Programmable Logic Controllers Flashcards
1
Q
PLCs were first used by what industry?
A
Automobile industry
2
Q
The PLC is designed to be operated in what type of environment?
A
Industrial
3
Q
Most PLCs are designed to be programmed with ____ _____ and _____ ______.
A
Relay schematic
Ladder diagrams
4
Q
Four basic parts of a PLC
A
Power supply
Central processing unit (CPU)
Program loader or terminal
I/O (input/output module)
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
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%
7
Q
The CPU is commonly referred to as the ?
A
Brain of the PLC
8
Q
The CPU contains?
A
Microprocessor chip and related integrated circuits to perform all the logic functions.
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
10
Q
The programming terminal is used to?
A
- load and modify the user program in a PLC
- troubleshoot the circuit
11
Q
The PLC has no NEMA symbols, it only recognizes ?
A
Open or closed contacts
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
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
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
15
Q
Components drawn with heavy dark lines a schematic indicate what?
A
That there is a complete path for current flow
16
Q
I/O Modules are used to?
A
Connect the CPU to the outside world
17
Q
Input modules carry info ____ the CPU.
A
To
18
Q
Output modules carry info ____ the CPU?
A
From
19
Q
The input and outputs may be ______ or ______ ?
A
Discrete (on/off)
Analog (variable)
20
Q
Most modules contain ___ to ___ inputs and outputs?
A
8 to 32
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
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
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
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 $$$)
25
The CPU of a PLC is extremely sensitive to voltage spikes and electrical noise and therefore most inputs and outputs use ?
Opto-isolation to separate the incoming signal from the CPU electrically
26
The output module is used to connect the CPU to the ?
Load
27
If the output is designed to control a DC voltage what is used to control the load?
Power transistor
28
If the output is designed to control an AC load what is used as the control device?
A TRIAC
29
Inductive loads such as relays, contactors, and solenoid valves should have what in order to prolong the life of PLC output circuits?
Surge Suppression
30
The surge suppression device should be placed in ____ with the load and ______ to the load as possible?
Parallel
| As close
31
Suppression devices include ?
RC networks
| Diodes
32
The actual logic of the control circuit is performed by?
Internal Relays
33
What is an internal relay?
- 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
34
The internal relays of PLCs can be used as?
Counters and Timers
35
As a general rule, PLCs with large I/O capacities will have a ____ memory and PLCs with small I/O capacities will have ____ memory.
Large
| Less
36
List the factors that must be considered when selecting a location for the PLC enclosure?
- cooling of the enclosed equipment
- protected from dust, dirt, and liquids
- vibration
- electromagnetic interference
- distance to the equipment to be controlled
- service access
37
When installing the equipment in the enclosure, it is recommended that the PLC be installed?
Near the bottom of the cabinet to protect the PLC from heat generated within the cabinet
38
The IEC uses a 2 digit code to describe the degree of protection offered by an enclosure, the term is?
Ingress Protection
39
The first and second digits of ingress protection refer to?
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
40
Power for the PLC should come from an ?
Isolation Transformer
41
All other power wiring for a PLC should be as _________ after entering the enclosure and be ___________ from other wiring in the cabinet.
short as possible
| separated as far as possible
42
Why is it essential to have an emergency stop circuit installed ?
In the case of PLC failure or an emergency that requires immediate shut down of all input an output devices.
43
Emergency stop circuits must be _______ using _________ components.
Hardwired
| Electromechanical
44
Why are wire runs for I/O circuits kept as short as possible?
A long wire run has more surface area of wire to pick up stray electrical noise
45
What type of cable is used for the installation of signal wiring?
Shielded cable
46
Why is shielded cable used signal runs?
A shielded wire is used for long runs to help reduce induced electrical noise ( Noise reduction ratio of 30 000:1)
47
Never run signal wire in the same conduit with ?
Power wiring
48
When it is necessary to cross power wiring, signal wiring should be installed at?
Right angle to power wiring
49
Input devices should never be wired in ____/____ arrangements.
- Series/Parallel
- Each input device requires its own set of input terminals
- Same rules for output devices
50
A PLC input is said to be a "sinking" input if the input energizes when a _________ is applied to the input terminal
High level (active high)
51
An electronic device often used to help overcome the problem of induced noise is the ?
Differential Amplifier
52
Explain the operation of a differential amplifier
Amplifies the difference only between 2 input signals and ignores induced electrical noise on the line.
53
What is the brute-force method of bonding?
is when two pieces of equipment are connected with heavy cable to force them to exist at the same ground potential.
54
What are the benefits of using a PLC?
- 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
Microprocessor based devices package binary bits into?
Nibbles - 4 bits
Bytes - 8 bits
Word - 16 bits
Double Word - 32 bits
56
What allowed the development of PLCs?
Microprocessors
57
What are the 4 steps of ladder logic?
1. Processor scanning
2. Read input status
3. Solve ladder logic program
4. Update the outputs
58
What does input image table do?
Keeps track of the input status of each input
59
What does output image table do?
Controls the status of each output
60
Scan time can be from ____ msec to ___ msec, depending on ?
1 mec to 200 msec
- # of I/O modules
- program length
- types of instructions
61
Inputs and outputs are electrically isolated using?
Opto-isolators
62
Input module specs
- 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
Output module specs
- 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
TRIAC outputs specs
- 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
If an input device switches + VDC to the input module, the device is _____ and the module is ______ .
Sourcing
| Sinking
66
If an input device switches - VDC, then the module is _____ and the device is _____.
Sourcing
| Sinking
67
Logic is solved ___ to ____, ____ to ____
left to right
| top to bottom
68
Internal bits are ______ internal to the PLC.
Registers
69
Internal bits are used for?
"one shots" or where the status of one rung is required for a subsequent rung.
70
What is Latching?
- Sets an output ON even when the logic in the run becomes FALSE.
- Latched outputs remain TRUE even during a power outage
71
What is OSR?
one shot rising
72
Explain the function for an OSR
an instruction that will be TRUE for 1 scan only
73
Applications for an OSR
- Incrementing the speed of a motor
- Toggle a condition on or off
- Capturing a fast counter
74
What does RTO stand for?
Retentive timer
75
Enable bit (EN)
this bit is set to 1 (TRUE) when the run condition becomes TRUE and reset (to O) when the run condition becomes FALSE
76
Timer Timing Bit (TT) On Delay
The bit will be TRUE when the rung condition becomes TRUE and the accumulated value is less than the preset value.
77
Done Bit (DN) On Delay Timer
The bit is TRUE when the rung condition is TRUE and the accumulated value is greater than or equal to the preset value.
78
Preset Value
the number representing how many counts of a time base it takes for the DONE BIT to be set.
79
Accumulated Value
This is incremented every counting timebase when the rung condition becomes TRUE.
80
Timer Timing Bit (TT) Off - Delay Timer
The bit is TRUE when the rung condition is FALSE and the accumulated value is less than the preset value
81
Done Bit (DN) Off Delay Timer
The bit is TRUE when the rung condition is TRUE and reset when the accumulated value equals the preset value.
82
Do not use a reset instruction with a ______
TOF
83
Fail safe wiring
Input device must be wired N.C or N.O.H.C
84
4 - 20mA Current Loop Input Specs
- 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
What type of bonding is achieved with large bonding conductors?
Brute force bonding
86
What is the purpose of a differential amplifier?
Provide noise cancellation
87
List the 5 types of power supplies in order from worst to best
1. Raw power
2. Isolating transformer
3. Voltage regulating transformer
4. Conditioned power
5. UPS
88
What is the purpose of a subroutine?
- make programs more efficient
| - improve organization for ease of understanding, interpretation and future editing for everyone involved
89
Instruction JSR
redirects logic execution from the current ladder file to the specified subroutine file
90
Instruction SBR
The first instruction of a subroutine
91
Instruction RET
Directs the processor scan to return to where it came from
92
To use a high speed instruction you must use?
A jumper
93
How does a HSC differ from a CTU or CTD?
HSC is a hardware counter as opposed to a software counter and that the HSC operates asynchronously to the ladder program scan
94
Inputs wired to the input module control the?
Input image table
95
List the three basic operations that the CPU section performs
1. Read the inputs
2. Solve the logic program
3. Write to the outputs
96
Modifying relay type process control circuits usually involves changing?
The input circuit modules
97
The power required to operate the logic circuits of the processor unit is?
Low voltage DC
98
The scan time is the time required to?
execute one cycle of the total program
