PLC Architecture & Component

Plc Architecture & Component

The PLC like a computer ,employs a microprocessors chip to the processing and memory chips to store the program. The PLC consist of :

1. A Processor                       2. Memory

3. Inputs                                 4. Output's

5. Programming Devices    6. Power Supply

The input and the Output are connected via proper interfaces. Each input and output has its own address. an interface a medium where two system come together and interface or communicate, i.e., communication between the plant being controlled and the PLC. Figure 1 shows the basic Architecture of a PLC. it contains a Processor e.g. a microprocessor chip and memory chips. It also contains all the input and output interfacing. The programming devices can be either handheld or desktop which are remote from the PLC .

1.Processor :-

Very small microprocessors - integrated circuits with the tremendous computing capacity and also controlling capacity-provide the intelligence of today's programmable controllers. The processor unit contain a microprocessors chip e.g , ZigLog Z80,8085,8086 or any 16 or 32 bit microcontroller and necessary circuit to communicate with the device inside the PLC as well as outside the PLC. This word length effect the speed at which the Processor perform most operation. 

For example :- a 32-bit microprocessor can manipulate data faster than a 16 -bit Microprocessors.

Learn More about PlC click on below link PLC Input and Output

2. Memory

The memory system is area in PLC's CPU ,where all the sequence of instruction , or programs , are stored and executed by the Processor to Provide the desired control of Feild devices. PlC memory contain RAM, and EEPROM in varying amount depending upon the design and the use of PLC.  PlC memory, generally, is divided into five major application. Specifying the right amount of memory can save costs of hardware and time associated with adding additional Memory capacity later. 

These five type of memories are:

1. Executive Memory.   2 .System Memory

3. I/O Status Memory.   4.Data Memory

5. User Memory

Memory :-

The executive memory is a collection of permentally stored program that are consider part of the PlC itself. These  supervisory program direct all system activities, such as executive of the control program and communication with the peripheral devices. Executive program are nothing but the operating system which are placed in EPROM and these rarely need any change later on. The function of executive program are :-

(a) It instruct microprocessor to examine each user instruction.

(b)It convert high -level instruction into machine level instructions.

(c) If the user program required data from other area of memory, the executive instructs the microprocessor to collect those requested data.

(d)It provide all the system status and operating condition to the microprocessor.

(e) It solve the user logic program.

(f) It update all Output device accordingly.

In short, the executive program is a special machine level program that literally runs the PlC.

2. Status Memory

While performing various functions, the executive requires a place to store intermediate result and information. The system memory is allotted for the user of the executive only and not available to user.

3. I/O Status Memory 

During each scan, the executive instructs microprocessor to read the correct statuses of the input and then information is stored in "input image area". After solving user logic program, various Output device status are generated and are stored in" Output image area ". This area of memory always available for the user for monitoring and use. User van modify those statuses only through programming.

4. Data Memory

Random access memory does not retain it's contents of power is losed; therefore ,it is a volatile type of memory. Random access memory normally uses a battery backup to sustain it's contents in the event of power outage .

RAM is allocated for the timer, counter ,math and data function to store.

 5.User Memory

This is allocated for storage of user program. The executive has to scan this area to find the user instruction. If ASCII messages or some special subroutines are there , then Processor can allocate portion of this area for storage of those special programs.

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