programmable controller logic

7/30/2019 Programmable Controller Logic

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PROGRAMMABLE CONTROLLER LOGIC


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What is a Programmable Logic

controller ?

A PLC consists of :

.a CPU contain an application Program,

and

. Interfacing input and output modules.CPU retrieves, decode, store, and processinformation. It executes the controller programstored in the PLC memory.

CPU is brain of a PLC and it function like a CPU

of a regular computerexcept thatCPU of aPLC uses special instructions and coding toperform its function.


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PLC BLOCK DIAGRAM

Physical parts of a PLC

CPU has three parts:

. the processor:

It codes, decodes and

compute data.

. the memory system:

It stores both control

program and the

equipment connectedto the PLC.

. the power supply: Itprovides voltage & current

PLC needs to operate


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MEMORY SYSTEM OF PLC


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Memory system

It holds the operating system anduser memory

Operating system is system

software that co-ordinates the PLCladder program.

User memory stores Time & counter

Available memory choices -depending on user need: RAM, ROM& EPROM.


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RAM, ROM & EPROM

Random Access Memory (RAM)

. Where data can be directly

addressed at any address.

. Data can be written in read from RAM.

. It is temporary storage, volatile (lost if

power is lost.

. It required batter back up to retain data

in the event of power loss.


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Read Only Memory (ROM)

ROM is nonvolatile memory. It is used

to protect data or program from

accidental erasure.

Rom normally store programs the

define the capability of the PLC.


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Erasable Programmable Read Only Memory

(EPROM)

It provides security against

unauthorized and unwanted changes

in a program.

EPROMs are designed to store data

in them but not easily altered.

They may be UVPROMs or

EEPROMs


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Firmware

User or specific program burned into

EPROM.

Firmware gives the PLC its basic

functionality.


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INPUT/OUTPUT SYSTEM


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Input/Output (I/O) System

Section of PLC to which all of the field devices areconnected.

If CPU is thought as brain of a PLC, then I/O systemcan be thought its arm and legs.

I/O system physically carries out the control commandsfrom the program stored in the PLC memory.

I/O system has two main parts

. the rack: Enclosure attached to CPU to insert I/O

modules

. I/O modules: Circuit boards with connection

terminals to which field devices are wired.

I/O system creates physical connection between the fieldequipment and PLC.

In small PLC I/O modules are packaged as single unit.


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Input Devices

Devices that supply signal/data to

PLC e.g. push buttons, switches and

measuring devices.

Input devices tells PLC Hay,

something is happening there.

Input devices may be either

Discrete or digital Input Devices, or

Analog Input Devices


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Discrete or digital input devices

They provide two bitinformation.

Examples are pushbuttons,toggle switches, limitswitches, proximity

switches and contactclosures of relay

They are connected to PLCdigital input terminals onthe I/O module.

On condition is referred asa logic 1(HIGH) and OFFcondition as logic 0 (LOW).


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Analog input devices

They provide informationas continuous, variablesignal.

Typical analog input mayvary from 0 to 20 mA, 4 to20 mA or 0 to 10 Volts.

Examples are leveltransmitter monitoring thelevel of liquid in a tank.pushbuttons, toggleswitches, limit switches,proximity switches and

contact closures of relayThey are connected to PLCanalog input terminals onthe I/O module.


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Output Devices

They await signal/data from PLC to

perform their control.

Examples: lights, horns, motors and valve

They state in their previous state until PLC

says you need to turn on now or youd

better open up your valve a little more etc.

Input devices may be either. Discrete or digital Input Devices, or

. Analog Input Devices


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Discrete or digital Output devices

They are either in ON orOFF condition

Examples are digilalactuators are solenoids,contactor coils, lamps etc.

They are connected to PLCdigital input terminals onthe I/O module.

On condition is referred asa logic 1(HIGH) and OFFcondition as logic 0 (LOW).


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Analog Output devices

They operate incontinuous, variablemanner.

Their output may vary from0 to 20 mA, 4 to 20 mA or0 to 10 Volts that drive an

analog meter.Examples are speed,weight, temperature orlevel of liquid in a tank.pushbuttons, toggle

Complex output device

may be a current topneumatic transducer withair-operated flow-controlvalve.


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PLC SCAN CYCLE


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Scan Cycle

Monitor-Execute-Update

SCAN Cycle: PLC programis executed as a part ofrepetitive process calledSCAN.

1. CPU reads the status ofinputs

2. application programexecuted using

the status of the inputs.

3. Program completed, CPUperforms internaldiagnostics andcommunication tasks

4. scan cycle ends withupdating the outputs

Cycle Time depends on thesize of the program, thenumber of I/Os, and theamount of communicationrequired.

Monitor ..Checks inputs

Inputs

Scan Execute .. Executesthe

Program control program

Change .. and updatesOutputs the oputputs


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Programming Languages

Program: Set of

instructions to

accomplish a task.

Types of Programs

Ladder Logic (LAD)

uses components

resembling elements

used in the diagram

format to describehardwired control.


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Programming Languages

Left Vertical Line:Power conductor

Right Vertical Line:Return path

Rungs refer tonetworks having singleof several elementsbut only one outputcoil in each rung.

Ladder logic diagramare read from left-to-right, top-to-bottom.


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Example LCD Program

1st Instructioncombination: I0.0,I0.1and Q0.0 represent ifinputs I0.0 and I0.1are energized output

relay Q0.0 energizes.2nd instructionCombination: I0.4,I0.5and Q1.1 represents ifeither

input 10.4 or 10.5 areenergized, outputrelay Q1.1 energizes.


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Statement List (STL)

Another view of set of Instruction

Network 1

Operand I/O address

LD I0.0

A I0.1

= Q0.0

Network 2

LD I0.4

O I0.5

= Q0.1


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Functional Block Diagram (FBD)

Another view of set of Instruction

Each function designated as a task

FUNCTIONS are indicated by rectangles

with input shown on its left-hand side andoutput shown on right-hand side.

Network 1

I0.0

I0.1

Network 2

AND


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Selection of PLC

This decision will depend on the basic criteria

listed belowNumber of logic inputs and outputs-often 1k andup. Need is dictated by size of ladder diagramprogram. A ladder element will occupy only a fewbytes and will be listed in the manufacture

documentation.Number of special I/O modules- forexoticapplications- a large number add-on special cardsmay be required.

Scan Time -Typical 1 ms/ladder instruction.

Shorter the scan time, higher the cost of PLC.Communication-Serial and networked connectionallow the PLC to be programmed and talk to theother PLC.

Software- Programming and other software

determine the ease of programming anddebugging.


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PLC MEMORY TIME ESTMATE- Part A

Project ID ------------------------

Name ------------------------------

Date: ------------------------------

Instructio

n type

Time Max Time

min

Instruction

memory (words

Instruction Data

(words)

Instruction Count Total

Memory

(words)

Min Time Max.

time

Contacts

Outputs

Timesa

counters


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System Architecture


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PLC Input & Output Circuits


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Basic PLC Programming

Programming may be done offline or online

Offline Programming Mode: User program

editing, maintenance tasks. Need not to be

connected to programming device.Online Programming Mode: Programming

device connected required

Changes are downloaded in the PLC.Status of the I/Ov devices monitored


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I/O Addressing

(SIEMENS S 7-200 SERIES)

The inputs and outputs are labeled at the wiringterminations and next to the status indicator.

Alphanumeric symbols identification of the I/Oaddress to which device is to be connected.

I = Discrete or digital inputQ = Discrete or digital input

For example:

I0.0 = Byte 0, Bit 0

I0.1 = Byte 0, Bit 1I1.0 = Byte 1, Bit 0

I1.1 = Byte 1, Bit 1


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I0.0 Input 1

I0.1 Input 2

I0.2 Input 3

I0.3 Input 4

I0.4 Input 5

I0.5 Input 6

I0.6 Input 7

I0.7 Input 8

I1.8 Input 9

0.9 Input 10

I0.10 Input 11

I0.0 Input 12

I0.0 Input 13

I0.0 Input 14

programmable controller logic

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