Designing systems

Abstract

This chapter considers how programs are designed and how these programs and a programmable logic controller (PLC) system can be tested. It involves consideration of both the hardware and the software. Some of the steps involved in a systematic design technique include a definition of what is required with the inputs and outputs specified, a definition of the algorithm to be used, translation of algorithms into instructions that can be inputted to the PLC, and testing and debugging of the program. To test input devices such as switches, they can be manipulated to give the open and closed contact conditions and the corresponding LED on the input module observed. It should be illuminated when the input is closed and not illuminated when it is open. Failure of an LED to illuminate could be due to the fact that the input device is not correctly operating, there are incorrect wiring connections to the input module, the input device is not correctly powered, or the LED or input module is defective. For output devices that can be safely started, pushbuttons can be installed, so that each output can be tested. Another method that can be used to test inputs and outputs is termed forcing. This involves software, rather than mechanical switching on or off, being used with instructions from the programming panel to turn off or on inputs/outputs.