1.What are the main advantages of PLC?
 Modular design
 High reliability
 Economy
 Size
 Ease of programming
 Rugged construction and designed to withstand vibrations,temperature,humidity,noise
 Ability to communicate with computer
2.What are the major areas of application of PLC?




Sequence control,timing,counting,and data calculation
Batch or continuous process control
Precise position/motion control
Open loop or feed back control ,process data acquisition and display
3.Write about the relay
With the relay type,the signal from the PLC output is used to operate a relay and so is able to
switch currents of the order of a few amperes in an external circuit.The relay isolates the PLC
from the external circuit and can be used for both D.C and A.C.switching. Relays are how
ever relatively slow to operate.
4.What are the three sizes of PLCs
 Small
 Medium
 Large
5.What is the general application of the PLC?
PLCs are mostly applied in steel mills,paper and pulp plants,chemical and automotive and
power plants.
6.Write the key factors in selecting a PLC?
The key factor in selecting a PLC is establishing exactly what the unit is supposed to do.In
general it is not advisable to buy a PLC system that is larger than current needs dictate.How
ever,future conditions should be anticipated to ensure that the system is the proper size to fill
the current and possibly future requirements of an application
7.Define memory
Memory is where the control plan or program is held or stored in the controller.The
information stored in the memory relates to how the input and output data should be
processed
8.List the types of memories
 Read only memory(ROM)
 Random Access Memory(RAM)
 Program Read-Only Memory(PROM)
 Erasable Programmable Read only Memory(EPROM)
 Electrically Allocable Read Only Memory(EAROM)
 Electrically Erasable Programmable Read only Memory(EEPROM)

Magnetic Core Memory
9.Explain the functions of PLC input and Output modules
 Input interface modules accept signals from the machine and convert them in to
signals that can be used by the controller
 Output interface modules convert controller signals in to external signals used to
control machine or process
10.Expalin On delay and OFF delay timer
ON delay timer:
This increment while it receives power flow and resets to zero when power flow and resets to
zero when power flow is stopped. The On delay timer will turn ON the output device after
delay
OFF delay timer:
OFF delay timer increments while power flow is off and resets to zero when power flow is
on. The Off delay timer will turn OFF the output device after delay.
11.What is a PLC?
A programmable logic controllers are industrially hardened micro computers designed to
replace electromechanical relays that perform discrete or continuous functions in a variety of
processing plant and factory environments.
12.Why the term logic is used?
The term logic is used because the programming is primarily concerned with implementing
logic and switching operations
13.Write the operating principle of the logic controllers
Input devices and output devices are connected to the PLC. The controller monitors the
inputs and outputs according to the program stored in the PLC by the operator and controls
the machine or process
14.Write about the architecture of PLC
It consists of a CPU,memory,input/output circuitry.The CPU controls and processes all the
operations with the PLC.It is supplied with a clock with a frequency between 1 and 8 MHz.It
also has a bus system,memory and input/output units a system ROM for permanent
storage,RAM for the users program and temporary buffers.
15.What happens when the power supply is switched off?
When power supply is lost PLC will loose its programmed content. To prevent the loss of the
programs,when the power supply is switched off,a battery is likely to be used in the PLC to
maintain the RAM contents for a period.
16.Write about the input /output channels
The input/output channels provide signal conditioning and isolation functions so that sensors
and actuators can be generally directly connected to them without the need for other circuitry.
Common input voltages are 5V and 24V.Common output voltages are 24 V and 240V
17.What are optoisolators
Opto isolators are used with transistor switches to provide isolation between the external
circuit and the PLC. They are used to provide isolation.
18.List out the programming languages in PLC
 Ladder Logic(LAD)
 Statement List(STL)
 Function block diagram(FBD)
19.What is mean by power flow in PLC?
If there is electrical current flow through the relay contacts in a rung,the output relay coil will
be turned ON.This is termed power flow in the ladder rung
20.List the part of PLC
 Central processing unit
 The input/output section
 Programming device
21.Define program ladder
Program ladders are used to record and store the program or to load pre-programmed
instructions in to the processor.There are two types of program ladders
 Magnetic cassette
 Disk recorder and electromagnetic modules
22.List three possible functions of a PLC programming device
 The programming device provides the primary mean by which the user can
communicate with the circuits of controller.
 The programming device allows the user to enter, change, or monitor a PLC
controller program
23.Explain the function of an optical isolator
 There are three main functions
 Separate the higher ac input voltage from the logic circuits
 Prevents damage to the processor due to line voltage transients.
 Helps to reduce the effect of electrical noise, which can pass entire operation of the
processor
24.What is the function of processor in PLC?
The processor reads the inputs, executes logic as determined by the application program,
performs calculations, and controls the outputs accordingly. It controls the operating cycle or
processor scan
25.List the relay type instructions
Instruction set is composed of contact symbols so the ladder diagram language is also
referred as contact symbology
26.List three possible functions of a PLC programming device?
•Easy to use programming equipment is an important feature of the PLC.
•The programming device provides the primary mean by which the user
•Can communicate with the circuits of controller.
•The programming device allows the user to enter ,change, or monitor a PLC controller
program
27.Explain the function of an optical isolator?
There are three main functions . They are
•Separate the higher ac input voltage from the logic circuits.
•Prevents damage to the processor due to line voltage transients.
•Helps top reduce the effect to electrical noise which can pass entire operation of the
processor.
28.List the conditions for drawing the ladder logic ?
•Contacts may be always inserted in the upper left .
•Coils must be inserted at the end of a rung.
•All contacts must run horizontally (i.e)no vertically oriented contacts.
•The contacts must be nested .
•Flow must be from left to right.
•Contact progression should be straight across.
29.List the sequence of operations carried out in PLC programming?
•Processor memory organisation
•Program scan
•PLC programming language
•Relay type instruction
•Instruction addressing
•Branch instruction
•Internal relay instruction
•Programming examine ON and OFF instructions
•Entering the ladder diagram.
•Modes of operation.
30.What is meant by program scan?
During each operating cycle the processor reads all the inputs,takes their values and
according to the user program energies or de_energies the output .This process is known as
program scan.
31.What is meant by PLC programming languages?
PLC programming languge refers to the method by which the user communicate information
to PLC
32.List the relay type instructions?
Instruction set is composed of content symbols so the ladder diagram langue is also referred
as contact symbology
1.EXAMINE ON instructions
2.EXAMINE OFF instruction
3.OUTPUT ENERGISE
33.What is meant by internal relay instructions?
Most PLCS have an area of memory allocated for what are known as internal storage bits
.These storage bits are also called internal outputs ,internal coils,internal control relays are
just internals .The internal outputs does not directly control an output device.
34.List the modes of operations in PLC?
There are 5 different modes of operations .They are
1.Connect PLC with PC
2.Clering the Ram
3.Store the logic in the Ram
4.Run
5.Continous until stop mode is selected.
35.What are the applications of PLC?
1.Sequence control, timing, counting, data calculation
2.Batch (or)continuous process control
3.Pricise position/motion control
4.Open loop or feedback control ,process data acquisition and display
36.What are the word transfer instruction in Allen Bradley PLC5?
Bit distribute(BTD)
Move(MOV)
Masked move(MVM)
37.What are the arithmetic operations
ADD-addition
SUB-subtraction
MUL-Multiplication
DIV-Division
38.What are the data comparison instructions
Equal to-EQU
Less than-LES
Greater than-GRT
39.Name some program control instruction
Master control relay-MCR
Jump(IMP)
Label-LBL
40.What are the three inputs in PLC sequencer function
Step circuit
Reset circuit
Enable circuit
41.What are the uses of PC
Maximizes plant yields
Increases plant capacity and productivity
Ensures the repeatability an the product quality
Reduces raw material and energy costs
Increases safety of plant operation and improved profitability
42.What are the applications of PC
Electric oven temperature control
Reheat furnace temperature control of mill in an integrated steel plant
Thickness and flatness control system for metal rolling
Automation of hot strip mill in an integrated steel plant
Control of Electric Power Generation plant
43.What are the uses of PLC
Used for logic-based sequencing operations
Used for counting, timing and complex mathematical operations
Used to perform PID, feed forward and other control functions
Large scale PLCs having data highway capabilities function well in DCS environment
44.What is PID
PID stands for Proportional, Integral, Derivative controller. PID is an effective control system
for continuous processes, that performs two control tasks. First PID control keeps the output
at a set level even though varying process parameters may tend to cause the output to vary
from the desired set point. Second, PID promptly and accurately changes the process level
from one set point level to another set point level.
45. What is MCR?
The master control relay (MCR) instruction is used in pairs to activate or deactivate the
execution of a group of zone of ladder rungs.
46. What is JMP?
The jump instruction (JMP) allows the normal sequential execution to be altered so the CPU
will jump to a new position in the ladder program.
47. What LBL?
The label (LBL) is to identify the Ladder rung that is the destination of a jump instruction.
48. What are the three subroutine instructions?
1.The jump to subroutine (JSR).
2.The Subroutine (SBR)
3.The return (RET)
49. What is EQU?
The equal to (EQU) instruction is used to test whether two values are equal.
50. What is LES?
Much like the equal to instruction, the less then (LES) instruction tests the contents of the
value of one location (source A) to see if it is less than the value stored in a second location
(source B).
51.Define SCADA?
Supervisory Control And Data Acquisition system is computerized hardware and software
based project that provides a single integrated view of all control and information resources,
enable engineers, supervisors, managers, operators to view and interact with the working of
an entire operation through graphical representation of their product process.
5 2.What is CIMPLICITY?
CIMPLICITY is a SCADA package. It is based on a client-server architecture consisting of
servers and viewers. Servers are responsible for the collection and distribution of data.
Viewers connect into servers and have full access to the collected data for viewing and
control actions.
5 3.State the requirements of CIMPLICITY HMI plant edition?
CIMPLICITY HMI Plant Edition provides an extraordinary selection of features that enables
to configure comprehensive and robust project. It gives a quick tour which provides links to
the related subject in the documentation.
5 4. What is a DDC?
The DDC (Direct Digital Control) directly interface to the process for data acquisition and
control purposes.
55. List the tasks performed by microprocessor in DDC?
• It reads the various variables from different transmitters through multiplexer and ADC.
•It determines the error for each control loop and executes control strategy for each loop.
•It outputs the correction value of control valve through DAC.
56.Mention the two algorithm used in DDC software?
There are two algorithms for programming a three mode PID control loop
•Position algorithm
•Velocity algorithm
5 7.Define AI?
Artificial Intelligence has been defined as the branch of computer science which deals with
the software and hardware techniques to solve symbolic problems as against ‘number
crunching’ problems solved by EDP machines.
58.What is heuristics?
The learned facts, experience and institution are three basic inputs which enable decision
making collectively these are known as heuristics. In AI systems the decisions are taken on
the basis of heuristics.
59.Mention the various categories under the AI system.
•Natural language system
•Perception system
•Expert or knowledge based system
60.List the various types of information presented to expert system.
The information presented to the expert system may be factual, incomplete, judgmental,
speculative, experimental, uncertain, fuzzy and intuitive. Expert system should derive
conclusions based on such incomplete information just like real life human expert.
61.What is domain knowledge?
The domain knowledge is stored in the knowledge base of system. The domain knowledge
contains:
•Facts
•Relation between facts
•Heuristics
62.Differentiate the two reasoning mechanics.
Backward chaining:-stars with a goal i.e. conclusion which is most portable and then tries to
go backward and match the left hand side of rule i.e. conditions.
Forward chaining:-is a top down approach. It proceeds in the direction until the goal i.e.
conclusion is reached.
63.List the goals for an ideal expert controller.
 Ability to control a large class of processes which may be time varying, non linear,
with variety of disturbance.
 Requirement of minimum prior knowledge about the process. Ability to improve its
performance with time, as it acquires more knowledge.
64.Differentiate between analog controller and digital controller.
Analog controller though are faster than digital controllers, the later is preferred because the
changes in the parameters values are possible in digital controller while not in analog
controller.
65.List down the methodologies used in knowledge representation
It includes
Sematic networks
In this,nods and arcs form graphical notation to represent objects and relationship among
them
Frames
Frames are templates or modules for holding clusters of related knowledge about a particular
narrow subjects
Rules
Rules may be in one of the three forms
a)If {this happens} then do {that} or
b)If(this is the case} then {that is true} or
c) If { this is the case} then do {that}
66.What are the operational objectives that a control system is called upon to achieve?
 Ensuring the stability of the process
 Supporting the influence of external
 Optuimising the economic performance of a plant or
 A combination of the above
67.What are the main constituents of the fuzzy controller
 Fuzzifier
 Knowledge base
 Decision strategy
 Defuzzifier
68.What are the advantages of neural network?
 Good fit for non linear models
 Ability to adapt,generalize and extrapolate results
 Speed of execution in recall mode
 Ease of maintenance
69.What are the disadvantages of neural networks?
 Cannot handle constraints
 Cannot optimize
 Needs of lots of data
 Needs lots of CPU power in training session
 Are unpredictable for utilization in untrained areas
70.What is the important concept in fuzzy logic
It is the grade of membership-A number which describes the extent to which an element is in
a set.It also describes the truth value of a particular statement.
71.What is meant by membership function?
The grade of membership of fuzzy set is defined by membership function.The membership
function provides a direct linkage between fuzzylogic and fuzzy sets.It is defined by
mA(x).Where x is a member of A and A is some proportional or predicate class.
72.What is fuzzy controller?
A controller which is used to control the complex processes is to investigate the control
stratergies followed by a human operator which are based on institution and experience.
73.What is mean by fuzzification
In fuzzy controller,in order to include the control statergy of human operator the ensor
readings which are non-fuzzy should be converted to fuzzy from by a process called
fuzzification
74.What are the manipulated variables to be used to control the process
Ususally in a process, there are number of inputs,variables adjusted freely.Which one we
select to use as a manipulated variables is a crucial question,as the choice will affect the
quality of control actions
75.What are the measure problems in velocity algorithm?
 Controller drift
 Integral overshoot
76 What is the significance of DCS?
Used for interfacing and computing functions and also provides the means of
communication between the other devices. It consists of local control unit, low level
interface, high level interface, shared communication facility. Etc
77. Mention any two advantages of DCS
•more reliable
•small expensive
•cost is lower than centralized system which performs the same function.
78. What is LCU?
Local control unit is the small collection of hard ware in the system that can do closed
loop control. Local control unit interfaces directly to the process.
79. What is the significance of LLHI?
This is the device that allows the operator or instrumentation engineer to interact with
the LCU to change set point, control modes, control configuration, or tuning using a direct
connection. LLHI can also interface directly to the process.
80. What is the function of HLOI?
It has the functions similar to the LLHI with increased capacity and user with
increased capability and user friendliness. It interfaces to other devices only over the shared
communication facilities. Operator oriented program at this level is called HLOI. It is an
instrument engineer oriented program.
81. What is shared communication?
One or more communication hardware and associated software that allow the sharing
of data among all devices in the distributed system. Shared communication facilities do not
include dedicated communication channels between specific devices.
82. What is the need can be satisfied in designing an industrial grade LCU?
•flexibility of changing the control configuration
•Ability to use the controller without being a computer expert.
•Ability to by pass the controller in case it fails. So that the process still can be
controlled manually.
•Ability of the LCU to communicate with other LCUs and other elements in system.
83. What is the architecture parameter to be considered while selecting LCU?
•size of controller
•functionality of controller
•performance of controller
•communication channels out of controller
•Controller output security.
84. Compare the configuration of the controller
Architecture
parameter
Controller size
Controller
functionality
Controller scalability
Configuration A
Configuration B
Number of function
needed for single
PID loop or motor
controller
Uses both continues
and logic function
Includes
and I/O
High
degree
scalability
Configuration C
functions Equivalent to small
DDC system
Uses both continues Uses both continues
and logic function and logic function
Split between the
controller
of Requires
both Not scalability
controller types
85. What are the security requirements of LCU?
• Maximize the availability of the automatic control functions of the system.
•During failure of the controller allows the operator to take the manual control of that
process?
•Operator can shut the process down in an orderly and safe manner.
86. Write objectives of DCS.
1. Safe operation of plant
2. Lowest cost of operation
3. Longest equipment life
4. Minimum environment effect
5. Maximum efficiency
87. What is distributed system?
A distributed system is one in which components located at networked computers
communicate and coordinate their actions only by passing messages.
88. What is DCS?
A distributed control system (DCS) refers to a control system usually of a
manufacturing system, process or any kind of dynamic system, in which the controller
elements are not central in location (like the brain) but are distributed throughout the system
with each component sub-system controlled by one or more controllers. The entire system of
controllers is connected by networks for communication and monitoring.
89. Write benefits of DCS?
High reliability – reduce human error
Improved response time
– allowing easier identification of bottle-necks
Improve operator interface to plant
– better production scheduling, making maximum use of the production
facilities
Improve accessibility of plant data to engineering & management personals
- Faster identification of faults in both product and processing machinery
Historical storage & retrieval system.
90. Write DCS elements1.
Local Control Unit: This unit can handle 8 to 16 individual PID loops.
Data Acquisition Unit: Digital (discrete) and analog I/O can be handle.
Batch Sequencing Unit: This unit controls a timing counters, arbitrary function
generators, and internal logic.
Local Display: This device provides analog display stations, and video display for
readout.
Bulk Memory Unit: This unit is used to store and recall process data.
91. Write DCS elements2.
General Purpose Computer: This unit is programmed by a customer or third party
to perform optimization, advance control, expert system, etc
Central Operator Display: This unit typically contains several consoles for operator
communication with the system, and multiple video color graphics display units
Data Highway: A serial digital data transmission link connecting all other
components in the system. It allow for redundant data highway to reduce the risk of
data loss
Local area Network (LAN
92. What are the Advantages of DCS?
 Access a large amount of current information from the data highway.
 Monitoring trends of past process conditions.
 Readily install new on-line measurements together with local computers.
 Alternate quickly among standard control strategies and readjust controller
parameters in software.
 A sight full engineer can use the flexibility of the framework to implement his
latest controller design ideas on the host computer.
93. List the Modes of Computer control.
1. Manual
2. Automatic
• PID with local set point
3. Supervisory
• PID with remote set point (supervisory)
4. Advanced
94. What are the two methods that control the flow of data across communication links?
1) Stop and wait
2) sliding window
95. What is the mechanism of stop-and-wait flow control?
In the stop and wait method of flow control sender sends one frame and waits for an
acknowledgement before sending the next frame.
96. What is the mechanism of sliding window flow control?
In the sliding window flow control; the sending of data is constrained by imaginary
window that expands and contracts according to the acknowledgement received by the
sender.
97. What does the term error control mean in the data link layer?
Error control in the data link layer is based on the automatic repeat request which
means retransmission of data in three cases: damaged frame lost frame, lost acknowledgment.
98. What is the mechanism of poll/select?
A poll is sent to the secondary device by the primary to determine if the secondary has
to send. The secondary can respond by sending a acknowledgement or a data frame. A select
frame is sent from the primary device to the secondary device to tell the secondary to prepare
to receive data. The secondary responds with an ACK or a NAK
99. Define protocol.
A protocol in data communications is the set of rules or specifications used to
implement one or more layers of the OSI model.
100. What are the two types of data link protocol?
1).Asynchronous protocol
2).Synchronous protocol
101. What is manual backup?
In this case, each LCU is designed to implemented only one or two control loops , and
reliable is placed on the operator to take over manual control in case of a failure of LCU.
10 2. What is redundant control mode?
In this case, the LCU is backed up by another LCU that takes over if the primary
controller fails. In this way, full automatic control is maintained even under failure
conditions.
103. What are the approaches in designing a redundant LCU architecture?
a) CPU redundancy,
b) One-on-one redundancy,
c) One-on-many redundancy,
d) Multiple active redundancy.
104. What is meant by geographically centralized and geographically distributed
control system?
All the LCUs are located in a central equipment room area; it is called as
geographically centralized control system. Each LCU is located in the plant area closest to
the portion of the process that it controls. It is called as geographically distributed control
system.
105. What is functionally distributed control system?
Several LCUs are used to implement the functions required in controlling the
process. Therefore it is called as functionally distributed control system.
106. What are multiple active controllers?
In this case, several LCUs are active at the same time in reading process inputs
calculating control algorithms and producing control outputs to the process. The multiple
active approaches is designed so that a failure of one of the controllers does affect the
automatic control function.
107. What are the redundant approaches of redundant controllers?
The redundant architecture should be kept as simple as possible. There is a law of
diminishing in redundancy design. At some point, adding more hardware will reduce system
reliability. The redundant nature of the controller configuration should be transparent to the
user that is the user should be able to deal with the system in the way as a non redundant one.
108. Mention the requirement of operator interface
•process monitoring
•process control
•process record keeping
109. What are the motivations for using LLOI?
•It provides an interface that is familiar to the operator trying to use panel board
instrumentation.
•Less expensive
•Provide manual back up in the case if high level operator interface fails.
110. Explain about different types of operator display.
•Plant display.
•Area display.
•Group display.
•Lop display.
•Graphic display.
•Trend display.
•Tabular display.
111. What is the function of engineering interface?
•system configuration
•operator configuration
•system documentation
•system failure diagnosis
112. What is data transparency?
Data transparency in data communication means we should be able to send any
combination of bits as data. The combination of bits as data is not confused with the control
information.
113. Define BSC.
Binary synchronous communication is a well-known character oriented protocol
.BSC operates in half-duplex mode using stop and wait ARQ in a point to –point or
multipoint configuration.
114. What is bit stuffing?
Bit stuffing is the process of adding one extra ‘0’ whenever there are five
consecutive ‘1’s in the data. So that the receiver doesn’t mistake the data for a flag.
115. What do you mean by HDLC?
High-level data link control is a bit oriented data link designed to support both halfduplex and full-duplex communication over point-to point and multi point links.
116. How do the three HDLC frame types differ from one another?
a)I-frame -for data transmission and control
b)S-frame-control
c)U-frame –for control and management
117. What are the types of modes in HDLC?
a) Normal response mode (NRM)
b) Asynchronous response mode (ARM)
c) Asynchronous balanced mode (ABM)
118. What is piggy bagging?
Piggy backing means combining data to be sent and acknowledgement of the frame
received in one single frame.
119. Name the four types of s-frames?
a) Receive ready(RR)
b) Receive not ready (RNR)
c) Reject (REJ)
d) Selective reject (SREJ)
120. Name the five categories of U-frame.
1) Mode setting
2) Unnumbered exchanging
3) Disconnection
4) Initiation
5) Miscellaneous
1 21. What are the types of sliding window ARQ error control?
1) go-back-n
2) Selective –reject
122.Distinguish X-modem and Y-modem?
X-modem
1.the data is 128bytes
2one CANS signal is used to abort a
transmission
3.CRC error checking is used
4.It is a half-duplex stop and wait ARQ
protocol
Y-modem
1.the data unit is 1024bytes
2.two CAN signals are used
3.ITU-T CRC is used for error checking
4.here multiple files can be sent
simultaneously
123. What are the disadvantages of asynchronous transmission?
1).speed of transmission is less
2)It leaves unpredictable gaps of time between each character
3).the addition of start and stop bits and the insertion of gaps into the bit stream make
asynchronous transmission slower.
124. What are the two types of synchronous protocol and explain it?
1) Character oriented protocol
2) bit-oriented protocol
125. What is routing table?
A routing table has columns for at least three types of information: the network ID
and ID of the next router. The network ID is the final destination of the packet. The cost is
the number of hops a packet must make packet to get there. And the next router is the router
is the router to which a packet must be delivered on its way to a particular destination.