Introduction to PLC Interview Questions and Answers

Programmable Logic Controller is a class of industrially hardened devices that acts as an interface for input and output sensors. The i/p field elements include elements like switches, sensors, push a button and final elements like actuator, solenoid/control valves, drives, hooters etc. PLC has an important role when it comes to automation. PLCs, take all tasks related to monitoring and control actions care. The main job of PLC is to sense input through modules, process the logic through CPU, and memory should be providing a necessary output through output modules.

Now, if you are looking for a job related to PLC, you need to prepare for the 2020 PLC Interview Questions. Every interview is indeed different as per the different job profiles. Here, we have prepared the important PLC Interview Questions and Answers, which will help you succeed in your interview.

In this 2020 PLC Interview Questions article, we shall present the 10 most important and frequently asked PLC interview questions. These interview questions are divided into two parts are as follows:

Part 1 – PLC Interview Questions (Basic)

This first part covers basic PLC Interview Questions and Answers.

Q1. Explain the role of I/O Modules?

Answer:
To control the process in a machine, input and output devices are used for interfacing with electronic plug-in units. The input modules receive data from input devices like Pushbutton, Switches, and Transmitters, which send it to the processor. The output module receives all data from processors and sends it to output devices like a relay, valves, etc. The devices can be as follows:

Analog: It is used for sending and receiving variable input and output signals.

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Digital/Discrete: This is used for sending and receiving on/off signals.

Q2. What is SCADA? Name any four editors who are available in Scada Package?

Answer:
Supervisory Control and Data Acquisition (SCADA) is computerized hardware and a software-based project that will provide a single integrated view. This view has all control and information resources, enable engineers, supervisors, operators to view and interact with the working of entire operations. It provides a graphical representation of their production processes.

The four editors are as follows:

Inkscape SAGE – SCADA Animation Graphic Editor
b. Generic Logic
c. TRACE MODE
d. UCanCode

Let us move to the next PLC Interview Questions.

Q3. Define SCADA and list down its benefits?

Answer:
Supervisory Control and Data Acquisition (SCADA) is computerized hardware and a software-based project that will provide a single integrated view. This view has all control and information resources, enable engineers, supervisors, operators to view and interact with the working of entire operations. It provides a graphical representation of their production processes.

The benefits of SCADA are:

It preserves your capital investment.
It helps in integrating your business systems together.
In order to reduce your operating and maintenance costs, SCADA is very beneficial.
SCADA helps in improving the quality of your product
When you use SCADA, productivity will be maximized.
It is reliable and robust.
It provides the ability to view and adjust the points and set those remotely.
It helps in predictive maintenance and allows proactive planning rather than reactive planning.

It also has higher reliability with more flexibility and enables all work at lower costs.

Q4. Explain the difference between PLC and DCS?

Answer:
These are the common PLC Interview Questions asked in an interview. The difference between PLC and DCS are explained below:

PLC: Programmable Logic Controller is a system that has a processor and input-output system that has some functional units which use basic modules, communication modules, etc. It makes use of SCADA for visualization and also does not use a central database.
DCS: DCS mainly uses multiple processors with a central database, and the functionality is distributed evenly. Here the control functions are performed by the control subsystem. The history node is used to connect to data; also, the IMS node provides reports, and the operator station provides a good HMI. In addition to this, the engineering station allows engineering changes to be made.

Q5. Explain the role of rack or chassis in the PLC system?

Answer:
A rack or chassis is a hardware assembly that helps in housing the processor, communication and input-output modules. It works in the following ways:

It distributes power evenly.
It contains different input and output modules.
It acts as a communication link between the I/O module and the CPU.

In different PLC systems, there are many different slots for chassis that are available. If a user wants additional chassis, they can be connected by using chassis interconnecting cables.

Part 2 – PLC Interview Questions (Advanced)

Let us now have a look at the advanced PLC Interview Questions and Answers.

Q6. What are the non-fatal errors that are available in Siemens PLC?

Answer:
The errors that are compiled and runtime errors are known as non-fatal errors. Non-fatal errors ought to degrade the performance of your PLC. It does not hamper the execution of the program or any updating in the I/O modules.

Runtime Errors: These are the errors that are detected at runtime. These are reflected in special memory bits of your program, which user can monitor and evaluate.
Compile Errors: These are the errors that are detected at compile-time, and they are detected at download. When this error is encountered, the compilation halts and the previous program is retrieved.

Q7. Explain the difference between instance data blocks and shared data blocks?

Answer:
The instance data block is the block that is assigned to any logical block. It works as a function block. The instance block usually has data where it is stored in function blocks in the variable declaration table.

A shared data block, on the other hand, is never assigned to any logical block. The usually contains values required by the plant or the machine that is going to be installed. It is called directly at any point in the program.

Let us move to the next PLC Interview Questions.

Q8. List the advantages of PLC over Relays?

Answer:
PLC is beneficial over Relays with the below aspects

PLC consumes less power than Relays
When necessary, they can also be high consumption PLC.
PLCs can be easily changed and expanded through programs.
Whenever system requirements are changing, the wiring needs to be changed in the relay. This is not the case in PLC.
There is no hard wire requirement in PLC.

Q9. What is Simplicity?

Answer:
This is the most popular PLC Interview Questions asked in an interview. SCADA has a package named simplicity. Some servers are responsible for taking care of data, its collection and distribution. It is based on client-server architecture, having servers and viewers. The viewers that are there cannot connect to servers, and hence they cannot have complete access to the data that is collected and cannot perform control actions.

Q10. Where is SCADA used?

Answer:
SCADA is used in different industrial processes like chemical plants, power generation, distribution, nuclear processes, and metallurgical actions. It can also be used to process nuclear waste, reactors, etc.