PLCTraining.pptVIP

WHAT EXACTLY IS A PLC ?;CPU ;The Input/Output System;The rack is an enclosure with slots in it that is connected to the CPU. I/O modules are devices with connection terminals to which the field devices are wired. Together, the rack and the I/O modules form the interface between the field devices and the PLC. When set up properly, each I/O module is both securely wired to its corresponding field devices and securely installed in a slot in the rack. This creates the physical connection between the field equipment and the PLC. In some small PLCs, the rack and the I/O modules come prepackaged as one unit.;All of the field devices connected to a PLC can be classified in one of two categories: ? inputs ? outputs Inputs are devices that supply a signal/data to a PLC. Typical examples of inputs are push buttons, switches, and measurement devices. Basically, an input device tells the PLC, “Hey, something’s happening out here…you need to check this out to see how it affects the control program.” Outputs are devices that await a signal/data from the PLC to perform their control functions. Lights, horns, motors, and valves are all good examples of output devices. These devices stay put, minding their own business, until the PLC says, “You need to turn on now” or “You’d better open up your valve a little more,” etc.;There are two basic types of input and output devices: ? discrete ? analog Discrete devices are inputs and outputs that have only two states: on and off. As a result, they send/receive simple signals to/from a PLC. These signals consist of only 1s and 0s. A 1 means that the device is on and a 0 means that the device is off. Analog devices are inputs and outputs that can have an infinite number of states. These devices can not only be on and off, but they can also be barely on, almost totally on, not quite off, etc. These devices send/receive complex signals to/from a PLC. Their communications consist of a variety of signals, not just 1s and 0s.;The control program i