INTERNAL ARCHITECTURE OF PLC(programmable logic controller)

 

In this blog, we will learn the basic internal architecture of PLC that includes CPU of PLC, different buses in PLC system then memory unit of PLC and input/output unit of PLC system.

The internal architecture of PLC consists of the CPU which contains the system microprocessor, memory storage and other integrated circuits that are used to execute the control program and input output circuitry that communicate to external devices. Central processing unit control and processes all the operations within the PLC and it is supplied with a clock which has a frequency which is typically between 1 and 8 MHz and this frequency decide the operation speed of the PLC, which provides timing and synchronization for all the elements in the system. The information is carried out within the PLC is by digital signals that are in 1 and 0’s and these digital signals flow on internal paths that path are called buses. The buses mean in physical sense it is a just a number of carriers or conductors along which electrical signals can flow and this flow might be the tracks on a printed circuit board or wires. The CPU uses different-different buses for to send data or to send control signals to other elements. So, there are four buses which are data bus is used to send data between component elements, address bus it carries the addresses of locations for the accessed stored data, control bus it Carrie different signals for internal control action and the system bus is used for communications between the input/output ports and the input/output unit.

Internal architecture of PLC

The CPU

·   ALU (Arithmetic and Logic Unit) ALU is responsible for manipulating data and it carries out the different arithmetic operations like addition, subtraction, multifaction, division and logical operations like AND operation, OR operation, NOT operation and Exclusive-OR operations.

·   Registers which are located within the microprocessor and register is used to store data or address of memory location and instruction that is involved in the program execution. There are different register present in the CPU like a memory data register that used to pass data between memory or processor then instruction register that holds the instruction and another register memory address register It holds the address of the location.

·   Control unit controls the timing of the operations. It generates a sequence of signals to carry out the different operations within some sequence.

 

The Buses

The buses are the paths that used for to communicating with the different elements that are in within PLC and the information is transfer by digital signal that is in binary form that consists of 1 and 0 which shows states which are on or off 1 means on and 0 means off and when they are in a group that is called bits, these group of bits when they carries some information is called word which a term of that. So that means an 8-bit word could be a binary number of 00101010 and each of these bits are communicating simultaneously along its own wires. The systems have four buses which are

·   The Data bus carries the data that data used in the processing carried out by the CPU. If the CPU consists of 8 bit microprocessor, then that microprocessor has internal data bus that can handle 8-bit numbers. it can perform operations that are between 8-bit numbers and deliver results as 8-bit values.

·   The Address bus holds the addresses of the memory locations, so that each word can be located in the memory and every memory location has a unique given address. This can be accessed by the CPU for to perform read operation or write operation on memory.

·   Control bus carries out the signal that are used by the CPU for to control the all the internal operations that happening between different elements. For example like if CPU needs some data from memory so CPU need to perform some operations so for that control unit generate the sequence of signal that signal held by or transfer through the control bus.

·   System bus is used for the communications between the input/output ports and the input and output unit.      

The Memory

Memory is the important component of PLC system that stores information, programs and data in a PLC. PLC system performs the different operations on memory like read data from memory and write data on memory and the storage capacity of a memory unit is determined by how many numbers of binary words that it can store. like, if memory size is 256 words so it can store 256×8 that is 2048 bits if 8-bit words are used and 256×16 that is 4096 bits if 16-bit words are used. so there are several memory elements in the PLC system that are

·   ROM stands for Read-Only-Memory as what name indicates it can only read data. ROM is non-volatile memory. ROM is also a storage medium that gives a permanent storage for the OS (operating system) and fixed data that data used by the CPU.

• RAM which is Random-Access-Memory is a fast and highly cost effective memory which is used for the user program. This memory is also used for the data and where the information stored on the status of input devices and output devices and values of timers or counters and other internal devices. The RAM data is sometimes referred to as a data table or register table. So the RAMs are read/write memories and can be easily programmed and modified and access time of RAM is independent of the address. So the access time is same for the last location and first location of RAM. The disadvantage of RAM is that it is volatile memory so the program stored in the RAM is lost when the power goes off.

·   EPROM stands for Erasable and Programmable Read-Only Memory which is used to store programs permanently and program can be erased and reused.

  

The Input/Output Unit

  The input/output unit provides with an interface between the system and the outside world which allowing for connections to be made through either the input/output channels. so basically in simple word I/O module is mediator between processor and input output devices. so, input devices such as sensors, switches etc and output devices such as valves, pumps, motors, relay etc. so input module take the input from sensors and switches and send to processor then processor process that input and send an output signal to the output module so this communication happens through a system bus. Every input point or output points have a unique address that can be used by the CPU. The input channels and output channel provides an isolation and signal conditioning functions so the sensors can often be directly connected to them without the need for other circuitry. So generally, 5v dc digital signal is compatible with PLC microprocessor.