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Introduction

Programmable controllers were originally designed to replace relay-based control systems and solid-state, hard-wired logic control panels. However, the modern programmable controllers system is far more complex and powerful.

The most basic function performed by programmable controllers is to examine the status of inputs and in response, control some process or machine through outputs. The logical combinations of inputs to produce an output or outputs are called control logic. Several logic combinations are usually required to carry out a control plan or program. This control plan is stored in memory using a programming device to input the program into the system. The processor, usually a high-speed microprocessor, in a predetermined sequential order, periodically scans the control plan in memory. The period required to examine the inputs and outputs, perform the control logic, and execute the outputs is called the "scan time."