Introduction to PLC

What is a PLC?

Programmable logic control (PLC) is an industrial computer control systems that monitor the state of input devices continuously and make decisions based on a custom program to control the state of output devices. 

Most of the function machine, production line or process can be improve by using this method. The main advantages of PLC are the ability to change and replicate the process/operation while collecting and communicating vital information. PLC system is a modular. So that, we can do mix and match different types of input and output devices that suit our requirement.

PLC consist a lot of hundreds/thousands of separate relays, timers, counters and also data storage locations. The counters, timers and etc. not really exists mean that they are simulated and consider as software counters, timer and etc. Mean that PLC has the three basic components to operate such as CPU, Location to storage data and the suitable circuits to receive input/output data.

PART	DESCRIPTION
INPUT RELAYS (contacts)	Connected to the outside world. They are physically exist and receive signals from switches, Sensors and etc.Typically they are not relays but rather they are transistors.
INTERNAL UTILITY RELAYS (contacts)	Do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored.
COUNTERS	Do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed. Some manufacturers also include high-speed counters that are hardware based. We can think of these as physically existing. Most times these counters can count up, down or up and down.
TIMERS	Do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.
OUTPUT RELAYS (coils)	Connected to the outside world. They physically exist and send on/off signals to solenoids, lights and etc. They can be transistors, relays, or triacs depending upon the model chosen.
DATA STORAGE	Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary

Application of a PLC

PLC used in many industries such as oil refineries, manufacturing lines, conveyor systems and etc.