Programmable Logic Controllers

Question 1

What are 3 basic components of a control system

Answer 1

Input devices
Controller
Output devices

Question 2

What does the acronym PLC mean?

Answer 2

Programmable Logic Controller

Question 3

List 4 advantages of PLC’s over relay control.

Answer 3

Lower overall cost

Programmability

Reliability

Expandability

Question 4

How are PLC’s selected

Answer 4

According to the number of input and output points (I/O points)

Question 5

What are connected to the input points?

Answer 5

Switches and sensors

Question 6

What are connected to the outputs?

Answer 6

Pilot lights, contactors & relays that operate motors and valves.

Question 7

How many I/O points can a small PLC have?

Answer 7

Fewer than 10

Question 8

How many I/O points can a large PLC have?

Answer 8

The may have more than 120000

Question 10

As PLC systems get larger what also increases?

Answer 10

Programming
Memory
Communication

Also increase

Question 11

Input devices do what?

Answer 11

Sense specific conditions of a process or system

Question 12

A controller does what?

Answer 12

Receives information and decides a response either by a hardwired arrangement or software program

Question 13

Output devices do what?

Answer 13

Control operating devices to bring about appropriate changes in the system

Question 14

Summarize the advantage of cost

Answer 14

PLCs are cheaper long-term – Fewer parts, easier troubleshooting, less space, and lower energy than relay systems.

Question 15

Summarize the advantage of programmability

Answer 15

Easy to change and test – Programs can be updated without rewiring and reused for multiple processes. Great for troubleshooting and automation.

Question 16

Summarize the advantage of reliability

Answer 16

More durable than relays – No moving parts means less wear, better for harsh environments, and faster switching.

Question 17

Summarize the advantage of expandability

Answer 17

Modular and scalable – Easy to add more modules or link PLCs using data networks.

Question 18

What are the 4 components of a modular PLC system

Answer 18

Power supply
Backplane
Processor (CPU)
Input/Output (I/O) Modules

Question 19

What kind of voltage is required by the PLC

Answer 19

Clean, filtered & regulated DC voltage.

Question 20

Does the PLC power supply feed the field outputs?

Answer 20

NO

Question 21

What’s connected to but electrically isolated from the backplane?

Answer 21

Input and output devices

Question 22

What’s the backplane?

Answer 22

The power and communication bus for the PLC components

Question 23

The system of an existing PLC can be modified by simply

Answer 23

Adding hardware devices

Question 24

True or False:
Input modules provide electrical isolation between the processor and the field device

Answer 24

True

Question 25

PLC’s have battery backup to ensure

Answer 25

The contents of PLC memory are retained during power down

Question 26

Battery backup is provided in the PLC to

Answer 26

Keep the program in the processor memory during failure

Question 27

The voltage required for the field wiring of magnetic starters from the output modules of programmable controllers depends on the:

Answer 27

Voltage rating of the coil in the magnetic starter

Question 28

What are the primary functions of a PLC processor

Answer 28

Read and store the status of all input devices Execute the user program by solving the programs logic Update the status of inputs

Question 29

What is processor scan

Answer 29

How the processor carries out its functions in sequence

Question 30

What are the different processor memories

Answer 30

RAM ROM EEPROM

Question 31

What’s non volatile memory?

Answer 31

Retains data and programs after the electrical power source has been removed It’s considered permanent

Question 32

What’s volatile memory

Answer 32

Cleared of all memory if it loses its power source.

Question 33

RAM is

Answer 33

Random Access Memory Volatile memory

Question 34

ROM is

Answer 34

Read Only Memory Non volatile memory Fixed at manufacturing

Question 35

EEPROM is

Answer 35

Electrically Erasable Programmable Memory Non volatile memory that can be reprogrammed using a PLC’s programming device.

Question 36

A data highway is a type of:

Answer 36

LAN (Local Area Network)

Question 37

A remote I/O is used to:

Answer 37

Reduce the amount of cabling between the PLC and I/O devices

Question 38

What are discrete inputs?

Answer 38

Switches, relay contacts or sensors that are either off or on. They are not powered by the power module of the plc

Question 40

What’s the minimum signal for analog? Why is it so?

Answer 40

4mA 4mA-20mA This allows the PLC to tell the difference between zero percent signal and a broken wire condition

Question 41

How many times is a shield grounded and where

Answer 41

Grounded once and at one end of the cable only

Question 42

What’s the two most common standards of analog signals?

Answer 42

0-10Vdc 4mA-20mA

Question 43

What’s the 5 main types of languages used for programming PLCs

Answer 43

GRAPHIC BASED: Ladder diagrams Function block diagrams Sequential functions TEXT BASED: Structured text Instruction lists

Question 44

Sequential function charts are used for?

Answer 44

Large process controls where several complex functions take place in sequence

Question 45

Discrete input modules provide which types of isolation?

Answer 45

Optoisolation Relay isolation Filter isolation

Question 46

Analog can best be described as

Answer 46

A signal which can vary continuously in value