PLC control system development through ladder diagram basically just used for modernizing of electromechanical relay control systems into the form of software.
For a new project, the representation of the control system through ladder diagram certainly will be very difficult to obtain (except for cases of automation or a simple control system).
To overcome this, a designer of control systems must be able to represent his ideas into the form of images relatively easy to understand. What is suitable for these cases? The answer is a State Diagram.
In this tutorial will illustrate how to build a model state diagram for a control system. An example is the flip-flop,
Specification of the system work very simple: Every 0.5 seconds, the situation will change lights alternately.
Now we represent the system through state diagrams. For this case the system obviously has only two states, see the image above. The system will transition every 0.5 seconds delay, the transition state in this case, of course, triggered by a timer (e.g. timer On Delay). A complete state diagram for the case you can see in the picture below.
From the picture above to know how to transform the state diagram into its equivalent ladder form:
When the PLC is first turned on, then the instructions will be executed PLC initialization (only once), then, by using instruction blocks if... then, we implement the control logic through instruction MCR (Master Control Relay). See details at the picture below.