99 PLC SCADA Interview Questions and Answers 4

PLC SCADA Interview Questions and Answer

PLC SCADA Interview Questions 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

What is the Difference between  Micro Logic and SLC  Micrologix

1. Has limited 1/0                 Large capacity of 1/0
2. Use DFI only                      Use PID , DH+


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