Describe the basic structure of PLC and describe its components in detail.

The basic block diagram of PLC system is shown below.

1. CPU :- CPU is heart of PLC. RTOS help in continuously looping the program instead of serial communication. The work of CPU is to analyze the input given by user and execute the logic and do the calculations. After the calculations are done it controls the required O/P. The CPU work is divided into two parts i.e. Program files & Date files.

2. Mounting Rack :- It is a metal frame with PCB that provides means for mounting the PLC (I/O) modules & processor. The mounting rack provides data & power connections to the processor & modules via the backplane.

3. I/O modules :- I/O modules are associated with respect to application. There are discrete analog high speed control or register type. Analog I/P and O/P modules are specified according to the desired resolution. Thermocouple is directly accepted as analog module

4. Power Supply :- Depends upon the manufactures. A Power supply is capable of delivering all required power to the system as furnished part of processes. These system having inbuilt power supply where as there are system having required excessive power supply which is externally provided.

5. Programming unit :- It allows the user to enter and edit the program and also execute the same. There can be hard held device with keypad viewing the program steps. Communication with programmable controller with this system is done via cable connected to a special programing port.

6. Data Storage :- Data storage is divided into five different parts. Basic division is in two parts RAM and ROM. But the PLC has five different types such as Executive memory, System memory RAM, I/O status memory RAM, Data memory RAM, user memory RAM, EPROM. Depending upon its use the size of memory is allocated.