structured text
{{Short description|Programming language for programmable logic controllers}}
{{For|the lightweight text markup languages|StructuredText|reStructuredText}}
Structured text, abbreviated as ST or STX, is one of the five languages supported by the IEC 61131-3 standard, designed for programmable logic controllers (PLCs).{{cite web |last1=Bacidore |first1=Mike |title=Should I limit programming to ladder logic or use all standards within IEC 61131? |url=https://www.controldesign.com/articles/2018/should-i-limit-programming-to-ladder-logic-or-use-all-standards-within-iec-61131/ |date=16 May 2018 |website=Control Design}}{{cite web |last1=Stevic |first1=Tom |title=A very short history of PLC programming platforms |url=https://www.controldesign.com/articles/2017/a-very-short-history-of-plc-programming-platforms/ |date=5 May 2017 |website=Control Design}} It is a high level language that is block structured and syntactically resembles Pascal, on which it is based.{{cite journal |last1=Roos |first1=Nieke |title=Programming PLCs using Structured Text |publisher=Department of Computing Science, University of Nijmegen|citeseerx=10.1.1.49.2016 }} All of the languages share IEC61131 Common Elements. The variables and function calls are defined by the common elements so different languages within the IEC 61131-3 standard can be used in the same program.
Complex statements and nested instructions are supported:
Sample program
(* simple state machine *)
TxtState := STATES[StateMachine];
CASE StateMachine OF
1: ClosingValve();
StateMachine := 2;
2: OpeningValve();
ELSE
BadCase();
END_CASE;
Unlike in some other programming languages, there is no fallthrough for the CASE statement: the first matching condition is entered, and after running its statements, the CASE block is left without checking other conditions.
=Additional ST programming examples=
// PLC configuration
CONFIGURATION DefaultCfg
VAR_GLOBAL
b_Start_Stop : BOOL; // Global variable to represent a boolean.
b_ON_OFF : BOOL; // Global variable to represent a boolean.
Start_Stop AT %IX0.0:BOOL; // Digital input of the PLC (Address 0.0)
ON_OFF AT %QX0.0:BOOL; // Digital output of the PLC (Address 0.0). (Coil)
END_VAR
// Schedule the main program to be executed every 20 ms
TASK Tick(INTERVAL := t#20ms);
PROGRAM Main WITH Tick : Monitor_Start_Stop;
END_CONFIGURATION
PROGRAM Monitor_Start_Stop // Actual Program
VAR_EXTERNAL
Start_Stop : BOOL;
ON_OFF : BOOL;
END_VAR
VAR // Temporary variables for logic handling
ONS_Trig : BOOL;
Rising_ONS : BOOL;
END_VAR
// Start of Logic
// Catch the Rising Edge One Shot of the Start_Stop input
ONS_Trig := Start_Stop AND NOT Rising_ONS;
// Main Logic for Run_Contact -- Toggle ON / Toggle OFF ---
ON_OFF := (ONS_Trig AND NOT ON_OFF) OR (ON_OFF AND NOT ONS_Trig);
// Rising One Shot logic
Rising_ONS := Start_Stop;
END_PROGRAM
== Function block example ==
//=======================================================================
// Function Block Timed Counter : Incremental count of the timed interval
//=======================================================================
FUNCTION_BLOCK FB_Timed_Counter
VAR_INPUT
Execute : BOOL := FALSE; // Trigger signal to begin Timed Counting
Time_Increment : REAL := 1.25; // Enter Cycle Time (Seconds) between counts
Count_Cycles : INT := 20; // Number of Desired Count Cycles
END_VAR
VAR_OUTPUT
Timer_Done_Bit : BOOL := FALSE; // One Shot Bit indicating Timer Cycle Done
Count_Complete : BOOL := FALSE; // Output Bit indicating the Count is complete
Current_Count : INT := 0; // Accumulating Value of Counter
END_VAR
VAR
CycleTimer : TON; // Timer FB from Command Library
CycleCounter : CTU; // Counter FB from Command Library
TimerPreset : TIME; // Converted Time_Increment in Seconds to MS
END_VAR
// Start of Function Block programming
TimerPreset := REAL_TO_TIME(in := Time_Increment) * 1000;
CycleTimer(
in := Execute AND NOT CycleTimer.Q,
pt := TimerPreset);
Timer_Done_Bit := CycleTimer.Q;
CycleCounter(
cu := CycleTimer.Q,
r := NOT Execute,
pv := Count_Cycles);
Current_Count := CycleCounter.cv;
Count_Complete := CycleCounter.q;
END_FUNCTION_BLOCK