Introduction to PLC

Programmable logic controllers abbreviated as PLC are said to be industrial computers that help monitor inputs, make relevant decisions on the basis of their programs and thereby control outputs so as to automate processes or machines. It is said to have an in-built operating system (OS). This OS under consideration is said to be highly specialized so as to handle the incoming events in the real-time scenarios or during the times of their occurrences.
It can be characterized as a digital computer that has been designed for having multiple inputs as well as output arrangements, extended ranges of temperature, immunity to electrical noises and resistance to vibrations and impacts. A PLC is truly known to be an example of real-time system.
The PLC is equipped with having a set of input lines where the sensors are connected so as to provide notifications upon events occurring (such as; temperatures rising above or below a certain threshold value, liquid level reached, etc.) and with output lines in order to signal any reaction or as an effect to the incoming events (such as; starting an engine, opening or closing a valve, etc.).
It makes use of a programmable memory so as to store any instructions along with specific functions that take into consideration an on or off control, sequencing, data handling, arithmetic and time counting.
PLC scan cycle and scan time:
At the time of PLC running, the scanning process incorporates four phases within it that are continuously repeated as separate individual cycles of the operation. They are as follows;
Input Scan – This is the stage where states of the inputs are scanned.
Program Execution – This phase includes the processing as well as execution of the program logic.
Housekeeping – It takes in internal diagnostics, communication, etc.
Output Scan – It is all about energizing/de-energizing the outputs.
“Scan time” is said to be the time required to implement a scan cycle. This scan time is composed of program scan time that is the time which is needed to solve the control program and input/output update time or the time needed to read the inputs as well as update the outputs.

PHASE 4
Output Scan
PHASE 3
Housekeeping
PHASE 2
Program Execution
PHASE 1
Input Scan

PLC Scan Cycle

O M
U O
T D
P U
U L
T E

I M
N O
P D
U U
T L
E
PROGRAMMING DEVICE
POWER SUPPLY

PROCESSOR

From
Sensors To Output

Basic Architecture of PLC

Major components of a common PLC:
Input Modules – These accept and then convert the signals from the sensors to a logic signal. Examples include switches, push buttons, etc.
Output Modules – These help in converting the control instructions of a signal that can be used by the actuators. Examples include alarms, lamps, etc.
Central Processing Unit (CPU) – It is considered as the brain of PLC and helps govern all the activities of entire PLC systems. This component helps perform scan cycle for the PLC. It incorporates various blocks such as; arithmetic logic unit (abbreviated as ALU), internal memory residing in CPU, internal timers (which is used to add a certain amount of delay in programming circuit by adding a certain amount of time to each of the circuits), counters (which runs on the method of counting number of pulses) and flags.
Memory – This is the component that helps in storage of information, data and programs in any PLC. The types of memories that are used in the PLCs are RAM and ROM.
Power Supply – This helps in the provision of voltage which is needed in order to run primary PLC components.
Programming Device – The programming terminal can be put to use in order to program the PLC as well as monitor or sequence the PLC’s operation.

Advantages of PLC:
Increased accuracy, flexibility as well as reliability
Easier during troubleshooting cases
Remote control capability
Communication capability
Handling much more complicated systems
Easy programming
Lower cost
Faster responses
Inculcation of less and simple wiring
Disadvantages of PLC:
Initial costs tend to be high
There is too much work and effort involved while connecting wires
Increase in unemployment rates

Difference between SCADA and PLC

The PLC component is said to be a part of the SCADA system so as to help SCADA carry out its functions. PLC has got direct contacts with the devices or equipments operating on the fields or floors of industries whereas SCADA does not tend to have any direct links among the field devices. PLC is considered to have a hardware presence while that of SCADA is considered to have a software existence.
PLC is like a microcontroller that makes use of a microprocessor. The program residing within PLC can be duly modified without having much or no knowledge of any assembly language. This is mainly used for making the logical flow of a process. SCADA on the other hand controls two or more PLCs for successfully carrying out process automation.
PLC performs more than a few operations in circuits by making use of timers and counters while SCADA is more of a process that is undertaken for monitoring as well as controlling the devices through the use of PLC from a single place.
PLC is used to control certain processes according to or in relation with predefined programs that are saved in the PLC itself. The programs so mentioned can be modified or changed as per the requirement basis. SCADA is equipped for monitoring and controlling along with graphical displaying features to the human operators. Through the help of SCADA, small changes can be done during the process executions along with features of storing as well as communicating to other servers. The programming done for SCADA is on the basis of scripting language.
PLC being a device provides direct control on the operations of the field devices based on the instructions given or fed to it while SCADA though not having direct control over the field devices, makes the devices run according to its purpose via the PLCs. Data storage is not completely possible by PLC while in the case of SCADA, it gathers the information (such as, status information, some definite values, etc.) and stores all of it at one place.
PLC is a hardware that would read sensor physical signals, solve the algorithms and give the controlled outputs whereas SCADA is software interfaced with the PLC to read and represent the information in graphical or animated forms.
PLC is a solid state device which controls the output of the process through the program given in the ladder diagrams. The input and output are represented in normal open, normal close and coil contacts. Each component involved is defined using address. SCADA on the other hand being software is used to monitor, control and acquire data from field devices even from remote locations. Here input and output are represented in images. Each object is defined using name.
For example; controlling the level is done by PLC and info graphics of the level with information about it (level, critical levels, alarms, etc.) from last 3 years or more is done by SCADA.