PLC SCADA DCS Interview Questions and Answers 1

 PLC SCADA DCS Interview Questions and Answers 1

PLC Interview
Questions and Answers

SCADA Interview
Questions and Answers

DCS Interview
Questions and Answers

Friends this the most awaited post I am writing for all the
freasher in the field of Automation and those are looking for job and preparing
for interviews. It is difficult to find the PLC interview questions and answers
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So here we go.
Question : What is
a PLC?
Answer: Programmable Logic
Controllers are industrially hardened micro computers, designed to replac e
electromechanical relays, used to control machines and processes.
Question : What are
the main advantages of PLC?
1.      Modular design
2.      Increased
3.      Lower cost
4.      Compact size
5.      Ease of programming
6.      Rugged construction
and designed to withstand vibrations, temperature, humidity and noise
7.      Ability to
communicate with computer
Question : What are
the major areas of application of PLC?
1.       Sequence control, timing, counting, and data
2.       Batch or continuous process control
3.       Precise position/motion control
4.       Open loop or feedback control, process data
acquisition and display
Question :  What are the advantages of PLC over Relays?
1.       They have to be hard-wired to perform a
specific function
2.       No hard-wire requirement
3.       When the system requirement change, the relay
wiring has to be changed or modified
4.       Can be easily changed or expanded through
5.       Higher power consumption
6.       Lower power consumption
Question :  Explain
the classification of PLC.
There are five major classification of PLC based on number of
inputs and outputs, cost, functionality and physical size. They are
Nano : 16 I/O pins
Micro : 32 I/O pins
Nano or micro PLC could used on applications such as elevators,
car washes or mixing machines.
Small : 128 I/O pins,2Kbytes memory, capable of simple to advanced
level of machine control
Medium: 2048 I/O pins, 32Kbytes memory, used for process control
Large: 8192 I/O pins, 750 Kbytes memory, mostly used to control
individual production process or entire plants like paper and pulp, chemical
and automotive and power plants.
 Question :  Explain the functions of PLC-input and
output modules
1. Input interface modules accept signals from the machine or
process devices (120Vac) and convert them into signals (5Vdc) that can be used
by the controller.
2. Output interface modules convert controller signals (5Vdc) in
to external signals (120Vdc) used to control machine or process.
Question :  List
out the programming languages in PLC.
a. Ladder Logic Diagram
b. Sequential Function Charts
c. Instruction List
d. Function Block
e. Structured Text Diagram
Question: What is Ladder LogicDiagram?
Ladder Logic
Diagram is a symbolic language used in PLC. This diagram consist of series of
symbols interconnected by lines to indicate the flow of current through the
various devices. In ladder logic diagram the sides of the ladder forms the
power sources and current flows through the various –logic input devices that
form the rungs of the ladder.
Question: List the conditions for
drawing thw ladder logic.
a.      Contacts may be
always inserted in the upper left.
b.      Coils must be
inserted at the end of the rung.
c.      All contact must
run horizontally.
d.      The number of
contacts per network are limited
e.      Only one outout may
be connected to a group of contacts
f.       Flow must be from
left to right.
g.      The contact must be
h.     Contact progression
should be straight cross.
Question : Explain On-Delay and
Off-Delay Timer instruction in PLC
a)      The on delay timer
operates such that when the rung containing the timer is true, the timer tiomed
out period commences. At the end of the timer time out period an output is made
b)     The Off delay timer
operation will keep the output energized for 
time period after the rung cotaining the timer has gone false.
Question: What is PLC SCAN ?
During each
operating cycle the processor reads all the inputs , takes the values and
energizes or de-energizes the output according to the user program. This
process is known as PLC Scan . A single PLC scan consists of the I/O scan and
the program scan.

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