2.1 Step 1: Identify States, Inputs, and Outputs

The first step in solving a state machine is to identify the possible states, inputs and outputs of the system. In the case of a basic traffic light system there are five states. These states, along with their corresponding encodings, are summarized in Table 2-1.

Table 2-1. Traffic Light State Machine State Table
State	Description	Binary Encoding
North/South (N/S) GO	North/South bound traffic can proceed. East/West traffic must stop.	`000`
North/South (N/S) SLOW	North/South traffic is about to transition to stop state. Traffic should slow down and proceed with caution.	`001`
East/West (E/W) GO	East/West traffic can proceed. North/South traffic must stop.	`010`
East/West (E/W) SLOW	East/West traffic is about to transition to stop state. Traffic should slow down and proceed with caution.	`011`
Error	An emergency or error has occurred and traffic should stop at the intersection and proceed with caution	`100`

The outputs of the system can also be summarized as in Table 2-2.

Table 2-2. Traffic Light State Machine Output Table
Name	Description
NSG	1 = North/South Green and East/West Red ON 0 = North/South Green and East/West Red OFF
NSY	1 = North/South Yellow and East/West Red ON 0 = North/South Yellow and East/West Red OFF
EWG	1 = East/West Green and North /South Red ON 0 = East/West Green and North/South Red OFF
EWY	1 = East/West Yellow and North/South Red ON 0 = East/West Yellow and North/South Red OFF
ERR	1 = All red lights flashing ON/OFF 0 = No error state

This traffic light system will only have one input to switch the system into the emergency state:

Table 2-3. Traffic Light State Machine Input Table
Name	Description
E	0 = Emergency is not present 1 = Emergency is present