PLC Interview Questions and Answers
1. The PLC was invented by .
a) Bill Gates
b) Dick Morley
c) Bill Landis
d) Tod Cunningham
2. The first company to build PLCs was .
a) General Motors
b) Allen Bradley
c) Square D
d) Modicon
3. Which of the following statements is not correct?
a) The PLC rung output [-( )-] is a discrete output instruction or bit in memory.
b) Each rung of the ladder logic represents a logical statement executed in software – inputs on the right and outputs on the left.
c) Input and output instructions in ladder logic do not directly represent the switches and actuators.
d) PLC input instructions are logical symbols associated with voltage at the input module terminals.
4. Which of the following statements is correct?
a) Ladder logic is a PLC graphical programming technique introduced in the last 10 years.
b) A ladder logic program is hard to analyze because it is totally different when compared with the equivalent relay logic solution.
c) The number of ladder logic virtual relays and input and output instructions is limited only by memory size.
d) The number of contacts for a mechanical relay is limited to number of coils on the relay.
5. Which of the following statements is NOT correct?
a) The status of each input can be checked from one location and outputs can be forced on and off.
b) All symbols in the RLL represent actual components and contacts present in the control system.
c) PLCs are not as reliable as electromechanical relays in RLL.
d) Input (-| |-) and output (- ( ) -) instruction symbols in the ladder logic represent only data values stored in PLC memory.
6. Which of the following statements is NOT correct?
a) If a problem in a PLC module occurs, the module can be changed in a matter of minutes without any changes in wiring.
b) Outputs can be paralleled on the same rung.
c) The physical wires between the input and output field devices and the PLC input and output modules are the only signal wires required in the PLC system.
d) The cost and size of PLCs have increased significantly in the last 10 years.
7. Which of the following statements about a single pole double throw relay is NOT true?
a) It is called an SPDT type of relay.
b) It has one common contact.
c) It has two positions (NC and NO).
d) It has a center off position.
8. Which of the following statements about a single pole double throw relay is true?
a) Insulators are used in the armature to isolate the electrical switching contacts from the rest of the relay components.
b) The NC contact and the pole are in contact when the relay is off.
c) It has just one coil.
d) All of the above.
9. Which of the following statements about RLL is NOT true?
a) NO contact symbol has two parallel lines to indicate an open contact.
b) RLL stands for Relay Ladder Logic.
c) NC contact symbol has the same two parallel lines with a line across them to indicate closed contacts.
d) The right power rail is positive or the high side of the source, and the left power rail is the power return or ground.
10. In a current sinking DC input module _____
a) The current flows out of the input field device
b) Requires that a AC sources be used with mechanical switches
c) The current flows out of the input module
d) Currents can flow in either direction at the input module
11. Explain what are different comonents in PLC?
1. Input Interface
2. Memory Section
3. Central Processing Unit (CPU)
4. Programmable Language
5. Programming tool
6. An output Interface
12. Explain Advantages of PLCs than Hard wired Relay?
PLCs are highly reliable, easily programmable, Small and Inexpensive, PLCs can be designed with the communication capabilities so they can converse with the local or remote computer, They can sustain in robust environment less maintenance.
13. Explain what is the programmable language used in PLC?
The general language program consists of Ladder Diagrams. Relay logic control scheme is represented in Ladder diagrams. Alternative languages uses Boolean representation of these control schemes as base of the computer representation.
14. Explain what does Central Processing Unit (CPU) of PLC consists?
CPU is the brain of the system and consists of
Microprocessor: To carryout arithematic and logical operations
Memory: The area in the CPU in which the information is stored and reterived
Power Supply: The electrical supply that converts the ac voltage to various DC operating voltages.
15. Explain what is SCAN in PLC ?
The sequential operation of the controller that goes through the ladder diagram from top to bottom of the ladder. In this process it updates all the outputs corresponding to the inputs. SCAN takes place from left to right of each rung. Usually SCAN time is in milliseconds and it is a continuous process.
16. Difference between PLCs and Computers
Programmable Logic Controllers (PLCs) have basic architecture compared to normal general purpose computers. A normal computer can be converted to PLCs by providing a way that the computer can able to receive information or signal from the field devices such as push buttons, switches and valve positions. Computer requires a some software to process the information obtained from the input to generate an output which decide whether to close or open the valve position in the process side.
Some of the important features and characteristics that distinguish between the general purpose computers and Programmable Logic Controllers
(PLCs) are given below:
PLCs are designed to operate under industrial environments (PLCs have to operate under wide range of temperature conditions, humidity and other environmental conditions). They are least affected by the electrical noise and are inherent to electrical noise
Programming in PLCs is through Relay Ladder Logic or other easily learned language. PLCs comes with program language built in its memory. PLCs do not contain input and output devices such as keyboards, mouse, monitor, CD drives and other hard disks. It is in simple a self contained box with communication ports and set of terminals for input and output devices.
Unlike computers which performs numerous tasks simultaneously, PLCs execute a single program in an orderly and sequential manner from first instruction to the last instruction
PLCs have been designed for installation and maintenance by plant electricians. Programming in PLCs is simple (Relay Ladder Programming), it does not include any advanced code. Troubleshooting is simpler and many PLCs are designed to include fault details and written fault details on display screen.
17. PLCs Advantages or Benefits
Higher Reliability:
Once the program written and tested it can be easily downloaded into other PLC’s memory. It requires lesser and simpler wiring compared to conventional hard wired circuits employed. Hence reliability of the system increases significantly with PLCs.
More Flexibility:
It is easier to create a new program module or change an existing program in PLC compared to hard wired circuitry system. These software program modules can be changed whenever required. Use can modify the programs in the field and if required, security can be enhanced by hardware interlocks such as key locks and software features such as passwords
Lower Cost:
PLCs were originally designed to replace relay control logic which is not economical and complex especially for large control circuits. With PLCs the cost savings have been so significant that the relay control becomes uneconomical except for some power applications. Generally if the application consists of more than half a dozen control relays, PLCs are least expensive to install
Communication Capability:
Communication capability of PLC with the other controllers and computers in the system is one of the main advantages compared to relay control circuit. Functions such as Supervisory control, data acquisition from the field, monitoring devices and process parameters associated with the field and downloading and uploading of programs can be easily possible with the PLC compared to hardwired circuits
Faster Response:
PLCs are designed for high speed and for the real time applications.Response time for PLCs are much smaller compared to relay logic circuits. The programmable controllers operates in real time i.e, an event taking carrying out at field will result in execution of operation of output
Easy to Troubleshoot:
PLCs have inbuilt diagnostics and override functions that helps the user to easily trance the software and hardware errors.
18. what is Progrmmable Logic Controllers (PLCs)
Programmable Logic Controllers (PLCs):
Programmable Logic Controllers (PLCs) are the micro processor controller devices situated at the remote sites of the plant or at the field of the process systems. PLCs purpose is to monitor the process parameters located at the field and adjust the outputs based on the inputs received by the PLCs. PLCs will operate on any systems which will have input field devices that are On-Off type (discrete or digital ) or analog input devices. Similarly it will operate on the field devices which will be discrete or analog output type.
In simple words, Hence PLC acts as interface between the input and output devices at the process side of the industries. It monitors the inputs receive from the input devices and takes necessary output control functions by executing the programs stored in its memory.
The term Logic is used in Programmable Logic Controllers because all the program inside the PLC will be of Logic programming (eg: Ladder diagrams ).
In industrial applications hard wired relay logics were replaced by Programmable Logic Controllers due to its reliablity, simplicity, low cost, easier programming and many other functions.