PLC Exam

Controls and Instrumentations ENGR 363

What are the major parts of a plc?

The CPU, I/O, power supply, and programming device

Architecture

Refers to PLC hardware or software

Open architecture

Allows the system to be connected easily to devices and programs made by other manufacturers

Closed architecture

A system whose design is proprietary making it difficult to connect to other systems

Fixed I/O

Small PLC that comes in one package with not separate removable units. The processor and I/O are packaged together

Modular I/O

A PLC that is divided by compartments into which separate modules can be plugged

Power supply

Provides DC power to all modules that plug into the rack

CPU or processor

A microprocessor for implementing the logic and controlling the communications among the modules

PLC scan cycle

Reads inputs -\> executes program -\> diagnostics & communication -\> updates outputs

I/O (Input/Output) system

Forms the interface by which field devices are connected to the controller.

I/O (Input/Output) system

Forms the interface by which field devices are connected to the controller

Programming device

Used to enter the desired program into the memory of the processor

Personal computer (PC)

The most commonly used programming device

Program

A series of instructions that directs the PLC to execute actions

Relay ladder logic

The standard programming language, is based on electromagnetic relay control

Address

Where an input or output device is conncected

PC vs PLC

PLC is designed to operate in the industrial environment with wide ranges of ambient temperature and humidity

Human machine interface (HMI)

Software that allows the user to monitor and control the process

Programmable Automation Controllers (PACs)

Combine PLC ruggedness with PC

functionality

Nano

Smallest size with less than 15 I/O

Micro

15-128 I/O

Medium

128-512 I/O

Large

Over 512 I/O

Single ended application

One PLC controlling one process

Multitask application

One PLC controlling several processes

Control management application

One PLC controlling several others

Memory

Part of a PLC controller that stores data, instructions, and the control program

Memory size

Expressed in K values

1K =

1024

Amount of memory required depends on

• Number of I/O points

used

• Size of control

program

• Data-collecting

requirements

• Supervisory functions

required

• Future expansion

Input interface modules

accept signals from the

machine or process devices and convert them into

signals that can be used by the controller

Output interface modules

convert controller signals

into external signals used to control the machine or

process

chassis

hardware assembly that houses I/O modules,

processor modules, and power supplies

logical rack

an addressable unit consisting of 128

input points and 128 output points

remote rack

A rack that is located away from the processor module

address

a label or number that indicates where

a certain piece of information is located in a PLC's

memory

Rack/slot-based addressing schemes

used with

Allen-Bradley PLC-5 and SLC 500 controllers

Tag-based addressing

used with Allen-Bradley

ControlLogix controllers

Combination I/O

modules

can

have both input

and output

connections in

the same

physical module

points

refers to the number of inputs or outputs

available

discrete I/O

interface module

connects

field input

devices of the

ON/OFF nature

discrete I/O module is powered by

some field supplied voltage source

Backplane power

Used to power the electronics that

reside on the I/O module circuit board

optical isolator

used to provide electrical isolation

between the power and logic circuitry

Interposing relay

used

to control a larger load

or motor starter

Output modules can be purchased with

Transistor,triac, or relay output

If the module is a current-sourcing module then the

input or output device must be a

current-sinking device

If the module is a current-sinking module,

then the input or output device must be a

current-sourcing device

The two basic types of analog input modules are

voltage sensing and current sensing

Bipolar input modules

Accept signals that swing

between a negative and positive value

Unipolar input modules

Accept an input signal

that varies in the positive direction only

Resolution refers to

the smallest change in input

signal value that can be sensed

The main element of the analog

output module is an

digital-to-analog (D/A) converter

analog output module

converts digital signal into

a proportional voltage or current to control an

analog field device

High-speed counter modules

used to count

pulses from sensors, encoders, and switches

that operate at very high speeds

TTL module

allows the