Parts or Components of PLC

Hi All,

We have always seen the PLC in a compact form now lets unfold what is it made up of it would be shocking to know that it consists of nothing more than some sensors relays and a very well programmed microrcontroller.

What PLC does.



The PLC has two types of ports namely Input and Output. The input ports are connected various sensors which continuously monitor the states of the the devices and if the value exceeds the threshold they give rise to events It is these events which are monitored and controlled via the output ports via various controllers like the ones showed in the pic above which consists of the motors,shafts valves and other controllers name it and you have it with utmost accuracy.




The various parts of PLC are






The basic PLC has only a few parts. “First, a central processing unit (CPU), the CPU is the brain of the whole operation. A typical processor usually consists of a microprocessor for implementing the logic and controlling the communications among the modules. The processor requires memory for storing the results of the logical operations performed by microprocessor. Memory is also required for the program EPROM or EEPROM plus RAM. The CPU controls all PLC activity and is designed so that the user can enter the desired program is relay ladder logic.”[2] Next, a power supply that supplies power to other modules that are plugged into the rack. The input and output (I/O) portions are just that, the inputs and the outputs. “The I/O system forms the interface by which field devices are connected to the controller.


The purpose of this interface is to condition the various signals received from or sent to external field devices.Finally we have a programming device which is used to enter the desired program into the memory of the processor. “A personal computer (PC) is the most commonly used programming device. Most brands of PLCs have software available to that a PC can be used as the programming device. The PC communicates with the PLC processor via a serial or parallel data communications link, or Ethernet. If the programming unit is not in use, it may be unplugged and removed. A program is a user-developed series of instructions that directs the PLC to execute actions. A programming language provides rules for combining the instructions so that they produce the desired actions. Relay ladder logic (RLL) is the standard programming language used with PLCs.