(* Conveyor Counters: CTU, CTD and CTUD Parts Buffer — Conveyor scene
   X1 = Start (N.O.)   X2 = Stop (N.C., TRUE when not pressed)
   X3 = Left sensor (load end)   X4 = Right sensor (drop-off end)
   X5 = Part Removed   X6 = Reset Counts  (click these two in the I/O panel)
   Y1 = Motor run   Y2 = Direction (TRUE = reverse, towards the load end)
   Every arrival at the drop-off delivers one part to a buffer. The conveyor
   holds at the load end when the batch of 10 is done or the buffer is full (3). *)
VAR
  ConveyorRun : BOOL;    // sealed-in run request
  Reverse     : BOOL;    // direction latch
  HoldAtLoad  : BOOL;    // do not send another part
  Delivered   : CTU;     // parts delivered, batch of 10
  Remaining   : CTD;     // parts left in the batch, counts down from 10
  Buffer      : CTUD;    // parts waiting in the buffer
END_VAR

(* Seal-in — X2 is N.C., so it is TRUE until Stop is pressed *)
ConveyorRun := (X1 OR ConveyorRun) AND X2;

(* Direction — reverse at the drop-off, forward again at the load end *)
IF ConveyorRun AND X4 THEN
  Reverse := TRUE;
END_IF;
IF ConveyorRun AND X3 THEN
  Reverse := FALSE;
END_IF;

(* CTU — counts every rising edge of X4. Q turns on at PV *)
Delivered(CU := X4, R := X6, PV := 10);

(* CTD — the IEC CTD has a load input: LD copies PV into CV, then each
   rising edge of CD counts down. Q turns on when CV reaches 0.
   Press Reset Counts (X6) once to load it. *)
Remaining(CD := X4, LD := X6, PV := 10);

(* CTUD — up on X4, down on X5, never below 0. QU = full, QD = empty *)
Buffer(CU := X4, CD := X5, R := X6, PV := 3);

(* Hold at the load end when the batch is done or the buffer is full *)
HoldAtLoad := X3 AND (Delivered.Q OR Buffer.QU);

Y1 := ConveyorRun AND NOT HoldAtLoad;
Y2 := Reverse;