Main Content

Model a Traffic Light by Using Moore Semantics

This example uses:

Open Model

This example shows how to use Moore semantics to model a traffic light. Moore charts compute outputs only in states, not in transitions. For more information, see Design Considerations for Moore Charts.

Logic of the Moore Traffic Light

In this example, the traffic light model contains a Moore chart called Light_Controller, which operates in five traffic states. Each state represents the color of the traffic light in two opposite directions, North-South and East-West, and the duration of the current color. The name of each state represents the operation of the light viewed from the North-South direction.

This chart uses temporal logic to regulate state transitions. The after operator implements a countdown timer, which initializes when the source state is entered. By default, the timer provides a longer green light in the East-West direction than in the North-South direction because the volume of traffic is greater on the East-West road. The green light in the East-West direction stays on for at least 20 clock ticks, but it can remain green as long as no traffic arrives in the North-South direction. A sensor detects whether cars are waiting at the red light in the North-South direction. If so, the light turns green in the North-South direction to keep traffic moving.

The Light_Controller chart behaves like a Moore machine because it updates its outputs based on current state before transitioning to a new state:

Stop — Traffic light is red for North-South, green for East-West.

  • Sets output y1 = RED (North-South) based on current state.

  • Sets output y2 = GREEN (East-West) based on current state.

  • After 20 clock ticks, active state becomes StopForTraffic.

StopForTraffic — Traffic light has been red for North-South, green for East-West for at least 20 clock ticks.

  • Sets output y1 = RED (North-South) based on current state.

  • Sets output y2 = GREEN (East-West) based on current state.

  • Checks sensor.

  • If sensor indicates cars are waiting ([sens] is true) in the North-South direction, active state becomes StopToGo.

StopToGo — Traffic light must reverse traffic flow in response to sensor.

  • Sets output y1 = RED (North-South) based on current state.

  • Sets output y2 = YELLOW (East-West) based on current state.

  • After 3 clock ticks, active state becomes Go.

Go — Traffic light has been red for North-South, yellow for East-West for 3 clock ticks.

  • Sets output y1 = GREEN (North-South) based on current state.

  • Sets output y2 = RED (East-West) based on current state.

  • After 10 clock ticks, active state becomes GoToStop.

GoToStop — Traffic light has been green for North-South, red for East-West for 10 clock ticks.

  • Sets output y1 = YELLOW (North-South) based on current state.

  • Sets output y2 = RED (East-West) based on current state.

  • After 3 clock ticks, active state becomes Stop.

Design Rules in Moore Traffic Light

This example of a Moore traffic light illustrates these Moore design rules:

  • The chart computes outputs y1 and y2 in state actions.

  • The chart tests the input sens in conditions on transitions.

  • The chart uses temporal logic, but no asynchronous events.

Related Topics