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5CHAPTER 2 LITERATURE REVIEW 2.1Basic of Programmable Logic Controllers Programmable logic controller (PLC) is a digital electronic device that uses a programmable memory to store instruction and to implement specific functions such as logic, sequence, timing, counting and arithmetic to control machines and process. Figure 2.0 shows Block Diagram of PLC.  Figure 2.0: Block Diagram of PLC. 2.1.1 The power supply The power supply gives the needed voltage and current to each component in the system. If the there have no power, the PLC does not operate. This power supply basically have a special characteristic which usually have the back up unit that give the power while the electricity cut down suddenly. Programming Console PowerInput interfaceMemoryCentral ProcessingUnit (CPU) Output interfaceSignals from Switches, Sensors etc Signals to Solenoids, Motors etc Programming
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62.1.2Control Processing Unit (CPU) The Central Processing Unit (CPU) is a microprocessor that co-ordinates and monitor all the activities of the PLC system. It executes the program which stored in the memory. There have one communication bus system which brings the data to or from CPU to the input or output memory units which govern by CPU. CPU have been provided with a clock which it frequency control by external quartz crystal or oscillator RC. The typical frequency was depends on the type of microprocessor and usage. Clock determines the speed of PLC operation which provides timing or synchronization for all elements in the system.  2.1.3 Memory There are various type of memory unit; Read-Only Memory (ROM), Random Access Memory (RAM), Erasable Programmable Read Only Memory (EPROM) and Electrical Erasable Programmable Read Only Memory (EEPROM). It is the area that holds the operation system and user memory. The operating system is actually a system software that co-ordinates the PLC. Ladder program, Timer and Counter Values are stored in the user memory.  2.1.4Output and Input Interface Input devices (e.g. mechanical contact, proximity switches) and output devices (e.g. motor, solenoids) from the machine or process to be controlled are connected to the PLC. A user enters a sequence of instructions (known as the program) into the PLC’s program memory. Then, the controller monitors the state of the inputs and switches outputs according to the users program. The stored program can be modified so the new control features can be added or old ones changed without rewiring the input and output.