Structured text

<h2 id="sample-program">Sample program</h2>
(* simple state machine *)
TxtState := STATES[StateMachine];

CASE StateMachine OF
   1: ClosingValve();
      StateMachine := 2;
   2: OpeningValve();
ELSE
    BadCase();
END_CASE;

<p>Unlike in some other programming languages, there is no <a href="/facts/Switch_statement/oQCWnGWh">fallthrough</a> for the CASE statement: the first matching condition is entered, and after running its statements, the CASE block is left without checking other conditions.
</p>
<h3>Additional ST programming examples</h3>
// 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

<h4>Function block example</h4>
//=======================================================================
// 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

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Bacidore, Mike (16 May 2018). "Should I limit programming to ladder logic or use all standards within IEC 61131?". Control Design. <a href="https://www.controldesign.com/articles/2018/should-i-limit-programming-to-ladder-logic-or-use-all-standards-within-iec-61131/" target="_blank">https://www.controldesign.com/articles/2018/should-i-limit-programming-to-ladder-logic-or-use-all-standards-within-iec-61131/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Stevic, Tom (5 May 2017). "A very short history of PLC programming platforms". Control Design. <a href="https://www.controldesign.com/articles/2017/a-very-short-history-of-plc-programming-platforms/" target="_blank">https://www.controldesign.com/articles/2017/a-very-short-history-of-plc-programming-platforms/</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Roos, Nieke. "Programming PLCs using Structured Text". Department of Computing Science, University of Nijmegen. CiteSeerX 10.1.1.49.2016. {{cite journal}}: Cite journal requires |journal= (help) <a href="/wiki/CiteSeerX_(identifier)" target="_blank">/wiki/CiteSeerX_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Roos, Nieke. "Programming PLCs using Structured Text". Department of Computing Science, University of Nijmegen. CiteSeerX 10.1.1.49.2016. {{cite journal}}: Cite journal requires |journal= (help) <a href="/wiki/CiteSeerX_(identifier)" target="_blank">/wiki/CiteSeerX_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
</ol>

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