Programmable Logic Controller

A Programmable Logic Controller (PLC or programmable controller) is a somewhat dedicated application of a programmable computer. It is used to control and monitor machines. Everything from simple level and temperature indicators to complex positioning systems to machine vision can be handled from a PLC.

The earliest PLCs expressed all decision making logic in simple [Ladder Logic]. This was chosen mainly to reduce the apprehension of the existing technicians. These technicians were quite able to trace out circuit problems with schematic diagrams and ladder logic leveraged that ability.

Today, the line between a programmable computer and a PLC is thinning. With the [IEC-1138] standard, it is now possible to program these devices with flow charts, structured programming languages (such as C), and ladder logic.