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). It is a high level language that is block structured and syntactically resembles Pascal, on which it is based. 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)
- Functions (SQRT(), SIN())
(* 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
- Bacidore, Mike (16 May 2018). "Should I limit programming to ladder logic or use all standards within IEC 61131?". Control Design.
- Stevic, Tom (5 May 2017). "A very short history of PLC programming platforms". Control Design.
- Roos, Nieke. "Programming PLCs using Structured Text". Pennsylvania State University. Department of Computing Science, University of Nijmegen.