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The PLC consists of the following components:

• Processor module (CPU)
• Memory,
• Power supply,
• Input/output modules.

THE PROCESSOR MODULE (CPU):  The principal function of the processor is to govern the activities of the entire system. It performs this function by interpreting and executing a collection of system programs. The system program is permanently stored in the processor and is considered a part of the controller itself.

Processors are programmable, which means they are told what to do by a set of instructions, compiled to form a program. When the microprocessor is to carry out a different task, a new program is written and fed to it.

In addition to a microprocessor, the CPU also contains at least an interface to a programming device and may contain interfaces to remote I/O and other communication networks.

PROCESSESOR MODULE


The Processor Module performs the following functions:

• It performs arithmetic and logic operations.
• Data handling executions.
• Performs diagnostic routines that were not possible with relays.
• Peripheral and external device communication.
• Memory management.
• Program execution and I/O bus traffic control.
• Scanning.
• Resource sharing.

MEMORY:  The memory unit of a PLC is the registry where the programs are stored. The fundamental unit of memory is the word. Words are made up of bits. A bit is a single piece of data. It contains information on only two states (ON/OFF or YES/NO). Longer words contain more information within. Programs are combination of words that produce control logic.

To operate the PLC system there is a need for it to access the data to be processed and instructions, that is, the program, which informs it how the data is to be processed. Both are stored in the PLC memory for access during processing.

MEMORY UNIT




ROM and RAM are the most common types of memory used in PLCs.

READ ONLY MEMORY (ROM): stores programs and data and cannot be changed after the memory chip has been manufactured. ROM memory is non-volatile, meaning that its contents will not be lost if power is lost. ROM is used by the PLC for the operating system.

RANDOM ACCESS MEMORY (RAM): is designed so that information can be written into or read from the memory. RAM is used as a temporary storage area of data that may need to be quickly changed. RAM is volatile, meaning that the data stored in RAM will be lost if power is lost. A battery backup is required to avoid losing data in the event of a power loss

POWER SUPPLY: PLCs and their constituting modules are equipped with power generated from a power supply. The power supply converts power line voltages into those required by the solid-state components. It may be integral or separately mounted which provides the isolation necessary to protect solid-state components from high-voltage line spikes. The power supply also POWER SUPPLY provides the heat dissipation required for the plant floor operation. It drives the Central Processor Unit, I/O logic signals, memory unit and some peripheral devices. The expansion of I/O has led to increased power requirement of some PLCs. Hence, they are furnished with additional power supplies.

INPUT/OUTPUT MODULES:  I/O modules physically connect to field devices, that is they provides the interface between the system and the outside world, allowing for connections to be made through input/output channels to input devices. Since there are different types of input and output devices, the process determines the type of input or output module selected, digital or analog.

PIC Inputs give the controller real-time status of variables in the form of real world signals. These variables can be analog, register or discrete. Typical analog inputs can be from thermocouples, RTDs, flow, pressure, and temperature transmitters. These inputs are transmitted over the I/O bus to the central processor unit after being converted into digital data. The input module is entrusted with the operation of converting electrical signals flowing in from input field devices like push buttons to electrical signals that the PLC can understand. Examples of input modules include limit switches, proximity switches and push buttons, photo sensors, and temperature sensors,motion control or high-speed counters