PLC communicate with other devices through various types o communication links such as fiber optics, twisted pair, serial cables and coaxial. Twisted pair involves sending the use of two wires (twisted) for sending and receiving information.The two wires are twisted in order to reduce the chances of stray electro-magnetic signals. PLC systems also use Coaxial cable which has an inner conductor surrounded by a concentric conductor separated by a layer of insulation. The outer conductor is usually grounded. Besides twisted pair and coaxial cables, PLC also uses fiber optic as a communication link. Fiber optic involves pulsing of light signal to carry digital signal over a long distance. It is capable of transmitting more information at a higher speed as compared to copper wires used in twisted pair and coaxial cable.The PLC CPU consists of three major subsystems; Arithmetic and Logic Unit (ALU) , Control Unit (CU), and the Registers. It works by fetching instructions from a specified location into instruction registers and decoded by the control unit. ALU is involved with the arithmetic and logic operations of the CPU. The results are then stored in a special memory location. ALU does the mathematical operations such as addition, subtraction and basic logic operations like AND /OR. It’s a collection of logic gates with inputs, outputs and control bits for determining the type of operation to be done.Combinational logic systems are implemented by Boolean and lack memory since their output entirely depends on the input while sequential logic system uses memory and their output not only depends on the input but also the results of the previous input. Sequential logic system uses flip flops thus have states unlike combinational logic system.Resolution in the conversion of analogue input in to digital output indicates the number of discrete values that can be produced over a range of analog values. Analogue input are continuous and non discrete unlike digital outputIn star network topology, there is a central node to which all the other nodes are connected. Failure in one node does not affect the other nodes unless it is the central node. Star topology enables the administrator to centrally monitor all the networked devices via the central hub.Bus topology on the other hand is the least complicated given that it utilizes a single transition media from which the other devices are
Rehg, James A., and Glenn J. Sartori. Programmable logic controllers. 2nd ed. Upper Saddle
River, N.J.: Pearson Prentice Hall, 2009.
Evans, William Ted. Programmable logic controllers: fundamentals and applications.
Champaign, Ill.: Stipes Pub., 2006.
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