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Model a Traffic Light by Using Moore Semantics

This example shows how to use Moore semantics to model a traffic light.

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:

When initial state Stop is active. 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.

In active state 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.

In active state 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.

In active state 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.

In active state 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 in state actions.

  • The chart tests inputs in conditions on transitions.

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

  • The chart defines chart inputs (sens) and outputs (y1 and y2).

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