Programmable Logic Circuits

Question 1

What does PLC stand for?

Answer 1

Programmable Logic Controller

Question 2

What is the main function of a PLC?

Answer 2

Industrial control and automation

Question 3

How is the logical relationship between Input and Output defined in PLCs?

Answer 3

In software, as opposed to hardwired Relay Ladder Logic

Question 4

What are the advantages of using PLCs over hardwired systems?

Answer 4

• Better response times
• Large scale networking between distributed facilities

Question 5

What is a typical configuration of a rack-mounted PLC?

Answer 5

• CPU with integrated power supply
• Standardised snap-in slots to plug in IO modules
• modules selected according to the application

Question 6

How is IO implemented in PLC-based control and PC-based control respectively?

Answer 6

PLC-based control uses ** IO modules with standardised field wiring terminals**.
PC-based control uses IO cards with separate terminal block

Question 7

What does the IO memory in a PLC map to?

Answer 7

One-to-one to the CPU memory addresses

Question 8

What is the purpose of the backplane in a PLC?

Answer 8

Carries power and the communication bus

Question 9

Describe the backplane in a PLC.

Answer 9

IO modules snap together with the CPU to make contact with the backplane.
Backplane is a continuous connection between the PLC components.
Backplane is composed of:
- address lines
- data lines
- control lines

Question 10

What types of modules can a typical PLC include?

Answer 10

• CPU module
• Discrete IO modules
• Timer and counter modules
• PID controller modules
• Network communication modules

Question 11

Describe what is meant by “Each IO point must have a unique address on the PLC bus”.

Answer 11

PLCs typically have a CPU rack and supports three to five expansion racks.
Each rack may have between four and fifteen slots.
In each slot, there can be a single IO module.
The address of each IO module is determined based on rack number, slot number, and channel number. Each IO module has a unique address that can be used to incorporate the specific IO module in the system logic.

Question 12

What is the scan mode in PLC application development?

Answer 12

The entire program is scanned for executions every time

Question 13

What does the scan time in a PLC typically measure?

Answer 13

Milliseconds or less per thousand lines of ladder logic code

Question 14

What do the standard reserved keywords in Ladder Logic include?

Answer 14

• I: discrete inputs
• O: discrete output
• T: timer functions
• C: counter functions
• F: Flags

Question 15

What is the representation for a discrete input channel state in Ladder Logic?

Answer 15

Two vertical lines

Question 16

How is Logic AND implemented in Ladder Logic?

Answer 16

By putting two contacts in series

Question 17

How is Logic OR implemented in Ladder Logic?

Answer 17

By putting two contacts in parallel

Question 18

What type of data structures does Ladder Logic support?

Answer 18

• Bits for discrete IO
• Bytes and words for analog IO and character data

Question 19

What is the function of timers and counters in Ladder Logic?

Answer 19

Represented by rectangular shapes

Question 20

What is the role of the PLC vendor in application development?

Answer 20

Supplies a software development tool for communication and debugging

Question 21

True or False: The PC is part of the real-time control in PLC systems.

Answer 21

False

Question 22

What is the typical range of IO points for discrete modules?

Answer 22

8, 16 or 32 IO points

Question 23

What types of signals can Discrete IO modules handle?

Answer 23

• DC signals
• AC signals

Question 24

What does the term ‘rung’ refer to in Ladder Logic?

Answer 24

A sequence in a Ladder Logic program

Question 25

Fill in the blank: PLC programs usually run in _______.

Answer 25

scan mode

Question 26

What is the significance of the address in a PLC bus?

Answer 26

Each IO point must have a unique address

Question 27

What is a PID controller module used for?

Answer 27

Control processes in automation

Question 28

What are the primary functions of the communication bus in a PLC?

Answer 28

* Address lines * Data lines * Control lines

Question 29

Are PLC-based control and PC-based control different based on the environment for which it is designed?

Answer 29

Yes. PLC: rugged design for harsh industrial environments (resistant against temperature variations, dust and vibration) PC: sensitive equipment

Question 30

How do PLC-based control and PC-based control differ based on their programming?

Answer 30

PLC: programmed using simple universally understood ladder logic diagrams. PC: Powerful programming language (Python, for example)

Question 31

What are PLC-based control and PC-based control used for respectively?

Answer 31

PLC: used to implement **control logic and IO support** in **real time** (real-time kernel and user program is stored in PLC memory). PC: used as **networking** and **user-interface devices** - off-line program development - on-line debugging and monitoring

Question 32

What mode do PLC programs run in?

Answer 32

Scan mode

Question 33

What is the sequence of operations in PLC program execution?

Answer 33

Update inputs -> Execute LLD -> Update outputs -> Repeat

Question 34

How does scan time in PLC programs compare to high-level programming languages on a PC?

Answer 34

Scan time is constant

Question 35

What are typical scan times for PLC programs?

Answer 35

In the order of milliseconds or less per thousand lines of ladder logic code

Question 36

What are PLC programs generally benchmarked against?

Answer 36

Basic logic functions (AND, OR, NOT etc.)

Question 37

What types of functions take longer to execute in PLC programs?

Answer 37

Special functions like trigonometric functions and PID control algorithms

Question 38

Can development tools measure scan time in PLC applications?

Answer 38

Yes

Question 39

What are PLCs mostly programmed in?

Answer 39

Ladder Logic Diagrams (LLD) ## Footnote LLD is a widely used programming language for PLCs due to its familiarity among field technicians.

Question 40

What does Ladder Logic Diagrams emulate?

Answer 40

The same structure of hardwired relay logic diagrams ## Footnote This emulation makes it easier for technicians who are accustomed to traditional relay systems.

Question 41

Why is Ladder Logic Diagrams most used?

Answer 41

Field technicians are familiar with it ## Footnote Familiarity among technicians contributes to its widespread adoption in industrial settings.

Question 42

What are alternatives to Ladder Logic Diagrams?

Answer 42

Boolean and flowchart languages ## Footnote These alternatives can be used for programming but are less common than LLD.

Question 43

What does LLD focus on?

Answer 43

Industrial control and logic ## Footnote LLD is designed for practical applications in industrial environments rather than high-level programming.

Question 44

What types of data structures are supported in LLD?

Answer 44

Bits, bytes, and words ## Footnote Bits are used for discrete I/O, while bytes and words are utilized for analog I/O and character data.

Question 45

Fill in the blank: LLD focuses on industrial control and logic as opposed to _______.

Answer 45

high level, object oriented data structures ## Footnote This distinction highlights the practical application of LLD in industrial settings.

Question 46

True or False: Output channels can appear at the end of a rung only once.

Answer 46

True

Question 47

True or False: Each rung must begin with an input channel or local data.

Answer 47

True