(* Motor start/stop station: 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 *)
  EStop_OK AT %IX0.2  : BOOL; (* Emergency stop, normally closed, 1 when healthy *)
  Motor    AT %QX0.0  : BOOL; (* Motor contactor *)
  Run_Lamp AT %QX0.1  : BOOL; (* Running indicator *)
END_VAR

PROGRAM Main
  (* Start with a seal-in. Stop_PB and EStop_OK are normally closed, *)
  (* TRUE while healthy, so a broken wire stops the motor too.       *)
  Motor := (Start_PB OR Motor) AND Stop_PB AND EStop_OK;
  Run_Lamp := Motor;
END_PROGRAM