(* Traffic light junction: reference solution in IEC 61131-3 Structured Text. *)

VAR_GLOBAL
  NS_Red   AT %QX0.0  : BOOL; (* North-south red lamp *)
  NS_Amber AT %QX0.1  : BOOL; (* North-south amber lamp *)
  NS_Green AT %QX0.2  : BOOL; (* North-south green lamp *)
  EW_Red   AT %QX0.3  : BOOL; (* East-west red lamp *)
  EW_Amber AT %QX0.4  : BOOL; (* East-west amber lamp *)
  EW_Green AT %QX0.5  : BOOL; (* East-west green lamp *)
END_VAR

TYPE Stage : (NS_GO, NS_WAIT, EW_GO, EW_WAIT); END_TYPE

PROGRAM Main
VAR
  stage : Stage := NS_GO;
  timer : TON;
  hold : TIME;
END_VAR
  (* How long each phase lasts. *)
  CASE stage OF
    NS_GO, EW_GO: hold := T#5s;
    NS_WAIT, EW_WAIT: hold := T#2s;
  END_CASE;

  timer(IN := TRUE, PT := hold);
  IF timer.Q THEN
    timer(IN := FALSE);
    CASE stage OF
      NS_GO: stage := NS_WAIT;
      NS_WAIT: stage := EW_GO;
      EW_GO: stage := EW_WAIT;
      EW_WAIT: stage := NS_GO;
    END_CASE;
  END_IF;

  (* One lamp per head, decided only by the stage, so two greens cannot happen. *)
  NS_Green := stage = NS_GO;
  NS_Amber := stage = NS_WAIT;
  NS_Red := stage = EW_GO OR stage = EW_WAIT;
  EW_Green := stage = EW_GO;
  EW_Amber := stage = EW_WAIT;
  EW_Red := stage = NS_GO OR stage = NS_WAIT;
END_PROGRAM