PLC Exam

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What are the major parts of a plc?

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Controls and Instrumentations ENGR 363

420 Terms

1

What are the major parts of a plc?

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

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2

Architecture

Refers to PLC hardware or software

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3

Open architecture

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

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4

Closed architecture

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

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5

Fixed I/O

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

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6

Modular I/O

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

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7

Power supply

Provides DC power to all modules that plug into the rack

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8

CPU or processor

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

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9

PLC scan cycle

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

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10

I/O (Input/Output) system

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

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I/O (Input/Output) system

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

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12

Programming device

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

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13

Personal computer (PC)

The most commonly used programming device

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14

Program

A series of instructions that directs the PLC to execute actions

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15

Relay ladder logic

The standard programming language, is based on electromagnetic relay control

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Address

Where an input or output device is conncected

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PC vs PLC

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

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18

Human machine interface (HMI)

Software that allows the user to monitor and control the process

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Programmable Automation Controllers (PACs)

Combine PLC ruggedness with PC

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functionality

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Nano

Smallest size with less than 15 I/O

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Micro

15-128 I/O

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Medium

128-512 I/O

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Large

Over 512 I/O

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Single ended application

One PLC controlling one process

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Multitask application

One PLC controlling several processes

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Control management application

One PLC controlling several others

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Memory

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

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Memory size

Expressed in K values

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1K =

1024

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Amount of memory required depends on

• Number of I/O points

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used

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• Size of control

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program

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• Data-collecting

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requirements

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• Supervisory functions

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required

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• Future expansion

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Input interface modules

accept signals from the

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machine or process devices and convert them into

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signals that can be used by the controller

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Output interface modules

convert controller signals

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into external signals used to control the machine or

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process

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chassis

hardware assembly that houses I/O modules,

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processor modules, and power supplies

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logical rack

an addressable unit consisting of 128

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input points and 128 output points

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remote rack

A rack that is located away from the processor module

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address

a label or number that indicates where

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

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memory

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Rack/slot-based addressing schemes

used with

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Allen-Bradley PLC-5 and SLC 500 controllers

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Tag-based addressing

used with Allen-Bradley

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ControlLogix controllers

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Combination I/O

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modules

can

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have both input

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and output

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connections in

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the same

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physical module

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points

refers to the number of inputs or outputs

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available

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discrete I/O

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interface module

connects

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field input

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devices of the

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ON/OFF nature

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discrete I/O module is powered by

some field supplied voltage source

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Backplane power

Used to power the electronics that

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reside on the I/O module circuit board

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optical isolator

used to provide electrical isolation

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between the power and logic circuitry

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Interposing relay

used

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to control a larger load

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or motor starter

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Output modules can be purchased with

Transistor,triac, or relay output

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If the module is a current-sourcing module then the

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input or output device must be a

current-sinking device

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If the module is a current-sinking module,

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then the input or output device must be a

current-sourcing device

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The two basic types of analog input modules are

voltage sensing and current sensing

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Bipolar input modules

Accept signals that swing

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between a negative and positive value

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Unipolar input modules

Accept an input signal

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that varies in the positive direction only

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Resolution refers to

the smallest change in input

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signal value that can be sensed

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The main element of the analog

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output module is an

digital-to-analog (D/A) converter

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analog output module

converts digital signal into

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a proportional voltage or current to control an

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analog field device

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High-speed counter modules

used to count

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pulses from sensors, encoders, and switches

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that operate at very high speeds

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TTL module

allows the

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