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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.

Note: Structured design is very important in engineering, but many engineers will write software without taking the time or effort to design it. This often comes from previous experience with programming where a program was written, and then debugged. This approach is not acceptable for mission critical systems such as industrial controls. The time required for a poorly designed program is 10% on design, 30% on writing, 40% debugging and testing, 10% documentation. The time required for a high quality program design is 30% design, 10% writing software, 10% debugging and testing, 10% documentation. Yes, a well designed program requires less time! Most beginners perceive the writing and debugging as more challenging and productive, and so they will rush through the design stage. If you are spending time debugging ladder logic programs you are doing something wrong. Structured design also allows others to verify and modify your programs.

Axiom: Spend as much time on the design of the program as possible.

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