(* Sorting conveyor: reference solution in IEC 61131-3 Structured Text. *)

VAR_GLOBAL
  Start_PB     AT %IX0.0  : BOOL; (* Start pushbutton, normally open *)
  Stop_PB      AT %IX0.1  : BOOL; (* Stop pushbutton, normally closed, 1 when healthy *)
  PE_Entry     AT %IX0.2  : BOOL; (* Photo-eye at the feeder, 1 when a part is there *)
  PE_Pusher    AT %IX0.3  : BOOL; (* Photo-eye at the pusher station, sees every part *)
  Metal_Sensor AT %IX0.4  : BOOL; (* Inductive sensor at the pusher station, sees metal parts only *)
  Belt         AT %QX0.0  : BOOL; (* Conveyor motor contactor *)
  Pusher       AT %QX0.1  : BOOL; (* Reject pusher solenoid valve, 1 extends *)
END_VAR

PROGRAM Main
VAR
  arrived : R_TRIG;
  parts : UDINT; (* Parts seen at the entry since the last cold restart. *)
END_VAR
  Belt := (Start_PB OR Belt) AND Stop_PB;

  (* A metal part at the pusher station is pushed off. *)
  Pusher := PE_Pusher AND Metal_Sensor;

  arrived(CLK := PE_Entry);
  IF arrived.Q THEN
    parts := parts + 1;
  END_IF;
END_PROGRAM