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Programmable Logic Controllers (PLC) 1. About Programmable Logic Controllers (PLC) Everyone knows there’s only one constant in the technology world, and that’s change. This is especially evident in the evolution of Programmable Logic Controllers (PLC) and their varied applications. From their introduction more than 30 years ago, PLC has become the comer stone of hundreds of thousands of control systems in a wide range of industries. At heart, the PLC is an industrialized computer programmed with highly specialized languages, and it continues to benefit from technological advances in the computer and information technology worlds. The most prominent of which is miniaturization and communications. Programmable logic controllers I/O channel specifications include total number of points, number of inputs and outputs, ability to expand, and maximum number of channels.? Number of points is the sum of the inputs and the outputs. PLC may be specified by any possible combination of these values.? Expandable units may be stacked or linked together to increase total control capacity.? Maximum number of channels refers to the maximum total number of input and output channels in an expanded system.? PLC system specifications to consider include scan time, number of instructions, data memory, and program memory.? Scan time is the time required by the PLC to check the states of its inputs and outputs.? Instructions are standard operations (such as math functions) available to PLC software.? Data memory is the capacity for data storage.? Program memory is the capacity for control software. Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs.? Outputs for PLC include DC, AC, relay, analog, frequency or pulse, transistor, triac.? Programming options for PLC include front panel, hand held, and computer. Programmable logic controllers use a variety of software programming lang