PLC/Microcontrollers/Microprocessors Interview Questions & Answers

  1. Question 1. What Is Plc?

    Answer :

    PLC are solid state members of the computer family using integrated circuit instead of electromechanical devices to implement control functions.

  2. Question 2. What Are The Two Basic Sections Of Plc?

    Answer :

    • CPU
    • The Input/Output interface system

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  4. Question 3. Common Plc Programming Languages?

    Answer :

    • Ladder logic
    • Functional Block diagram
    • Sequential Function Chart

  5. Question 4. How To Select A Plc (selection Criteria )?

    Answer :

    • Cost of Hardware and Software
    • Reliability Flexibility Scalability 
    • Ease of database configuration 
    • Graphics development
    • Interlocks and batch processing
    • Integration of high level application

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  7. Question 5. What Is Sinking Input And Sourcing Input/output?

    Answer :

    • Sinking input / output provides a grounded connection to the load.
    • Sourcing input / output provides to the load.

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  9. Question 6. What Are The Different Types Of Modules Used In Plc?

    Answer :

    • Digital input
    • Digital Output
    • Analog input 
    • Analog output
    • Pulse input
    • Communication module
    • Cpu

  10. Question 7. Explain What Are Different Components In Plc?

    Answer :

    1. Input Interface
    2. Memory Section
    3. Central Processing Unit (CPU)
    4. Programmable Language
    5. Programming tool
    6. An output Interface

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  12. Question 8. Explain Advantages Of Plcs Than Hard Wired Relay?

    Answer :

    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. Question 9. Explain What Is The Programmable Language Used In Plc?

    Answer :

    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.

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  15. Question 10. Explain What Does Central Processing Unit (cpu) Of Plc Consists?

    Answer :

    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.

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  17. Question 11. Explain What Is Scan In Plc?

    Answer :

    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.

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  19. Question 12. Difference Between Plcs And Computers?

    Answer :

    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.

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  21. Question 13. What Are Plcs Advantages Or Benefits?

    Answer :

    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.

  22. Question 14. What Are The Various Criteria To Choose The Microcontroller?

    Answer :

    The important criteria to be considered in choosing micro controllers are:

    • Availability of software development tools like compilers, debuggers, assemblers
    • Meeting the computing needs of the task at hand efficiently at low-cost.
    • Wide availability
    • Reliable sources/manufacturers
    • The amount of RAM and ROM on chip
    • The number of I/O pins and the timer on the chip
    • Power consumption
    • Speed of the device
    • Packaging
    • Cost per unit.

  23. Question 15. List Some Of The 8051 Microcontroller Manufacturers?

    Answer :

    • Intel
    • Philips
    • Infineon
    • Maxim/Dellas semiconductor
    • Atmel

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  25. Question 16. What Is Difference Between Microprocessor And Microcontroller?

    Answer :

    The microprocessor has no ROM, RAM and no I/O ports on the chip itself.Whereas the microcontroller has a CPU in addition to a fixed amount of RAM,ROM, I/O ports and a timer all on a single chip.

  26. Question 17. List Out Some Of The Features Of The 8051?

    Answer :

    • ROM –  4K bytes
    • RAM – 128 bytes
    • Timer – 2 no
    • I/O Pins – 32
    • Serial Port – 1
    • Interrupt sources – 6

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  28. Question 18. What Are The Various Types Of Memories Used In Microcontroller/microprocessor?

    Answer :

    ROM – Read Only Memory
    RAM – Random Access Memory
    PROM – Programmable Read Only Memory
    EPROM – Erasable Programmable Read Only Memory
    EEROM – Electrically Erasable Programmable Read Only Memory

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  30. Question 19. What Is Meant By Flipflop?

    Answer :

    • A Flip-flop is a basic electronic circuit used for storing information in a digital machine.
    • It is a bistable device. It means it has two stable states.
    • It has one or more inputs and two complement outputs.
  31. Question 20. What Is A Bus?

    Answer :

    Group of lines connecting the microprocessor with other components of a computer system is called a bus. It is a communication path over which electrical signals representing binary digits ( 0, 1) are transmitted. For a single bit to transmit, one line is used. The width of the bus is number of lines that are used to constitute that bus.

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  33. Question 21. What Is Data Bus?

    Answer :

    Data Bus: Group of lines that transmit data in and out of the microprocessor is called the data bus. Here the data can flow in both directions between the microprocessor and the peripheral devices. Hence the data bus is bidirectional. The width of the data bus depends on the architecture of the microprocessor. For an example, an 8 bit processor can operate on 8 bits of  data at a time and has a 8 bit wide data bus.

  34. Question 22. What Is Address Bus?

    Answer :

    Address Bus:Group of lines used by the microprocessor to send the address of the device or the memory location which the microprocessor needs to access.The address bits flow in one direction, ie, from the microprocessor to the peripheral  devices.Hence the address bus is unidirectional.The width of the address bus determines the maximum number of memory locations that the microprocessor can access.

  35. Question 23. What Is Multiplexing?

    Answer :

    Multiplexing:Using a single bus for two different functions is called multiplexing.For an example, in 8085 microprocessor the low order 8 bits of the address and data are transmitted using AD7- AD0 lines.These lines are also used as low order address bus during execution of instructions.Thus the same lines are used for transmitting address and data.This is called as multiplexing.These lines are called as multiplexed address/data bus.

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  37. Question 24. What Is A Microprocessor?

    Answer :

    Microprocessor is a program-controlled device, which fetches the instructions from memory, decodes and executes the instructions. Most Micro Processor are single- chip devices.

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  39. Question 25. What Are The Flags In 8086?

    Answer :

    In 8086 Carry flag, Parity flag, Auxiliary carry flag, Zero flag, Overflow flag, Trace flag, Interrupt flag, Direction flag, and Sign flag.

  40. Question 26. Why Crystal Is A Preferred Clock Source?

    Answer :

    Because of high stability, large Q (Quality Factor) & the frequency that doesn’t drift with aging. Crystal is used as a clock source most of the times.

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  42. Question 27. In 8085 Which Is Called As High Order / Low Order Register?

    Answer :

    Flag is called as Low order register & Accumulator is called as High order Register.

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  44. Question 28. What Is Tri-state Logic?

    Answer :

    Three Logic Levels are used and they are High, Low, High impedance state. The high and low are normal logic levels & high impedance state is electrical open circuit conditions. Tri-state logic has a third line called enable line.

  45. Question 29. What Happens When Hlt Instruction Is Executed In Processor?

    Answer :

    The Micro Processor enters into Halt-State and the buses are tri-stated.

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  47. Question 30. Which Stack Is Used In 8085?

    Answer :

    LIFO (Last In First Out) stack is used in 8085.In this type of Stack the last stored information can be retrieved first

  48. Question 31. What Is Program Counter?

    Answer :

    Program counter holds the address of either the first byte of the next instruction to be fetched for execution or the address of the next byte of a multi byte instruction, which has not been completely fetched. In both the cases it gets incremented automatically one by one as the instruction bytes get fetched. Also Program register keeps the address of the next instruction.

  49. Question 32. What Are The Various Registers In 8085?

    Answer :

    Accumulator register, Temporary register, Instruction register, Stack Pointer, Program Counter are the various registers in 8085

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  51. Question 33. What Is 1st / 2nd / 3rd / 4th Generation Processor?

    Answer :

    The processor made of PMOS / NMOS / HMOS / HCMOS technology is called 1st / 2nd / 3rd / 4th generation processor, and it is made up of 4 / 8 / 16 / 32 bits.

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  53. Question 34. Name The Processor Lines Of Two Major Manufacturer?

    Answer :

    High-end: Intel – Pentium (II, III, 4), AMD – Athlon. Low-end: Intel – Celeron, AMD – Duron. 64-bit: Intel – Itanium 2, AMD – Opteron.

  54. Question 35. What’s The Speed And Device Maximum Specs For Firewire?

    Answer :

    IEEE 1394 (Firewire) supports the maximum of 63 connected devices with speeds up to 400 Mbps. Where’s MBR located on the disk? Main Boot Record is located in sector 0, track 0, head 0, cylinder 0 of the primary active partition.

  55. Question 36. Where Does The Cpu Enhanced Mode Originate From?

    Answer :

    Intel’s 80386 was the first 32-bit processor, and since the company had to backward-support the 8086. All the modern Intel-based processors run in the Enhanced mode, capable of switching between Real mode (just like the real 8086) and Protected mode, which is the current mode of operation.

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  57. Question 37. How Many Bit Combinations Are There In A Byte?

    Answer :

    Byte contains 8 combinations of bits.

  58. Question 38. Have You Studied Buses? What Types?

    Answer :

    There are three types of buses.

    • Address bus: This is used to carry the Address to the memory to fetch either Instruction or Data.
    • Data bus : This is used to carry the Data from the memory.
    • Control bus : This is used to carry the Control signals like RD/WR, Select etc.
  59. Question 39. What Is The Maximum Clock Frequency In 8086?

    Answer :

    5 Mhz is the Maximum clock frequency in 8086.

  60. Question 40. What Is Meant By Maskable Interrupts?

    Answer :

    An interrupt that can be turned off by the programmer is known as Maskable interrupt.

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  62. Question 41. What Do You Mean By The Term Embedded Controllers?

    Answer :

    The devices that has all the functional blocks on chip, including the program and data memory and there is no external data/address bus provided. For example, ATMEL89C2051.

  63. Question 42. Discuss The Advantages Of Microcontroller Over Microprocessor In Control Applications?

    Answer :

    The first main advantage is that the ALUs can be assembled together horizontally to form computers that can handle very large data at a time. Another advantage is bit slice design that makes use of possible bipolar chip technology that is very fast.

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  65. Question 43. What Are Risc And Cisc Processors?

    Answer :

    RISC chips requires lesser hardware implementations, which makes them simpler to design and hence lesser costs of production.

  66. Question 44. Discuss The Criteria For Selecting A Micro-controller Device?

    Answer :

    It must have ram and rom to support its inbuilt functions. It must have +5v supply, must be fast and should have programmable and data memory.

  67. Question 45. Discuss The Advantages And Disadvantages Of Harvard And Von Neuman Architectures?

    Answer :

    Harvard architecture uses separate memories for program and data memory whereas Von Neuman uses same program and data memory.Therefore Harvard is faster compared to other one.

  68. Question 46. Name 2 Register That Consists Of 16 Bit?

    Answer :

    1. Program counter
    2. Data pointer
  69. Question 47. Name Few General Purpose Registers?

    Answer :

    Accumulator, B-register, R0- R7

  70. Question 48. What Are Sfr?

    Answer :

    The 128 bytes of on-chip additional RAM locations from 80H to 0FFH are reserved for the special functions and therefore these are called as special function register.

  71. Question 49. Can An Rc Circuit Be Used As Clock Source For 8085?

    Answer :

    Yes, it can be used, if an accurate clock frequency is not required. Also, the component cost is low compared to LC or Crystal

  72. Question 50. Which Interrupt Has The Highest Priority?

    Answer :

    TRAP has the highest priority

  73. Question 51. What Are Hardware Interrupts?

    Answer :

    TRAP, RST7.5, RST6.5, RST5.5, INTR

  74. Question 52. What Does Quality Factor Mean?

    Answer :

    The Quality factor is also defined, as Q. So it is a number, which reflects the lossness of a circuit. Higher the Q, the lower are the losses.

  75. Question 53. What Is Stack Pointer?

    Answer :

    Stack pointer is a special purpose 16-bit register in the Microprocessor, which holds the address of the top of the stack.

  76. Question 54. What Are Level-triggering Interrupt?

    Answer :

    RST 6.5 & RST 5.5 are level-triggering interrupts.

  77. Question 55. What Is The Difference Between Primary & Secondary Storage Device?

    Answer :

    In primary storage device the storage capacity is limited. It has a volatile memory. In secondary storage device the storage capacity is larger. It is a nonvolatile memory. Primary devices are: RAM / ROM. Secondary devices are: Floppy disc / Hard disk.

  78. Question 56. Name 5 Different Addressing Modes?

    Answer :

    1. Immediate
    2. Direct
    3. Register
    4. Register indirect
    5. Implied addressing modes
  79. Question 57. What Is Clock Frequency For 8085?

    Answer :

    3 MHz is the maximum clock frequency for 8085.

  80. Question 58. What Is The Difference Between 8086 And 8088?

    Answer :

    The BIU in 8088 is 8-bit data bus & 16- bit in 8086.Instruction queue is 4 byte long in 8088and 6 byte in 8086.

  81. Question 59. Give Example For Non-maskable Interrupts?

    Answer :

    Trap is known as Non-Maskable interrupts, which is used in emergency condition.

  82. Question 60. What Is Sim And Rim Instructions?

    Answer :

    SIM: is Set Interrupt Mask. Used to mask the hardware interrupts.

    RIM: is Read Interrupt Mask. Used to check whether the interrupt is Masked or not.