Top 20 Interview Question and answer in PLC

            

Top 20 Interview Question and answer in PLC

1.                What is PLC

        A Programmable Logic Controller (PLC) is an Industrial Computer that accepts inputs from switches and sensors, evaluates these in accordance with a stored program and generates outputs to control machine and process

                              (Or)

It is a specialized computer used to control machines or process. It uses programmable memory to store instructions and execute special functions that include on/off control, timing, counting, sequencing, arithmetic and data handling.

It is microprocessor based device. 

2.                What is Difference Between PLC and Micro-Controller

	PLC	Micro-Controller
1.	It is a special computing device designed for use in industrial control systems and other systems where the reliability of the system is high.	It is a small, single-chip computing device containing one or more processing cores, with memory devices embedded alongside programmable dedicated and general purpose input and output (I / O) ports.
2.	PLCs can generally be called a high level microcontroller.	It is generally can be referred to as a low-level microcontroller.
3.	The processor module consists of the central processing unit (CPU) and memory. In addition to a microprocessor, the CPU also contains at least one interface with which it can be programmed (USB, Ethernet or RS232) together with communication networks.	The microcontroller implements everything on a single chip, from the CPU to the  I / O ports and interfaces required for communication with the outside world.
4.	The power supply is usually a separate module and the I / O modules are separate from the processor	There is no separate module in this power supply. It is built with processor
5.	PLCs are designed to communicate with industrial grade sensors, actuators and communication modules and thus get current and voltage ratings that are often incompatible with microcontrollers without additional hardware.	Microcontrollers have sensors, actuators and modules designed to meet their specific needs, which may be difficult to connect to a PLC.
6.	PLC is designed for use in industrial setups and thus has been reinforced to withstand various adverse conditions associated with that environment, such as extreme temperature ranges, electrical noise, rough handling and high vibration	Microcontrollers are not designed to serve as stand-alone devices such as PLCs. They are designed to be embedded in a system. This explains their less rugged appearance compared to PLCs.
7.	The PLC is designed for use by both highly trained automation experts and factory technicians with little or no formal training	Microcontroller designers must have a good understanding of electrical engineering principles and programming to design complementary circuits for the microcontroller.
8.	PLCs were originally designed to be programmed using a programming image that mimics the connections / schematics of logic relay diagrams. This reduced the training requirements for existing technicians.	It usually requires a high level of experience in the programming language used and a general understanding of the principles of firmware development. Programmers usually need to understand concepts such as data structures and a deep understanding of the microcontroller architecture is required to develop a very good firmware for the project.
	PLCs are the main control elements used in industrial control systems. They are used in the control of industrial machines, conveyor belts, robots and other machines in the production line. They are also used in SCADA-based systems and in systems that require a high degree of reliability and can withstand extreme conditions.	Microcontrollers are being used, they are now being adapted for certain applications for which microcontrollers were considered unsuitable, from mini-do-it-yourself computers to various complex operating systems.

3.                What are the data types used in Counter

A counter is a simple device aimed at one simple thing - counting.

   There are up-counters (they only count 1,2,3 ...). These are called CTU, (count up) CNT C or CTR. These counters are below (they only count 9,8,7, ...). These are usually called CTD (count down), when they are a separate instruction.

These are also up-down counters (they count 1,2,3,4,3,2,3,4,5, and down or down.) ..) These are usually UDC (up-down counters) It is said, when they give different instructions.

Typically counters can count from 0 to 9999, -32,768 to +32,767 or 0 to 65535. Why the weird numbers? Because most plc's have 16-bit counters. We'll get into what this means in a later chapter but for now suffice it to say that 0-9999 is 16-bit BCD (binary coded decimal) and that -32,768 to 32767 and 0 to 65535 is 16-bit binary.

               

          

4.                What is the Difference between Relay Logic circuit and PLC circuit

S.NO.	Relay Logic	PLC  Logic
1.	Relay logic is a hard wired control system using instrumentation, switches, timers, relays, contactors, motors and actuators. Traditional machine and process automation was accomplished using relay logic.	Relay logic in a PLC is the method of formulating logic expressions to automate machines and processes in industrial applications. The programming language used to create relay logic in a PLC is called Ladder Logic.
2.	Requires a lot of switchboard space.	Easier to develop complex logic expressions with ladder logic software.
3.	Less reliability compared to plc logic	Increased reliability with PLC life easily reaching + 10years,
4.	Solving problems is extremely difficult.	Solving problems is very easy.
5.	Changes to control functionality are complicated and seriously annoying.	Virtually maintenance-free compared to relays.

 

5.                What is the role of PLC in automation?

     In automation, PLC plays an important role in controlling and monitoring industrial applications. It has both hardware and software.

           This controller can be run or operated automatically and manually. For its operation it requires extra manpower, time, utility and precise operation in the system. It is useful to increase the reliability and stability of the system. It performs precise edits in a very short time. This is one of the reasons why PLC is widely used in industry today. 

6.                What are the different components of PLC?

PLC has several components. Each part is linked to specific PLC operations and functions.

 Here is the list of the useful basic components -

 Input and output modules 

These modules provide input to the PLC and output. Input and output can be both digital and analog.

 Power supply 

This part is responsible for providing AC or DC power to operate the PLC.

 Central processing unit (CPU)

 CPU stores and executes PLC software programs.

 Memory system

 As the name implies, the memory system is responsible for storing and retrieving information necessary for PLC operations.

 Communication protocol

Several devices can be connected to the PLC. Communication protocols are used to exchange information from one device to another.

 PLC programming (software proficiency) 

You must write a programming language to implement PLC logic.

These are the six basic components of the PLC.

 7.                What are the different PLC brands?

 The different companies have invented their own PLC brands. Here are some popular PLC brands.

ABB PLC

AB (Rockwell) PLC

Siemens

Delta PLC

Mitsubishi PLC

Honeywell PLC

Omron PLC

Schneider PLC

Hitachi PLC

Fatek PLC

Bosch PLC

GE (General Electric) PLC

  Siemens PLC and Allen-Bradley (AB) PLC are mostly used for project and

educational purposes and also in largest industries.

8.                What is LD in PLC?

          LD is short for the "Ladder Diagram". It is the most popular and universally accepted programming language for PLC. It is easy to implement PLC logic with LD as it provides a graphical user interface.

 9.                What is the rack or chassis?     

The modular PLC consists of the number of input or output modules, the controller processor and the communication controller. This hardware composition of these modules is called "rack or chassis".

10.             What is the PLC scan cycle?

When the PLC program is running, multiple repetitive processes take place. This is the whole process called a PLC scan cycle.

 

This scanning program consists of reading input data, running the programming logic and updating the output data in one cycle.

                      Scan cycle of PLC

The scan time is a time required for continuous scanning of programs. It will take about 10ms-15ms times.

 

11.            What is the MCR?

  MCR is the short abbreviation of “Master Control Relay”. It is a type of instruction used in Siemens PLC software. This instruction is used to shut down the process.

 There are some other types of MCR instruction like

 MCRA (Master Control Relay Active),

MCRD (Master Control Relay Deactivate),

MCR <(Master Control Relay Less),

MCR> (Master Control Relay Greater).

 

12.            What is a communication protocol?

The system communicates with the two or more communicating devices to transfer digital data or information. This communication is subject to a number of rules called "Communication Protocol".

 

      The standard definition communication protocol is a set of rules that govern data communication.

 

13.            What different types of communication protocols are used in PLC?

 In the PLC communication network, the different types of protocols are used to communicate with multiple connected devices.

 Here are some standard PLC communication protocols.

1	EtherNet / IP
2	Profibus
3	Modbus
4	Interbus
5	ProfiNet
6	ControlNet
7	DeviceNet
8	DirectNet
9	CompoNet
10	RAPInet
11	EtherCAT
12	MelsecNet
13	Optomux
14	DF-1 protocol
15	HostLink protocol
16	MECHATROLINK
17	DH- Data highway
18	PPI - Point to point
19	EtherNet Powerlink
20	MPI- Multi-Point Interface
21	EGD- Ethernet global data
22	AS-i- Actuator Sensor interface
23	OSGP- Open Smart Grid protocol
24	DNP3 - Distributed network protocol
25	SDS - Smart Distributed System Protocol
26	PieP- Process Image Exchange Protocol
27	SRTP- Service Request Transport Protocol
28	BSAP-Bristol Standard Asynchronous Protocol
29	FINS- Factory Interface Network Service Protocol
30	HART Protocol for addressable external transducer

 

Recommended standard protocol (RS-232, RS-422 and RS-485)

These are the main communication protocols used for the PLC and other network connections. These protocols are supported by various PLC software brands. The communication protocols depend on three fundamental parts, such as baud rate, network length and the number of modes.

 

14.            What types of sensors are used in automation?

  The sensor is an electronic device that detects various parameters from the physical environment and passes them as input to the PLC controller.

      The sensor can be either digital or analog depending on the type of signal it is passing through. 

Here are different types of sensors used to communicate the different parameters to the controller.

 

1.	Inductive proximity sensors
2.	Capacitive proximity sensors
3.	Optical sensors
4.	Ultrasonic sensors
5.	RTD sensors (Resistance Temperature Detectors)

           

.

 

 

 

 15.            What is sinking and sourcing?

          Sinking and Sourcing are very important concepts used to properly connect a PLC to the external environment. These two concepts are only applied to the DC modules.

 The zinc circuit provides a ground path for charging (-DC). And the sourcing circuit provides +24 V source to the load (+ DC).

 16.            Explain the term download and upload in PLC.

   These two terms refer to the data transfer between PLC and connected PC.

              Upload means data transfer from PLC to PC.

              Download means data transfer from PC to PLC.

 17.            Explain the functions of PLC.

        The functions of the PLC are,

                    1.  Successive automated control

                     2.   Monitoring of the system and the plant

                     3. Control the position or movement of the system

                    4.  Detection of the fault condition in the system                  

18.              What are the different types of counters used in PLC?    

The PLC counter is divided into three different types. 

1.     Up Counter

2.      Down counter

3.    Counter up / down

       The up / down counter is usually used in PLC programming.

19.            What are the types of PLC?

 There are two basic types of PLC bases for PLC construction and operation.

 1.     Compact PLC

2.      Modular PLC

Compact PLC: In Compact PLC the I / O capacity is defined and determined by the manufacturer. They are used for small-scale applications.

 A single case contains the multiple built-in modules. The integral PLC has an integrated power supply, CPU, I / O interfaces and other components are housed in a compact chassis. Some of the integrated PLCs make it possible to connect additional I / O.

 Modular PLC: A modular PLC consists of multiple modules such as CPUs, I / O modules, power modules connected to a popular rack. It can expand the capacities of I / O. The modular PLC is versatile and can be designed for this purpose. These modular PLCs have variable power, computing power, I / O connectivity, etc. In different sizes.

 20.            Who is PLC's father?

          Dick Morley is mentioned as the father of the programmable logic controller (PLC).In 1968, he created and developed the first industrial PLC i.e. Modicon PLC during his studies. The first Modicon PLC was built for General Motors.