{{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).<ref>{{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}}</ref><ref>{{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}}</ref> It is a high level language that is block structured and syntactically resembles Pascal, on which it is based.<ref name="nlpaper">{{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 }}</ref> 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:
* Iteration loops (REPEAT-UNTIL; WHILE-DO) * Conditional execution (IF-THEN-ELSE; CASE)<ref name="nlpaper"/> * Functions (SQRT(), SIN())
==Sample program==
<syntaxhighlight lang="pascal"> (* simple state machine *) TxtState := STATES[StateMachine];
CASE StateMachine OF 1: ClosingValve(); StateMachine := 2; 2: OpeningValve(); ELSE BadCase(); END_CASE; </syntaxhighlight>
Unlike in some other programming languages, there is no fall through 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=== <syntaxhighlight lang="pascal"> // 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 </syntaxhighlight>
==== Function block example ==== <syntaxhighlight lang="actionscript" line="1"> //======================================================================= // 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 </syntaxhighlight>
==References== {{reflist}}
<!--Categories--> Category:IEC standards Category:Programmable logic controllers Category:Programming languages Category:Pascal programming language family