Programmable Logic Controllers

Programmable logic controllers are commonly known throughout most of the world as PLCs. These controllers have taken automation to a new high since being introduced by Modicon, for General Motors, during the late 1960s. PLCs are microprocessors that have replaced miles of hard wiring and scores of relays and timers in factories.

These devices are the control hubs for various types of automated systems and processes. PLCs contain numerous inputs and outputs which use transistors and other electronic circuitry to simulate switches and relays to control equipment. You can program PLCs through standard computer interfaces and proprietary languages and network options.

Programmable logic controllers I/O channel specifications include the total number of points, the number of inputs and outputs, the ability to expand, and the maximum number of channels. The number of points is the total amount of the inputs and the outputs. PLCs may be specified by any possible combination of these values. Expandable units can be stacked or linked together to increase total control capacity. Maximum number of channels refers to the maximum number of input and output channels in an expanded system. Scanning time is the time required to verify the input and output states and the data memory tells you how much data can be stored.

You can use various inputs for PLCs such as AC, DC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs. Outputs that can be used for PLCs include AC, DC, relay, analog, frequency or pulse, transistor, and triac. The various programming options for PLCs include front panel, hand held, and computer.

You can also use numerous types of software programming languages for controlling PLCs. This means PLCs are compatible for different manufacturers and they can also be programmed in various languages. PLCs control items such as sensors, actuators, valves, cams, etc. using Boolean logic. Different values of voltage are interpreted by the PLC as: 1 equals the ON state and 0 equals the OFF state.

While selecting a PLC you need to consider the number of points, the maximum number of channels and how many instructions are to be run, scanning time, memory, etc. PLCs can be specified with a number of computer interface options, network specifications and features. PLC power options, mounting options and environmental operating conditions are all also important to consider.


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