop is initialized or when the secondary loop leaves the Cascade/Ratio
mode. In this case, move the state of the InitPrimary output and SP output from
the secondary loop to the CVInitReq input and CVInitValue input on the
primary loop.
The instruction does not initialize and the CVEU and CV values are not updated
if CVFault or CVEUSpanInv is set.
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
N/A
If CVFault and CVEUSpanlnv are set,see Processing Faults later in this instruction.
If CVFault and CVEUSpanlnv are cleared:
1. CVInitializing is set
2. If PVFault is set, PVSpanlnv and SPlimitsInv are cleared. See Processing Faults
in this instruction.
3. The PID control algorithm is not executed.
4. The instruction sets CVEU = CVInitValue and CV = corresponding percentage.
5. When CVInitializing and Manual AfterInit are set, the instructions misables
auto and cascade/ratio modes. If the current mode is not Override or Hand
mode, the instruction changes to manual ode.If ManualAfterInit is cleared, the
mode is not changed.
CVEu = CVInitValue
CV = CV = CVEU - CVEUMin
n-1
CVEUMax - CVEUMin
CVOper = CV
EnableIn and EnableOut bits are cleared to false.
Action Taken
See Prescan in the Function Block table.
See Tag.EnableIn is true in the Function Block table.
See Postscan in the Function Block table.
Chapter 1
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