Allen-Bradley LOGIX 5000 Reference Manual page 46

Chapter 1 Process Control Instructions
46
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
SD instruction is typically used to control on-off or open-close devices such as
pumps or solenoid valves. In this example, the D2SD instruction controls a
solenoid valve adding corn syrup to a batch tank. As long as the D2SD instruction
is in Program control, the valve opens when the AddSyrup input is set. The
operator can also take Operator control of the valve to open or close it if necessary
The solenoid valve in this example has limit switches that indicate when the valve
is fully closed or opened. These switches are wired into the FB0 and FB1 feedback
inputs. This allows the D2SD instruction to generate a FaultAlarm if the solenoid
valve does not reach the commanded state within the configured FaultTime.
Function Block
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018