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Electrical Level 3 and 4 > Programmable Logic Controllers > Flashcards

Programmable Logic Controllers Flashcards

1
Q

PLCs were first used by what industry?

A

Automobile industry

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2
Q

The PLC is designed to be operated in what type of environment?

A

Industrial

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3
Q

Most PLCs are designed to be programmed with ____ _____ and _____ ______.

A

Relay schematic

Ladder diagrams

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4
Q

Four basic parts of a PLC

A

Power supply
Central processing unit (CPU)
Program loader or terminal
I/O (input/output module)

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5
Q

The power supply is used to

A

lower the incoming AC voltage to the desired level, rectify it to DC, and then filter and regulate it

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6
Q

The internal logic circuits of PLCs operate on a voltage range of? And must be regulated to within __ % of the required voltage.

A

5V to 15V DC

5%

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7
Q

The CPU is commonly referred to as the ?

A

Brain of the PLC

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8
Q

The CPU contains?

A

Microprocessor chip and related integrated circuits to perform all the logic functions.

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9
Q

Most CPUs are designed so that once the program has been tested, it can be ?

A

Stored as an electronic or printed file

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10
Q

The programming terminal is used to?

A
  • load and modify the user program in a PLC

- troubleshoot the circuit

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11
Q

The PLC has no NEMA symbols, it only recognizes ?

A

Open or closed contacts

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12
Q

The number on the contact is used to?

A
  • Distinguish one contact from another

- Represent an input or output terminal screw to which a signal or control wire will be attached

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13
Q

Examine If Open (XIO) bit specs

A
  • Represented by a NC schematic symbol
  • It is “true” or it will be continuous and allow current flow, if no voltage is on the terminal screw associated with the memory address
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14
Q

Examine If Closed (XIC) bit specs

A
  • Represented by a NO schematic symbol

- is “True” and thus will exhibit electrical continuity when a voltage signal is present on the terminal

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15
Q

Components drawn with heavy dark lines a schematic indicate what?

A

That there is a complete path for current flow

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16
Q

I/O Modules are used to?

A

Connect the CPU to the outside world

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17
Q

Input modules carry info ____ the CPU.

A

To

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18
Q

Output modules carry info ____ the CPU?

A

From

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19
Q

The input and outputs may be ______ or ______ ?

A

Discrete (on/off)

Analog (variable)

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20
Q

Most modules contain ___ to ___ inputs and outputs?

A

8 to 32

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21
Q

The purpose of the input is to?

A
  • Handle different inputs from pilot devices such as push buttons float switches, inductive or capacitive prox switches, P/E, ultrasonic devices or limit switches.
  • Debouncing any switch contacts connected to it
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22
Q

The purpose of the output is to?

A

Control up to several external devices such as pilot lights, solenoids, contactors, or motor stater coils

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23
Q

I/O module operating voltages can be __ or __ and are generally ____ V AC or ____ V DC?

A

AC or DC
120V AC
24V DC

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24
Q

What is Scan Time on a I/O module?

A

The speed at which the processor can complete one scan of the I/O devices and implement the changes dictated by the logic in the program. (Faster scan time = more $$$)

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25
Q

The CPU of a PLC is extremely sensitive to voltage spikes and electrical noise and therefore most inputs and outputs use ?

A

Opto-isolation to separate the incoming signal from the CPU electrically

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26
Q

The output module is used to connect the CPU to the ?

A

Load

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27
Q

If the output is designed to control a DC voltage what is used to control the load?

A

Power transistor

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28
Q

If the output is designed to control an AC load what is used as the control device?

A

A TRIAC

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29
Q

Inductive loads such as relays, contactors, and solenoid valves should have what in order to prolong the life of PLC output circuits?

A

Surge Suppression

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30
Q

The surge suppression device should be placed in ____ with the load and ______ to the load as possible?

A

Parallel

As close

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31
Q

Suppression devices include ?

A

RC networks

Diodes

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32
Q

The actual logic of the control circuit is performed by?

A

Internal Relays

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33
Q

What is an internal relay?

A
  • An imaginary device that exists only in the logic of the computer.
  • Can have one to several hundred contacts
  • Contacts can be NO or NC
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34
Q

The internal relays of PLCs can be used as?

A

Counters and Timers

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35
Q

As a general rule, PLCs with large I/O capacities will have a ____ memory and PLCs with small I/O capacities will have ____ memory.

A

Large

Less

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36
Q

List the factors that must be considered when selecting a location for the PLC enclosure?

A
  • cooling of the enclosed equipment
  • protected from dust, dirt, and liquids
  • vibration
  • electromagnetic interference
  • distance to the equipment to be controlled
  • service access
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37
Q

When installing the equipment in the enclosure, it is recommended that the PLC be installed?

A

Near the bottom of the cabinet to protect the PLC from heat generated within the cabinet

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38
Q

The IEC uses a 2 digit code to describe the degree of protection offered by an enclosure, the term is?

A

Ingress Protection

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39
Q

The first and second digits of ingress protection refer to?

A

First - describes the degree of protection against access to hazardous parts and solid foreign objects

Second - describes the degree of protection the enclosure offers to the ingress of liquids

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40
Q

Power for the PLC should come from an ?

A

Isolation Transformer

41
Q

All other power wiring for a PLC should be as _________ after entering the enclosure and be ___________ from other wiring in the cabinet.

A

short as possible

separated as far as possible

42
Q

Why is it essential to have an emergency stop circuit installed ?

A

In the case of PLC failure or an emergency that requires immediate shut down of all input an output devices.

43
Q

Emergency stop circuits must be _______ using _________ components.

A

Hardwired

Electromechanical

44
Q

Why are wire runs for I/O circuits kept as short as possible?

A

A long wire run has more surface area of wire to pick up stray electrical noise

45
Q

What type of cable is used for the installation of signal wiring?

A

Shielded cable

46
Q

Why is shielded cable used signal runs?

A

A shielded wire is used for long runs to help reduce induced electrical noise ( Noise reduction ratio of 30 000:1)

47
Q

Never run signal wire in the same conduit with ?

A

Power wiring

48
Q

When it is necessary to cross power wiring, signal wiring should be installed at?

A

Right angle to power wiring

49
Q

Input devices should never be wired in ____/____ arrangements.

A
  • Series/Parallel
  • Each input device requires its own set of input terminals
  • Same rules for output devices
50
Q

A PLC input is said to be a “sinking” input if the input energizes when a _________ is applied to the input terminal

A

High level (active high)

51
Q

An electronic device often used to help overcome the problem of induced noise is the ?

A

Differential Amplifier

52
Q

Explain the operation of a differential amplifier

A

Amplifies the difference only between 2 input signals and ignores induced electrical noise on the line.

53
Q

What is the brute-force method of bonding?

A

is when two pieces of equipment are connected with heavy cable to force them to exist at the same ground potential.

54
Q

What are the benefits of using a PLC?

A
  • Can replace hundreds of relays and occupy a fraction of the space
  • Circuit logic can be changed without requiring extensive physical rewiring
  • No moving parts or contact to wear out
  • Downtime is less than an equivalent relay circuit
55
Q

Microprocessor based devices package binary bits into?

A

Nibbles - 4 bits
Bytes - 8 bits
Word - 16 bits
Double Word - 32 bits

56
Q

What allowed the development of PLCs?

A

Microprocessors

57
Q

What are the 4 steps of ladder logic?

A
  1. Processor scanning
  2. Read input status
  3. Solve ladder logic program
  4. Update the outputs
58
Q

What does input image table do?

A

Keeps track of the input status of each input

59
Q

What does output image table do?

A

Controls the status of each output

60
Q

Scan time can be from ____ msec to ___ msec, depending on ?

A

1 mec to 200 msec

  • # of I/O modules
  • program length
  • types of instructions
61
Q

Inputs and outputs are electrically isolated using?

A

Opto-isolators

62
Q

Input module specs

A
  • Usually discrete (on/off) (switches, PBs, floats etc.)
  • Sometimes analog (RTDs, thermocouple inputs, 4-20mA inputs, 1-5V etc.)
  • Use analog to digital (ADC), to convert varying inputs to a binary value
  • AC or DC, 24VDC, 120VAC, 10-60VDC, 5VDC etc.
63
Q

Output module specs

A
  • usually discrete (on/off)
  • AC or DC or Relay Outputs (dry contacts, potential free)
  • Analog outputs have a digital to analog converter (DAC) and may convert a 16 bit word to a 4-20mA signal or 1-5V etc.
64
Q

TRIAC outputs specs

A
  • 2A max
  • when off may have leakage current to activate high impedance (Small) load
  • may require “burden resistors” in parallel with load
  • or “interposing relays”
65
Q

If an input device switches + VDC to the input module, the device is _____ and the module is ______ .

A

Sourcing

Sinking

66
Q

If an input device switches - VDC, then the module is _____ and the device is _____.

A

Sourcing

Sinking

67
Q

Logic is solved ___ to ____, ____ to ____

A

left to right

top to bottom

68
Q

Internal bits are ______ internal to the PLC.

A

Registers

69
Q

Internal bits are used for?

A

“one shots” or where the status of one rung is required for a subsequent rung.

70
Q

What is Latching?

A
  • Sets an output ON even when the logic in the run becomes FALSE.
  • Latched outputs remain TRUE even during a power outage
71
Q

What is OSR?

A

one shot rising

72
Q

Explain the function for an OSR

A

an instruction that will be TRUE for 1 scan only

73
Q

Applications for an OSR

A
  • Incrementing the speed of a motor
  • Toggle a condition on or off
  • Capturing a fast counter
74
Q

What does RTO stand for?

A

Retentive timer

75
Q

Enable bit (EN)

A

this bit is set to 1 (TRUE) when the run condition becomes TRUE and reset (to O) when the run condition becomes FALSE

76
Q

Timer Timing Bit (TT) On Delay

A

The bit will be TRUE when the rung condition becomes TRUE and the accumulated value is less than the preset value.

77
Q

Done Bit (DN) On Delay Timer

A

The bit is TRUE when the rung condition is TRUE and the accumulated value is greater than or equal to the preset value.

78
Q

Preset Value

A

the number representing how many counts of a time base it takes for the DONE BIT to be set.

79
Q

Accumulated Value

A

This is incremented every counting timebase when the rung condition becomes TRUE.

80
Q

Timer Timing Bit (TT) Off - Delay Timer

A

The bit is TRUE when the rung condition is FALSE and the accumulated value is less than the preset value

81
Q

Done Bit (DN) Off Delay Timer

A

The bit is TRUE when the rung condition is TRUE and reset when the accumulated value equals the preset value.

82
Q

Do not use a reset instruction with a ______

A

TOF

83
Q

Fail safe wiring

A

Input device must be wired N.C or N.O.H.C

84
Q

4 - 20mA Current Loop Input Specs

A
  • have less influence from transient voltages, surges and spikes.
  • induced voltages do not affect currents like a 0-10V or 1-5V sensor and input module
  • shielded input wiring for sensors reduces ill effects of plant noise EMI and RFI
85
Q

What type of bonding is achieved with large bonding conductors?

A

Brute force bonding

86
Q

What is the purpose of a differential amplifier?

A

Provide noise cancellation

87
Q

List the 5 types of power supplies in order from worst to best

A
  1. Raw power
  2. Isolating transformer
  3. Voltage regulating transformer
  4. Conditioned power
  5. UPS
88
Q

What is the purpose of a subroutine?

A
  • make programs more efficient

- improve organization for ease of understanding, interpretation and future editing for everyone involved

89
Q

Instruction JSR

A

redirects logic execution from the current ladder file to the specified subroutine file

90
Q

Instruction SBR

A

The first instruction of a subroutine

91
Q

Instruction RET

A

Directs the processor scan to return to where it came from

92
Q

To use a high speed instruction you must use?

A

A jumper

93
Q

How does a HSC differ from a CTU or CTD?

A

HSC is a hardware counter as opposed to a software counter and that the HSC operates asynchronously to the ladder program scan

94
Q

Inputs wired to the input module control the?

A

Input image table

95
Q

List the three basic operations that the CPU section performs

A
  1. Read the inputs
  2. Solve the logic program
  3. Write to the outputs
96
Q

Modifying relay type process control circuits usually involves changing?

A

The input circuit modules

97
Q

The power required to operate the logic circuits of the processor unit is?

A

Low voltage DC

98
Q

The scan time is the time required to?

A

execute one cycle of the total program

Electrical Level 3 and 4 (10 decks)

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