(* Tank level control: reference solution in IEC 61131-3 Structured Text. *)

VAR_GLOBAL
  Auto_Mode   AT %IX0.0  : BOOL; (* Auto/manual selector, 1 is auto *)
  Level       AT %IW0    : INT; (* Level transmitter, 0 to 1000 is 0 to 100 percent *)
  LSH         AT %IX0.1  : BOOL; (* High level switch, 1 above 90 percent *)
  LSL         AT %IX0.2  : BOOL; (* Low level switch, 1 below 10 percent *)
  Fill_Valve  AT %QX0.0  : BOOL; (* Fill valve solenoid, 1 opens *)
  High_Alarm  AT %QX0.1  : BOOL; (* High level alarm lamp *)
  Level_Meter AT %QW0    : INT; (* Panel meter, 0 to 1000 *)
END_VAR

PROGRAM Main
  (* Level reads 0 to 1000 for 0 to 100 percent. Fill from 40 percent up to 80, *)
  (* and never past the high switch or out of auto.                          *)
  IF Auto_Mode AND Level < 400 THEN
    Fill_Valve := TRUE;
  END_IF;
  IF Level > 800 OR LSH OR NOT Auto_Mode THEN
    Fill_Valve := FALSE;
  END_IF;

  Level_Meter := LIMIT(0, Level, 1000);
  High_Alarm := LSH;
END_PROGRAM