04Level 2

Crossroad traffic lights

Two coordinated traffic lights driven by a single cycle timer and compare contacts on its elapsed time.

The simulator is designed for computers (screens 1024 px or wider).

Virtual plant

Shown at rest. In the simulator the plant moves according to your program's outputs.

Inputs and outputs

Address	Name	Description
I0.0	ON	Operation switch
Q0.0	RED_A	Red light, street A
Q0.1	AMBER_A	Amber light, street A
Q0.2	GREEN_A	Green light, street A
Q0.3	RED_B	Red light, street B
Q0.4	AMBER_B	Amber light, street B
Q0.5	GREEN_B	Green light, street B
T0	CYCLE	20 s cycle timer
M0.0	CYCLE_END	Pulse at the end of each cycle

Program

Ladder diagram (LD)

Rung 1 — Self-restarting 20 s cycle timer.
Rung 2 — Street A: green from 0 to 8 s.
Rung 3 — Street A: amber from 8 to 10 s.
Rung 4 — Street A: red from 10 to 20 s.
Rung 5 — Street B: red from 0 to 10 s.
Rung 6 — Street B: green from 10 to 18 s.
Rung 7 — Street B: amber from 18 to 20 s.

Structured text (ST)

(* Crossroad traffic lights — converted from Ladder *)

VAR
  CYCLE : TON;
END_VAR

(* Rung 1: Self-restarting 20 s cycle timer. *)
CYCLE(IN := ON AND NOT CYCLE_END, PT := T#20s);
CYCLE_END := CYCLE.Q;

(* Rung 2: Street A: green from 0 to 8 s. *)
GREEN_A := ON AND T0.ET < T#8s;

(* Rung 3: Street A: amber from 8 to 10 s. *)
AMBER_A := ON AND T0.ET >= T#8s AND T0.ET < T#10s;

(* Rung 4: Street A: red from 10 to 20 s. *)
RED_A := ON AND T0.ET >= T#10s;

(* Rung 5: Street B: red from 0 to 10 s. *)
RED_B := ON AND T0.ET < T#10s;

(* Rung 6: Street B: green from 10 to 18 s. *)
GREEN_B := ON AND T0.ET >= T#10s AND T0.ET < T#18s;

(* Rung 7: Street B: amber from 18 to 20 s. *)
AMBER_B := ON AND T0.ET >= T#18s;

Instruction list (IL)

(* Crossroad traffic lights — converted from Ladder *)

VAR
  CYCLE : TON;
  RUNG_1 : BOOL;
END_VAR

(* Rung 1: Self-restarting 20 s cycle timer. *)
  LD    ON
  ANDN  CYCLE_END
  ST    RUNG_1
  CAL   CYCLE(IN := RUNG_1, PT := T#20s)
  LD    CYCLE.Q
  ST    CYCLE_END

(* Rung 2: Street A: green from 0 to 8 s. *)
  LD    ON
  AND(  T0.ET
    LT    T#8s
  )
  ST    GREEN_A

(* Rung 3: Street A: amber from 8 to 10 s. *)
  LD    ON
  AND(  T0.ET
    GE    T#8s
  )
  AND(  T0.ET
    LT    T#10s
  )
  ST    AMBER_A

(* Rung 4: Street A: red from 10 to 20 s. *)
  LD    ON
  AND(  T0.ET
    GE    T#10s
  )
  ST    RED_A

(* Rung 5: Street B: red from 0 to 10 s. *)
  LD    ON
  AND(  T0.ET
    LT    T#10s
  )
  ST    RED_B

(* Rung 6: Street B: green from 10 to 18 s. *)
  LD    ON
  AND(  T0.ET
    GE    T#10s
  )
  AND(  T0.ET
    LT    T#18s
  )
  ST    GREEN_B

(* Rung 7: Street B: amber from 18 to 20 s. *)
  LD    ON
  AND(  T0.ET
    GE    T#18s
  )
  ST    AMBER_B

Step by step

1Press ▶ Run (F5) and turn on the ON switch (I0.0).
2Rung 1 is a 20 s timer that restarts itself: when it finishes, CYCLE_END turns on, its NC contact resets the timer and everything starts over.
3The other rungs compare the elapsed time (CYCLE.ET) with fixed limits: green A from 0 to 8 s, amber A from 8 to 10 s, red A from 10 to 20 s; street B does the opposite.
4Watch T0.ET in the monitor and check that two green lights are never on together. Try changing the times in the compare contacts.

Open in the simulator