99 PLC SCADA Interview Question and Answers 4

99 PLC SCADA Interview Question and Answers 

61.      What are the Types
of I/Os?
Ø  Local – These are the I/Os placed
in the PLC main rack containing CPU. These I/Os are connected to CPU through
Distributed – These are the I/O placed at remote location from the
main rack containing the CPU. These I/O’s are to be connected on communication
bus like control net, device net or FIP I/O.
62.      What is meaning of
resolution in I/O cards in PLCs?
It is the minimum change in i/p parameter which can sensed by the
i/p card. As far as Digital I/O is concerned it takes only one bit for
operation. In case of analog input the resolution determines how much bits are
used for input or output. For example a 12 bit resolution card means the input
will come as 0 to 4095 count (2
Ù12). For 16 bit data the counts will be from
0 – 65536(2
Ù16). More the resolution the data
will be more accurate.
63.      What is an Analog
Input Module?
An I/O module that contains
circuits that convert analog input signals to digital values that can be
manipulated by the processor. The signals for pressure, flow, level,
temperature transmitters are connected to this module. Typically the input
signal in 4-20 mA, 0-10 V
64.      What is Analog Output Module?
An I/O module that contains
circuits that output an analog dc signal proportional to a digital value
transferred to the module from the processor. By implication, these analog
outputs are usually direct (i.e., a data table value directly controls the
analog signal value).
65.      What is meaning of universal analog input card?
Normally there are different
cards for different signals. But in universal input card the same channels can
be configured for RTD, Thermocouple, Current or voltage input.
66.      Give examples of I/P and Output connected to PLCs
Ø  Digital I/P (Pushbutton, Switches)
Ø  Analog I/P (Temp, Pressure, Flow, Level)
Ø  Digital O/P (Solenoids valves, Contactors)
Ø  Analog O/P (Control Valves, Speed)
67.      Explain Source and Sink Concept?
Sinking Source type modules gives
out the current to the field digital devices while sins type modules draw
current when the device is in high sate.
Sinking – When active the output
allows the current to flow to a common ground.
Sourcing – When active, current
flows from a supply, through the output device and to ground.
68.      What is forcing of I/O?
Forcing the I/Os means making the
desired status of I/O in PLCs irrespective of its status coming from the field.
In certain cases when there is
problem in receiving field input/ output, we can force the i/Os so that the
logic takes desired state.
69.      Scan cycle of PLC
PLC’s can cycle follows following
path Scan cycle of PLC
Ø  Input Image Updation
Ø  Process Logic Execution
Output Updation
70.      What is meaning of scan time in PLC?
Scan time is the Time required to
read the I/P, Process the logic and update the output in one cycle.
71.      What is typical scan time in PLCs? What effects scan time?
Typically it is less than 10 ms.
It depends on the complexity of logic, PID algorithm etc.
72.      How to program PLCs?
Every PLC manufacturer have their
own software for programming the PLC. For example Siemens uses Simantic S7
Manager, Allen Bradley uses RS Logix and Modicon uses PLC pro programming
software. The programming language used is Ladder Logic (LD), Statement List
(STL), Functional Block Diagram (FBD), Sequential Foundation Chart (SFC),
Instruction List (IL) etc.
73.      What is ladder diagram?
This is a programming language,
which expresses a program as a series of “coils” and “contacts”, simulating the
operation of electromechanical relays. The resultant program is the equivalent
of an equation, which is executed continuously in a combinatorial manner. The
advantage of this language is the familiarity many electricians have with the
simple operation of relays. Disadvantages include the complexity of large,
cross-connected programs, and the difficulty of expressing such non-binary
functions as motion control and analog I/P.
74.      What is redundancy?
The capacity to switch from
primary equipment to standby equipment automatically without affecting the
process under control. Redundancy means provision for standby module. In case
of failure of one module is running process, the standby module takes over. Hot
redundancy means the changeover of control from active processor to standby
processor in less than 1 scan time.
75.      Need of redundancy
in critical processes, it is
important to run the plant without failure. In such case it is important to
have redundancy so that even in one system fails the redundant system can take
care without affecting plant.
76.      Types of redundancy
CPU redundancy : In case of CPU
failure the standby CPU takes care of the plant.
Power Supply redundancy : In case
the power supply fails the standby power supply takes control of the situation.
Communication : Multiple
communication channels are provided to take care of communication failure.
I/O Redundancy : Multiple I/O
channels are provided to take care of input or output failure.
77.      What are components of redundant PLC system?
Typical component on Schneider
Redundant PLCs
The backplane used is either 4
slot o66 slot with
Ø  Power Supply
Ø  Controller with built-in Modbus Plus and
Modbus ports
Ø  Optional dual cable Modbus Plus
Ø  Optional fiber optic Modubs Plus
Ø  CHS Hot Standby module
Ø  Dual cable Remote I/O Head
The master and Standby
configuration must be identical
78.      Commonly used Instructions in PLCs
Examine if Closed (XIC):- | | –
Examines if the bit is in ON
condition. If the bit is ON the instruction is tue.
Examine if Open (XIO):- |/| –
Examines if the bit is in OFF
condition. If the bit is OFF the instruction is true.
One short rising (OSR):- [OSR]-
When the conditions preceding
the instruction is true, makes the rung run for one program scan.
Note retentive Output instruction
Output Entergies (OTE):– ()–
If the rung is true, it turns
on the bit. If the rung goes or a power cycle occurs the bit turns off.
Retentive Output instruction
Output Latch (OTL):– (L)—
If the rung is true, turns ON a
bit. The bit stays ON until the rung containing an OUT with the same address
goes true.
Output Latch (OUT):– (U)—
If the rung is true, turns OFF
a bit. The bit stays OFF until the rung containing an OTL with the same
address goes true.
79.      Timers
Timer and Counter are used to
control operation based on time or number of events
Types of timers
TON – (Timer ON delay)
An output instruction that can
be used to turn an output ON or OFF after the timer has been timing for a
preset time interval.
TOF – (Timer OFF delay)
An output instruction used to
turn an output ON or OFF after its rung has been off for a preset time
RTO – (Retentive Timer)
An output instruction that can
be used to turn an output ON or OFF after the timer has been timing for a
preset time interval. Once it has begin timing, it holds the count of time
even when the rung continuity is lost. 
Timer Status Bit
DeviceNet: A low-cost communication link that connects
industrial devices to a network It is based on broadcast – oriented
communication protocol- the Controller Area Network (CAN).
Ethernet: The standard for local communications network
developed jointly by Digital Equipment Corp, Xerox, and Intel Ethernet base
band coaxial cable transmits data at speed up to 10 megabits per second.
Ethernet is used as the underlying transport vehicle by several upper-level
protocols, including TCP/ IP.

80.      What are the PLC ranges available in Rockwell?
Pico : Non
modular small PLCs
1000, 1200 and 1500 Series
5/01, 5/02, 5/03 ….
Logix Flex Logic and Soft PLC
Diff b/w Micro Logic and SLC   Micrologix
Has limited 1/0                 Large
capacity of 1/0
Use DFI only                      Use PID
, DH+

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