Again we dedicate this post to the beginner PLC programmers.
Almost all PLC programs use to follow sequential structure (on state following to another one).
In fact SFC language is based on this concept.

Unfortunately SFC is not always enough user friendly for beginners, and there´s no manuals who show that kind of tricks when you start programming.
So or you have a experienced teacher, or you´re going to expend a lot of time trying to find out the easy way to do it.

In this case, we want to make the thinks easy, so we made the typical traffic lights program following the Sequential structure.
Once you understand the way of programming, enter to SFC or even Function blocks it will be a piece of cake ;)

Step 1: Establish the different states

Step 2: Put the different states in order (flux diagram)

Step 3: Add transition conditions

Step 4: Program it!!

The advantage of this method, are the following:
1. If you programs freeze, you can find the bug easily checking the program index.
2. You can debug each state just changing the MOV value.
3. You can add middle steps easily.
4. you can reboot our program only changing the program index to 0.

I should recommend to try it by your own, and once you finish, you can download the solution
Programming Sequential Solution ISPsoft & WPLSoft

And let us know if it has been useful!