# Model a Traffic Light by Using Moore Semantics

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.