The basic idea was to design a hybrid PLC with comparable performance to dedicated industrial hybrid PLC’s designed by big companies. Designed hybrid PLC consists of five modules namely input and PLC module, output and PLC module, switch module, power supply module and relay module.
Input and PLC module consists of LPT parallel port inputs from the PC to the microcontroller through opto-couplers used for signals isolation. Output and PLC module consists of microcontroller outputs to the relay via serial resistors combination and CE transistors. The switch module consists switches which connect different AC loads and this hybrid PLC’s appliances. The power supply module gives the two different output levels of DC voltage of 5V and 12V. The relay module consists of relays which are activated DC signals with taken from the collector of CE transistors to control alternating current.
Infect microcontroller is the heart of hybrid PLC which is connected in between to the relays and PC while relays are made as intermediary components between as the PLC and loads. Parallel port can be programmed to write data out and to read data in. There are three locations of possibility in which the parallel printer port registers can reside namely, 0x278, 0x3BC and 0x378 (most common).
The time delay for parallel port in C++ language programming and for the microcontroller in the assembly language is set equal for correct switching of the relays. PC and the AC loads are vital and needed to be cautiously designed to fulfill the requirements of desired operation communication among the PLC. The AC loads and PLC are interfaced via eight relay contacts. Each relay was powered by a 12 VDC from external power supply. The signals are sent from the PC keyboard to trigger the PLC and its output are connected to each relay to the AC load’s switch.