Unit 22 Programmable Logic Controllers

Unit 22:

Programmable Logic Controllers

Unit code:

QCF level:

Credit value:


The aim of this unit is to investigate programmable logic controller (PLC) concepts and their applications in engineering.

Unit abstract

The unit focuses on the design and operational characteristics and internal architecture of programmable logic control systems. It examines the signals used and the programming techniques that can be applied. The unit also provides learners with the opportunity to produce and demonstrate a program for a programmable logic controller device (for example produce a programme for an engineering application, store, evaluate and justify approaches taken).

Learning outcomes

On successful completion of this unit a learner will:

1     Understand the design and operational characteristics of a PLC system

2     Understand PLC information and communication techniques

3       Be able to apply programmable logic programming techniques

4       Understand alternative implementations of programmable control.
Unit content

1      Understand the design and operational characteristics of a PLC system

Design characteristics: unitary; modular; rack-mounted

Input and output devices: mechanical switches; non-mechanical digital sources; transducers; relays

Communication links: twisted pair; coaxial; fibre-optic; networks

Internal architecture: central processor unit (CPU); arithmetic logic unit (ALU); storage devices; memory; opto-isolators; input and output units; flags; shift; registers

Operational characteristics: scanning; performing logic operations; continuous updating; mass input/output (I/O) copying

2      Understand PLC information and communication techniques

Forms of signal: analogue (0-10 v dc, 4-20mA); digital

Digital resolution and relationships: 9-bit; 10-bit; 12-bit

Number systems: decimal; binary; octal; hexadecimal; Binary-Coded Decimal (BCD)

Evaluate communication standards: comparison of typical protocols used in signal communication

Evaluate networking methods and standards: master to slave; peer to peer; ISO; IEE; MAP

Logic functions: writing programmes using logic functions based on relay ladder logic (AND; OR; EXCLUSIVE OR; NAND; NOR)

3      Be able to apply programmable logic programming techniques

Write programs: use of ladder and logic diagrams; statement lists; Boolean algebra; function diagrams; graphical programming languages; production of a PLC

Advanced functions: less than; greater than; binary to BCD conversion; proportional feedback control

Producing and storing text: contact labels; rung labels; programming lists; cross-referencing

Test and debug programs: forcing inputs, forcing outputs; changing data; comparing files (tapes, EPROM, disc); displayed error analysis

Associated elements: contacts; coils; timers; counters; override facilities; flip-flops; shift registers; sequencers

4      Understand alternative implementations of programmable control

PICs and other programmable devices: specification and use of PICs and other programmable devices; embedded controllers

PLC simulators: compare operation and functionality; advantages and limitations

Learning outcomes and assessment criteria

Learning outcomes
Assessment criteria for pass

On successful completion
The learner can:

of this unit a learner will:

LO1 Understand the design

evaluate the design characteristics of typical

and operational

programmable logic devices

characteristics of a PLC

describe different types of input and output device


evaluate the different types of communication link used

in programmable logic control systems

describe the internal architecture and operational

characteristics of the CPU of a typical programmable

logic device

LO2 Understand PLC

evaluate the different forms of signal used in

information and

programmable logic control


describe the resolution and relationship between


analogue inputs and outputs and word length

express numbers using different number systems

describe typical protocols used in signal communication

and evaluate networking methods and networking


LO3 Be able to apply

identify elements associated with the preparation of a

programmable logic

programmable logic controller program

programming techniques

write programs using logic functions based on relay

ladder logic

evaluate the range and type of advanced functions of

programmable logic controllers

use and justify methods of testing and debugging

hardware and software

LO4 Understand alternative

evaluate PICs and other programmable devices as

implementations of

programmable devices and embedded controllers

programmable control

compare the operation, functionality, advantages and

limitations of PLC simulators.



This unit may be linked to Unit 46: Plant and Process Control, Unit 49: Computer Control of Plant, Unit 58: Microprocessor Systems and Unit 71: Combinational and Sequential Logic.

Essential requirements

Centres delivering this unit must be equipped with, or have access to, industrial-standard programmable logic control units and development software.

Employer engagement and vocational contexts

Visits to industrial PLC installations will be of value to supplement the learning activities.