what is Programmable Logic Controller(PLC)?

A programmable logic controller (PLC) is a microcomputer which is designed to control processes and events in an industrial environment-that is,to take over the job previously done with relay logic controllers.so we can say PLC is the replacement of relay logic controller.
PLC  has some advantages-
Programmable Logic Controller (PLC)  is suitable for rugged design harsh industrial environments against   high temperature variations, dust, and vibrations
it has    Industry standard I/O interfaces;
capable of communicating with other PLCs, computers and intelligent devices.
Industry standard programming languages; easily learned and  understood. Programming is primarily concerned with logic, timing, counting and switching operations.
 Field programmable.
 Reduces hard wiring and wiring cost.                                                                                                            Monitoring, error checking and diagnostics capability.
Competitive in both cost and space requirements. 

Basic Components:
 PLC is essentially a microcomputer, tailored specifically for certain  control tasks.
 Hardware: consists of the actual device technology, i.e. the PCBs,  integrated modules, wires, battery, housing etc.
Firmware: is the software part, known as executive software, that   is permanently installed and supplied by the the PLC  manufacturer. Programs are usually stored in ROM or EPROM.
Software:is is the user program.user program usually stored in the RAM.
 What is Firmware? 
 PLC hardware does not differ significantly from computers. What  makes the PLC special is the executive software. It is the internal  program, provided by the manufacturer,which executes the user’s program.       The executive software determines                                                                 
-  what functions are available to the user’s program,
-  how the program is solved,                                                                      
-  how the I/O is serviced,
-  what the PLC does during power up and down and fault conditions.
                                                                                                                                                       Multitasking Capability
Some PLCs are capable of executing multiple tasks with a single processor.user  program assigns I/O for each task separately.
Multitasking may take several forms:
Time driven - it is possible to configure the processor to run each task on periodic time intervals.
Event driven or interrupt driven-user defines a particular event,such as input changing state or an output turning off,that causes each tasks to run.



Considerations in Choosing PLC :
1.Number and Types of input & output points required
2.Size and type of memory required
3.Speed and power required of CPU and instruction set
4.Manufacturer’s support and backup
PLCs basic hardware are-
power supply,memory,Central processing unit(CPU)/logic solver,I/O interface ,programming section.
Memory:The memory function of CPU stores program and data that's the CPU need to perform various operations.The memory is to organized into several sections-

• Executive memory
• Scratch pad
• Processor file

Central processing unit(CPU):
CPU execute a program stored in executive the memory is set by the manufacturer.It organize all control activities by receiving inputs,performing logical decisions according to the program,and control the output.CPU does not operate I/O directly,its works with the I/O image stored in the I/O image memory.The I/O interface is responsible for transferring the image outputs to the I/O system.A "watchdog " timer is provided to time the CPU to executed the user's program.