Process Control Instructions
34 Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
Structured Text
In Structured Text, EnableIn is always true during normal scan. Therefore, if the
instruction is in the control path activated by the logic it will execute.
Condition/State Action Taken
Prescan See Prescan in the Function Block table.
Normal Execution See Tag.EnableIn is true in the Function Block table.
Postscan See Postscan in the Function Block table.
Examples
The D3SD instruction is typically used to control 3-state devices such as
high/low/off feed systems. In this example, the D3SD instruction controls a feed
system consisting of a pair of solenoid valves adding vegetable oil to a batch tank.
One of the valves is on a large diameter feed pipe into the batch tank, and the
other valve is plumbed in parallel on a small diameter feed pipe. When oil is first
added, the D3SD instruction is commanded to the fast feed state (state 2) where
both valves are opened. When the oil added approaches the target amount, the
D3SD instruction is commanded to the slow feed state (state 1) where the "large
valve" is closed and the "small valve" is kept open. When the target is reached, the
D3SD instruction is commanded to go to the off state (state 0) and both valves are
closed. As long as the D3SD instruction is in Program control, the valves open
according to the CloseOilFeed, SlowOilFeed, and FastOilFeed inputs. The
operator can also take Operator control of the feed system if necessary. The
solenoid valves in this example have limit switches which indicate when the valves
are fully closed or opened. These switches are wired into the FB0, FB1, FB2, and
FB3 feedback inputs. This allows the D3SD instruction to generate a FaultAlarm
if the solenoid valves do not reach their commanded states within the configured
FaultTime.
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