Introduction 01 This is the first episode in an 8 part series. 02 This series is on programmable logic controllers, or PLCs as they are commonly known. What is a PLC you ask? In short, it is a general purpose programmable industrial control device. They are used all through industry in factories, utilities, and anywhere industrial machines need to be controlled. 03 They are a fully integrated system of hardware, software, and development environment tailored specifically for automatic industrial equipment. It is probably easiest to describe them by first giving a bit of historical background as to what preceded them. 04 The Early Days of Automation You have probably heard of the Jacquard loom of the early 19th century and its use of punch cards with respect to it being one of the technologies which eventually lead to computing. However, for our purposes here, its significance is that it was an early form of industrial automation control system, since it was after all controlling a machine in a factory. 05 For other types of machines, a common purely mechanical means of having the various parts of a complex machine move in a coordinated manner was through the use of shafts and cams. A series of cams located on a shaft or set of shafts connected by gears, could move parts of a machine, turn valves on or off, and generally coordinate the parts of a machine. With the spread of the use of electricity in factories in the early 20th century, it was now possible to use the new electrical technology to perform control and automation functions. 06 The Background of Relay Logic I will need to explain some electromechanical terminology here as it is necessary to understand these terms and the concepts behind them in order to understand PLCs, as the latter is an evolution of its predecessors and uses the same terms. What preceded PLCs was what was commonly known as "relay logic". 07 What is a Relay A relay is an electromechanical device that responds to and controls the flow of electric current. A relay has a "coil" which as the name implies is a coil of conductive wire. When you energize the coil, that is you apply electric current to it, it forms an electromagnet. 08 This electromagnet draws in an armature. The armature is a moving piece of metal which is attracted to the coil when the latter is energized. The relay being activated is referred to as closing. Deactivating is referred to as opening. Alternative names for these states are "pulls in" and "drops out" respectively. 09 The armature in turn is attached to one or more contacts. You can think of a contact as being like a switch. When the switch is turned one way, the electrical path is closed and current can flow. When it is turned the other way, the electrical path is broken and current flow is interrupted. 10 Contacts can be normally open, in which case the path is closed and current can flow when the relay turns on. Alternatively, they can be normally closed, in which case the path is closed and current can flow when the relay turns off. A relay can, and typically does, have multiple contacts, including both normally open and normally closed. Remember these terms, coil, contact, normally open, and normally closed. 11 What is a Latching Relay There is a special type of relay known as a