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Electrical - Level 4 > PLC's > Flashcards

PLC's Flashcards

1
Q

What allowed the development of PLC?

A

Microprocessors

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2
Q

What was the first PLC?

A

Modicon GM

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3
Q

How are PLC’s programmed?

A

using ladder logic diagrams or english

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4
Q

What do ladder logic programs use in their schematics compared to motor control schematics?

A

Ladder uses instructions

Motor control uses N.C. & N.O. contacts

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5
Q

What is ladder logic used to represent?

A

AND, OR, NOT plus arithmetic, timing, counting and memory functions

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6
Q

What are some advantages of PLC’s?

A
  • Good for noisy industrial environments
  • Has isolation
  • Accepts inputs from pilot devices
  • Outputs to contactors
  • Work in wide temp. ranges
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7
Q

What does EEPROM stand for?

A

Electrically Erasable Programmable Read Only Memory

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8
Q

What are opto-isolators used for?

A

So there is no electrical connection between input & output devices

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9
Q

How many bits is 1 Byte?

A

8 bits

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10
Q

how many bits is a nibble?

A

4 bits

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11
Q

What does the input image table keep track of?

A

the input status of each input

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12
Q

what does the output image table control?

A

the status of each output

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13
Q

How does processor scanning work?

A
  1. Read input status
  2. Solve ladder logic program
  3. update the outputs
  4. repeat
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14
Q

what is the approx time for a scan?

A

1 to 200 milliseconds

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15
Q

what does scan time depend on? (3)

A
  1. # of I/O
  2. Program length
  3. Types of instructions
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16
Q

When is a device “sourcing” and the module “sinking” ?

A

if an input device switches +VDC to the input module (module supplies the -Ve)

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17
Q

When is a module “sourcing” and device “sinking” ?

A

if an input device switches -VDC and the module supples the +Ve

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18
Q

What will happen if a faulty program is downloaded?

A

PLC may be bricked (locked-up)

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19
Q

What are internal bits?

A

Registers internal to the PLC (virtual coils)

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20
Q

Where are internal bits used?

A

in programming for “one-shots” or where the status of one rung is required for a subsequent rung

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21
Q

What does latching do?

A

Sets an output on even when the logic in the rung becomes false

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22
Q

What would happen to latches during a power outage? Is low voltage protection provided?

A

They remain true, and low voltage protection would not be provided unless a MCR is provided

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23
Q

What is a One Shot Rising Instruction?

A

An instruction that will be true for only one scan

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24
Q

Where is an OSR typically placed?

A

In a rung after other “trigger” input instructions

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25
Q

What happens to the output instruction associated with the OSR?

A

it will only be true for the duration of the scan

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26
Q

What does the OSR do after the completion of the scan?

A

It will hold the rung false even with prior instructions still true

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27
Q

What happens to an OSR when the input logic goes false?

A

it will be triggered again when the rung conditions become true

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28
Q

What are some applications for an OSR?

A
  1. incrementing speed of a motor equally with one press of a button
  2. One button used to toggle a condition on or off
  3. Capturing a fast counter which would otherwise be unreadable
29
Q

what are the 3 words for each timer?

A

Enable bit, timer timing bit, Done bit

30
Q

What does the Enable Bit do?

A

is set (to 1) when the rung becomes true and reset (to 0) when the rung becomes false

31
Q

What does the Timer Timing Bit do?

A

is set when rung becomes true and accumulated is less than preset value

32
Q

What does the Done Bit do? What is it used for?

A

is set when accumulated value is greater than or equal to preset value. Used to control output devices

33
Q

How does a Retentive timer work?

A
  • RTO timer retains the accumulated value when the rung conditions are false and when the timer is done
  • Must have a reset instruction addressed to it
34
Q

List 5 basic sections of a PLC?

A
  1. Input module
  2. Output module
  3. Opto-Isolators
  4. Central Processing Unit
  5. Power Supply
35
Q

Inputs wired to the input module control the _____________?

A

Input Image Table

36
Q

List 3 basic operations that the CPU section of the PLC performs?

A
  1. Read the inputs
  2. Solve the logic program
  3. Write to the outputs
37
Q

When will the output instruction in a rung become true?

A

If the rung conditions become true up to that point

38
Q

What is the max current for a Triac Output Module?

A

2 A

39
Q

What may Triac Outputs require and why?

A

May require a Burden Resistors in parallel with load or interposing relays. When off it may give enough leakage current to activate high impedance (small) loads

40
Q

What do analog output modules have?

A

A digital to analog converter

41
Q

Where can you not use a reset instruction?

A

Timer Off Delay

42
Q

What must be done for fail safe wiring?

A

Input devices shall not be wired normally open in the field. Fail safe = normally closed or NOHC

43
Q

What does shielded input wiring for sensors do?

A

Reduces ill effects of plant noise, EMI and RFI

44
Q

How must you bond shielded input wiring? Why?

A

Bond one end only to prevent ground loops

45
Q

How is brute force bonding achieved?

A

With large bonding conductors

46
Q

What does a differential amplifier do?

A

Provides electrical noise cancellation

47
Q

What can using interposing relays in Triac outputs do?

A

To avoid leakage current issue, and control loads with higher power requirements (higher voltage/current). Also used to interface different voltages (AC to DC)

48
Q

List the power supply options from least to most desirable (5)

A
  1. Raw power
  2. Isolating transformer
  3. Voltage regulating transformer
  4. Conditioned power
  5. Uninterruptible power supply
49
Q

Where does the main program run in a PLC?

A

In ladder file #2

50
Q

Which files are designated for subroutines?

A

Ladder files 3 thru 255

51
Q

What do subroutines do?

A
  • Make programs more efficient

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

52
Q

What does a JSR instruction do?

A

Redirects logic execution from current ladder file to specified subroutine file

53
Q

What is a SBR instruction?

A

The 1st instruction of a subroutine

54
Q

What is a RET instruction?

A

Directs the processor scan to return to where it came from

55
Q

Why does the CPU have a key switch?

A

To prevent the programmed logic from being changed accidentally

56
Q

What voltage do internal logic circuits of PLCs operate on? What must it be free from and regulated to?

A

5 - 15V DC

  • must be free of voltage spikes and other electrical noise
  • regulated to within 5% of the required voltage
57
Q

What do discrete and analog mean?

A

Discrete - on or off

Analog - variable

58
Q

What operating voltages are generally used for I/O modules?

A

120 Vac or 24 Vdc

59
Q

What should have surge suppressors to prolong the life of a PLC?

A

Inductive loads such as relays, contactors and solenoid valves

60
Q

What do surge suppressors do?

A

Reduce voltage transients and prevent electrical noise

61
Q

What must an enclosure for a PLC provide?

A

Method of cooling either by convection or cooling fans

62
Q

Where is it recommended a PLC be installed in a cabinet?

A

At the bottom

63
Q

What is shielded cable used for?

A

Signal wiring

64
Q

Where must signal cable never be ran?

A

In same conduit as power wiring

65
Q

How should input and output devices never be wired?

A

In series or parallel arrangements

66
Q

When is a PLC input said to be a sinking input?

A

If the input energizes when a high level voltage is applied

67
Q

When is a PLC input said to be a sourcing input?

A

If the input energizes when a low-level voltage is applied

68
Q

Why is there a problem using a sourcing input module?

A

If a ground develops between sensor and input module, the input will energize if the negative side of the power supply is grounded

Electrical - Level 4 (8 decks)

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