A PLC is a microprocessor based, user friendly, specialized computer that can control multi-functions of different complexity levels. It can operate any system with output devices that turn off and on, as well system with variable outputs. PLC has become widely used in industry due to its easy operation and powerful functions. The advantages of PLC it makes the brain of modern industrial control systems that if a logical relationship between outputs and inputs can be achieved then many potential faults and combinations of faults can be avoided and PLC provides this logic relationship between inputs and outputs.
This article describes how to control the different AC loads with PC. Parallel interfacing was made between the PC via printer port and the designed PLC. A designed PLC connected to PC or to another PLC is called hybrid PLC. Microcontroller is the brain of the PLC was used to connect the PC parallel port circuitry to the electromagnetic relay circuitry to which AC loads were attached.
Microcontroller transmits the signals to the output according to its incoming parallel port signals and its own programming which is made in language assembly while the parallel port was accessed in C++ language programming using outport instruction. Parallel port can be programmed and accessed in assembly language. Serial port sends data bit wise while parallel port transfers data byte wise 8 bit simultaneously.
The parallel port has five modes of communication operation. In the current study compatibility mode is used to interface the PLC. Because printer parallel port contains of eight pins therefore its any concerned pin can be partially activated by changing the data amount. Switch statement is used in C++ language programming and in its case the pin is turned on first and after some delays it is turned off again for the proper relays switching.