Basic PLC

What is PLC?

PLC is called Programmable Logic Control. The National Electrical Manufacturers Association (NEMA) defines a PLC as a "digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions, such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog I/O modules various types of machines or processes."

Block Diagram of PLC

Types of Input/Output

• Analog Input/Output

Analog input devices sense continuous parameters. The information that they provide is given as a continuous range of values, not just an on or off indicator. Common analog inputs are pressure, temperature, speed, etc. Analog output devices respond to a range of output values from the controller. Common analog output signals include motor speed, valve position, air pressure, etc.

Analog Input Device :

1. RTD
2. Pressure sensor
3. Temperature sensor

Analog Output Device :

1. Electric valve 

• Digital input/Output

Digital input devices are binary – they may be either on or off, open/closed. Common digital field input devices include pushbuttons, limit switches, and photoeyes. Common digital output devices include relays, motor starters, and solenoid valves. A digital signal is a signal that represents a sequence of discrete values. its value eithe 1 (high) or 0 (low).

Digital Input Device :

1. NO/NC Switch
2. Proximity sensor
3. Limit Switch
4. Reed Switch

Digital output Device : 

1. Alarms
2. Lights
3. Relays
4. Fans
5. Solenoids
6. Valves