Traditionally ladder logic programs have been written by thinking about the process and then beginning to write the program. This always leads to programs that require debugging. And, the final program is always the subject of some doubt. Structured design techniques, such as Boolean algebra, lead to programs that are predictable and reliable. The structured design techniques in this and the following chapters are provided to make ladder logic design routine and predictable for simple sequential systems.
Most control systems are sequential in nature. Sequential systems are often described with words such as mode and behavior. During normal operation these systems will have multiple steps or states of operation. In each operational state the system will behave differently. Typical states include start-up, shut-down, and normal operation. Consider a set of traffic lights - each light pattern constitutes a state. Lights may be green or yellow in one direction and red in the other. The lights change in a predictable sequence. Sometimes traffic lights are equipped with special features such as cross walk buttons that alter the behavior of the lights to give pedestrians time to cross busy roads.
Sequential systems are complex and difficult to design. In the previous chapter timing charts and process sequence bits were discussed as basic design techniques. But, more complex systems require more mature techniques, such as those shown in Figure 9.1 Sequential Design Techniques. For simpler controllers we can use limited design techniques such as process sequence bits and flow charts. More complex processes, such as traffic lights, will have many states of operation and controllers can be designed using state diagrams. If the control problem involves multiple states of operation, such as one controller for two independent traffic lights, then Petri net or SFC based designs are preferred.