TABLE OF CONTENTS This chapter considers how programs are designed and how they and a PLC system can be tested and faults found. This involves consideration of both the hardware and the software.
Whatever the language in which a program is to be written, a systematic approach to the problem can improve the chance of high quality programs being generated in as short a time as possible. A systematic design technique is likely to involve the following steps:
A definition of what is required with the inputs and outputs specified.
A definition of the algorithm to be used. An algorithm is a step-by-step sequence which defines a method of solving the problem. This can often be shown by a flow chart or written in pseudocode, this involving the use of the words BEGIN, DO, END, IF-THEN-ELSE, WHILE-DO.
The algorithm is then translated into instructions that can be inputted to the PLC. Because programs are often very long and can end up difficult to write as a long single block and even more difficult to later follow for fault finding and maintenance, it is advisable to break the program down into areas which are then further subdivided until manageable size blocks of program occur. This is termed top-down design.
The program is then tested and debugged.
The program is documented so that any person using or having to modify the program at a later date understands how the program works.
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