ABSTRACT

A programmable logic controller (PLC) or programmable controller is a

digital computer used for automation of electromechanical processes, such

as control of machinery on factory assembly lines, amusement rides, or

lighting fixtures. PLCs are used in many industries and machines. Unlike

general-purpose computers, the PLC is designed for multiple inputs and

output arrangements, extended temperature ranges, immunity to electrical

noise, and resistance to vibration and impact. Programs to control machine

operation are typically stored in battery-backed or non-volatile memory. A

PLC is an example of a real time system since output results must be

produced in response to input conditions within a bounded time. The PLC

was invented in response to the needs of the American automotive

manufacturing industry. Programmable logic controllers were initially

adopted by the automotive industry where software revision replaced the re-

wiring of hard-wired control panels when production models

changed.Before the PLC, control, sequencing, and safety interlock logic for

manufacturing automobiles was accomplished using hundreds or thousands

of relays, cam timers, and drum sequencers and dedicated closed-loop

controllers. The process for updating such facilities for the yearly model

change-over was very time consuming and expensive, as electricians needed

to individually rewire each and every relay.

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1. INTRODUCTION

The National Electrical Manufacturers Association (NEMA) as a

“digital electronic device that uses a programmable memory to store

instructions and to implement specific functions such as logic,

sequence, timing, counting and arithmetic operations to control

machines and processes” currently defines a programmable Logic

Controller. The PLC was invented in response to the needs of the

American automotive manufacturing industry. Programmable logic

controllers were initially adopted by the automotive industry where

software revision replaced the re-wiring of hard-wired control panels

when production models changed.Before the PLC, control, sequencing,

and safety interlock logic for manufacturing automobiles was

accomplished using hundreds or thousands of relays, cam timers, and

drum sequencers and dedicated closed-loop controllers. The process for

updating such facilities for the yearly model change-over was very time

consuming and expensive, as electricians needed to individually rewire

each and every relay.In 1968 GM Hydramatic (the automatic

transmission division of General Motors) issued a request for proposal

for an electronic replacement for hard-wired relay systems. The

winning proposal came from Bedford Associates of Bedford,

Massachusetts. The first PLC, designated the 084 because it was

Bedford Associates' eighty-fourth project, was the result. Bedford

Associates started a new company dedicated to developing,

manufacturing, selling, and servicing this new product: Modicon, which

stood for Modular Digital Controller. One of the people who worked on

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that project was Dick Morley, who is considered to be the "father" of

the PLC. The Modicon brand was sold in 1977 to Gould Electronics

and later acquired by German Company AEG and then by French

Schneider Electric, the current owner. One of the very first 084 models

built is now on display at Modicon's headquarters in North Andover,

Massachusetts. It was presented to Modicon by GM, when the unit was

retired after nearly twenty years of uninterrupted service. Modicon used

the 84 moniker at the end of its product range until the 984 made its

appearance.

1.2 DEVELOPMENT

Early PLCs were designed to replace relay logic systems. These PLCs

were programmed in "ladder logic", which strongly resembles a

schematic diagram of relay logic. This program notation was chosen to

reduce training demands for the existing technicians. Other early PLCs

used a form of instruction list programming, based on a stack-based

logic solver.

Modern PLCs can be programmed in a variety of ways, from ladder

logic to more traditional programming languages such as BASIC and C.

Another method is State Logic, a very high-level programming language

designed to program PLCs based on state transition diagrams.

Many early PLCs did not have accompanying programming terminals

that were capable of graphical representation of the logic, and so the

logic was instead represented as a series of logic expressions in some

version of Boolean format, similar to Boolean algebra. As

programming terminals evolved, it became more common for ladder

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logic to be used, for the aforementioned reasons. Newer formats such

as State Logic and Function Block (which is similar to the way logic is

depicted when using digital integrated logic circuits) exist, but they are

still not as popular as ladder logic. A primary reason for this is that

PLCs solve the logic in a predictable and repeating sequence, and

ladder logic allows the programmer (the person writing the logic) to

see any issues with the timing of the logic sequence more easily than

would be possible in other formats.

1.3 FUNCTIONALITY

The functionality of the PLC has evolved over the years to include

sequential relay control, motion control, process control, distributed

control systems and networking. The data handling, storage, processing

power and communication capabilities of some modern PLCs are

approximately equivalent to desktop computers. PLC-like programming

combined with remote I/O hardware, allow a general-purpose desktop

computer to overlap some PLCs in certain applications. Regarding the

practicality of these desktop computer based logic controllers, it is

important to note that they have not been generally accepted in heavy

industry because the desktop computers run on less stable operating

systems than do PLCs, and because the desktop computer hardware is

typically not designed to the same levels of tolerance to temperature,

humidity, vibration, and longevity as the processors used in PLCs. In

addition to the hardware limitations of desktop based logic, operating

systems such as Windows do not lend themselves to deterministic logic

execution, with the result that the logic may not always respond to

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changes in logic state or input status with the extreme consistency in

timing as is expected from PLCs. Still, such desktop logic applications

find use in less critical situations, such as laboratory automation and use

in small facilities where the application is less demanding and critical,

because they are generally much less expensive than PLCs.

In more recent years, small products called PLRs (programmable logic

relays), and also by similar names, have become more common and

accepted. These are very much like PLCs, and are used in light industry

where only a few points of I/O (i.e. a few signals coming in from the

real world and a few going out) are involved, and low cost is desired.

These small devices are typically made in a common physical size and

shape by several manufacturers, and branded by the makers of larger

PLCs to fill out their low end product range. Popular names include

PICO Controller, NANO PLC, and other names implying very small

controllers. Most of these have between 8 and 12 digital inputs, 4 and 8

digital outputs, and up to 2 analog inputs. Size is usually about 4" wide,

3" high, and 3" deep. Most such devices include a tiny postage stamp

sized LCD screen for viewing simplified ladder logic (only a very small

portion of the program being visible at a given time) and status of I/O

points, and typically these screens are accompanied by a 4-way rocker

push-button plus four more separate push-buttons, similar to the key

buttons on a VCR remote control, and used to navigate and edit the

logic. Most have a small plug for connecting via RS-232 to a personal

computer so that programmers can use simple Windows applications

for programming instead of being forced to use the tiny LCD and push-

button set for this purpose. Unlike regular PLCs that are usually

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modular and greatly expandable, the PLRs are usually not modular or

expandable, but their price can be be two orders of magnitude less than

a PLC and they still offer robust design and deterministic execution of

the logic.

2. THEORY

The main difference from other computers is that PLCs are armored for

severe conditions (such as dust, moisture, heat, cold) and have the

facility for extensive input/output (I/O) arrangements. These connect

the PLC to sensors and actuators. PLCs read limit switches, analog

process variables (such as temperature and pressure), and the positions

of complex positioning systems. Some use machine vision. On the

actuator side, PLCs operate electric motors, pneumatic or hydraulic

cylinders, magnetic relays, solenoids, or analog outputs. The

input/output arrangements may be built into a simple PLC, or the PLC

may have external I/O modules attached to a computer network that

plugs into the PLC.

2.1 TYPES OF PLC

There are two types of PLC

(a) Unitary PLCs and (b) Modular PLCs

2.1.1 Unitary PLC

A unitary PLC has a power supply, a CPU and a limited number of

inputs and outputs (20 inputs, 12 outputs, 32 I/O). It is sometime

called “Shoe-box type” and is mainly used for the control of a small

system.

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2.1.2 Modular PLC

A modular PLC is one that can be constructed using separate

modules of power supply, CPU, inputs, outputs, timers, counters,

ADC, DAC, expansion modules. These modular PLCs are sometimes

called “Rack-mounted type”. Modular PLC can be sub-divided into

the following types:

Small PLC

PLCs having less then 100 I/O are designed as a small PLCs. Out of

the I/Os, 20 inputs and 12 outputs are mounted locally with the

processor. Additional I/Os can be added through remote I/O racks to

accommodate the extra I/Os. These PLCs generally have a memory

of 2 KB to 10 KB to store the user’s logic programs.

Medium PLC

These have extended instruction sets that include mathematical

functions, file functions, PID process control etc. These PLCs can

have between 4000 to 8000 I/Os. They are also made to support wide

variety of special modules such as ASCII communication modules,

BASIC-programming modules, 16-bit multiplexing modules, analog

I/O modules and communication modules.

Large PLC

The purpose of introducing large PLCs was to provide enough user

memory space and I/O for complete factory automation. However, the

major disadvantage of these large PLCs is that the whole factory may

collapse if the PLC starts malfunctioning. The advent of Local Area

Networking (LAN) helped to introduce the concept of distributed

control, where small or medium PLCs are connected together through

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an appropriate network. The entire factory is brought under the control

of a number of PLCs, but failure in one system will not disturb any

other system.

2.2 BLOCKS OF PLC

It mainly consist of :-

CPU

I/O Module

Memory

Programmable device

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2.2.1 CPU

It understands input command instructions and status signal and

provides logic processing capability.

2.2.2 I/O Module

They are designed to interface directly with industrial equipment.

2.2.3 Memory

In a PLC the central program is stored in memory to tell itself output

commands. All I/O are updated per scan.

2.2.4 Program device

Transform the control scheme in to useful PLC logic understand by

CPU, then stored in memory. Ladder symbols are used for

programming.

CPU has following modules with their functions:

Memory module

It stores the user control program. It can be installed in rack sloat A

and B.

Register Module (RM)

It contains system I/O data table. Data table is delivered into three

areas I/O status, system status table and register tables.

Processor Module (PM)

It executes program stored in MM and handles arithmetic

computation and data movement instructions of all the modules in the

processor.

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System Control Module (SCM)

The functional extension of processor module co-orients the interaction

of all the modules in the processor control programs. Installed in rack

slot E.

I/O Control Module (IOCM)

Co-orients communication between processor and the 621 I/O systems

and formats the data flowing between I/O system and the processor. It

also monitors individual I/O modules and fault diagnostics.

Parallel Link Driver Module (PLDM)

The PLDM works with the IOCM module to control I/O

communications. The PLDM also controls system status by the mode

key switch and it determines I/O response to system fault and various

operating parameters.

Serial Link Module (SLM)

The SLM front has five LCDs indicating the status of module. Green

action light indicates that data being transmitted properly. The normal

state of the light is on during the transmission.

Communication Interface Module (CIM)

The CIM is optional in the 620-25/35 processor. They provide

interface to microcomputer or other serial device.

Highway Interface Module (HIM)

HIM produces a service facility for higher ordered device in the

system, such as computer and operator station to interface with the

IPC 620 processor and Honeywell’s TDC 3000 data highway.

Redundancy Control Module (RCM)

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The RCM is the primary component in the Honeywell IPC 620

processor redundancy system. The high availability control system

requires minimal hardware and installation effort.

Parallel I/O Module (PIOM)

The PIOM is located in the N slot in the full I/O rack or H slot of the

I/O half rack. This module acts as the interface to processor parallel

link driver module and to the PIOM. The PIOM has two 50-pin D-

type connectors. The male plug is the in port and female is the out

port. The green LEDs labeled active indicates proper communication

from preceding rack

2.3 Programming

PLC programs are typically written in a special application on a

personal computer, then downloaded by a direct-connection cable or

over a network to the PLC. The program is stored in the PLC either

in battery-backed-up RAM or some other non-volatile flash memory.

Often, a single PLC can be programmed to replace thousands of

relays. Under the IEC 61131-3 standard, PLCs can be programmed

using standards-based programming languages. A graphical

programming notation called Sequential Function Charts is available

on certain programmable controllers. Initially most PLCs utilized

Ladder Logic Diagram Programming, a model which emulated

electromechanical control panel devices (such as the contact and

coils of relays) which PLCs replaced. This model remains common

today.

Programming in the PLC can be sampled and straightforward when

approached in a systematic manner. This is achieved by:

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Draw a relay ladder schematic of the control scheme.

Assign an input address to each field input device (pressure switch,

temperature switch, etc). They are wired to input modules per their

respective address assignments.

Assign output relay coil numbers, where required for solenoid valve,

motors, indicators light alarms etc.

Then enter the program into the processor using the CT

programming panel.

Early PLCs, up to the mid-1980s, were programmed using

proprietary programming panels or special-purpose programming

terminals, which often had dedicated function keys representing the

various logical elements of PLC programs. Programs were stored on

cassette tape cartridges. Facilities for printing and documentation

were very minimal due to lack of memory capacity. The very oldest

PLCs used non-volatile magnetic core memory.

More recently, PLCs are usually programmed using special

application software written for use on desktop computers, and

connecting between the desktop computer and the PLC such as via

Ethernet or RS-232 cabling. Such software allows entry and editing

of the ladder style logic, and then may provide additional

functionality to assist debugging and troubleshooting the software,

for example by highlights portions of the logic to show current status

during operation or via simulation. Finally, the software may allow

uploading and downloading of the program between the computer

and the PLC, for backup and restoration purposes. Alternately,

specific devices known as programming boards are used to hard wire

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the logic into the controller by the use of a removable chip, such as

an EEPROM, where the program is transferred to the programming

board from the workstation via serial or other bus logic.

2.4 Communication

PLCs have built in communications ports, usually 9-pin RS-232, but

optionally EIA-485 or Ethernet. Modbus, BACnet or DF1 is usually

included as one of the communications protocols. Other options

include various field buses such as Device Net or Profibus. Other

communications protocols that may be used are listed in the List of

automation protocols.Most modern PLCs can communicate over a

network to some other system, such as a computer running a SCADA

(Supervisory Control And Data Acquisition) system or web browser.

PLCs used in larger I/O systems may have peer-to-peer (P2P)

communication between processors. This allows separate parts of a

complex process to have individual control while allowing the

subsystems to co-ordinate over the communication link. These

communication links are also often used for HMI devices such as

keypads or PC-type workstations.

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Fig:-2.1

2.5 User Interface

PLCs may need to interact with people for the purpose of configuration,

alarm reporting or everyday control.

A Human-Machine Interface (HMI) is employed for this purpose. HMIs

are also referred to as MMIs (Man Machine Interface) and GUI

(Graphical User Interface).

A simple system may use buttons and lights to interact with the user.

Text displays are available as well as graphical touch screens. More

complex systems use a programming and monitoring software

installed on a computer, with the PLC connected via a communication

interface.

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2.7 PLC compared with other control systems

PLCs are well-adapted to a range of automation tasks. These are

typically industrial processes in manufacturing where the cost of

developing and maintaining the automation system is high relative to

the total cost of the automation, and where changes to the system

would be expected during its operational life. PLCs contain input and

output devices compatible with industrial pilot devices and controls;

little electrical design is required, and the design problem centers on

expressing the desired sequence of operations. PLC applications are

typically highly customized systems so the cost of a packaged PLC is

low compared to the cost of a specific custom-built controller design.

On the other hand, in the case of mass-produced goods, customized

control systems are economic due to the lower cost of the components,

which can be optimally chosen instead of a "generic" solution, and

where the non-recurring engineering charges are spread over thousands

or millions of units.

For high volume or very simple fixed automation tasks, different

techniques are used. For example, a consumer dishwasher would be

controlled by an electromechanical cam timer costing only a few dollars

in production quantities.

A microcontroller-based design would be appropriate where hundreds or

thousands of units will be produced and so the development cost (design

of power supplies, input/output hardware and necessary testing and

certification) can be spread over many sales, and where the end-user

would not need to alter the control. Automotive applications are an

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example; millions of units are built each year, and very few end-users

alter the programming of these controllers. However, some specialty

vehicles such as transit busses economically use PLCs instead of custom-

designed controls, because the volumes are low and the development cost

would be uneconomic.

Very complex process control, such as used in the chemical industry,

may require algorithms and performance beyond the capability of even

high-performance PLCs. Very high-speed or precision controls may also

require customized solutions; for example, aircraft flight controls.

Programmable controllers are widely used in motion control, positioning

control and torque control. Some manufacturers produce motion control

units to be integrated with PLC so that G-code (involving a CNC

machine) can be used to instruct machine movements.

PLCs may include logic for single-variable feedback analog control loop,

a "proportional, integral, derivative" or "PID controller." A PID loop

could be used to control the temperature of a manufacturing process, for

example. Historically PLCs were usually configured with only a few

analog control loops; where processes required hundreds or thousands of

loops, a distributed control system (DCS) would instead be used. As

PLCs have become more powerful, the boundary between DCS and PLC

applications has become less distinct.

PLCs have similar functionality as Remote Terminal Units. An RTU,

however, usually does not support control algorithms or control loops. As

hardware rapidly becomes more powerful and cheaper, RTUs, PLCs and

DCSs are increasingly beginning to overlap in responsibilities, and many

vendors sell RTUs with PLC-like features and vice versa.

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2.7. Digital and analog signals

Digital or discrete signals behave as binary switches, yielding simply an

On or Off signal (1 or 0, True or False, respectively). Push buttons,

limit switches, and photoelectric sensors are examples of devices

providing a discrete signal. Discrete signals are sent using either voltage

or current, where a specific range is designated as On and another as

Off. For example, a PLC might use 24 V DC I/O, with values above 22

V DC representing On, values below 2VDC representing Off, and

intermediate values undefined. Initially, PLCs had only discrete I/O.

Analog signals are like volume controls, with a range of values between

zero and full-scale. These are typically interpreted as integer values

(counts) by the PLC, with various ranges of accuracy depending on the

device and the number of bits available to store the data. As PLCs

typically use 16-bit signed binary processors, the integer values are

limited between -32,768 and +32,767. Pressure, temperature, flow, and

weight are often represented by analog signals. Analog signals can use

voltage or current with a magnitude proportional to the value of the

process signal. For example, an analog 4-20 mA or 0 - 10 V input

would be converted into an integer value of 0 - 32767.

Current inputs are less sensitive to electrical noise (i.e. from welders or

electric motor starts) than voltage inputs.

2.8. Example

As an example, say a facility needs to store water in a tank. The water is

drawn from the tank by another system, as needed, and our example

system must manage the water level in the tank.

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Using only digital signals, the PLC has two digital inputs from float

switches (Low Level and High Level). When the water level is above

the switch it closes a contact and passes a signal to an input. The PLC

uses a digital output to open and close the inlet valve into the tank.

When the water level drops enough so that the Low Level float switch

is off (down), the PLC will open the valve to let more water in. Once

the water level rises enough so that the High Level switch is on (up),

the PLC will shut the inlet to stop the water from overflowing. This

rung is an example of seal-in (latching) logic. The output is sealed in

until some condition breaks the circuit.

| |

| Low Level High Level Fill Valve |

|------[/]------|------[/]----------------------(OUT)---------|

| | |

| | |

| | |

| Fill Valve | |

|------[ ]-----| |

| |

| |

Fig 2.3:-Ladder diagram

An analog system might use a water pressure sensor or a load cell, and

an adjustable (throttling) dripping out of the tank, the valve adjusts to

slowly drip water back into the tank.

In this system, to avoid 'flutter' adjustments that can wear out the valve,

many PLCs incorporate "hysteresis" which essentially creates a

“deadband” of activity? A technician adjusts this deadband so the valve

moves only for a significant change in rate. This will in turn minimize

the motion of the valve, and reduce its wear.

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2.9 Different types of PLC according to Manufacturer

2.9.1Siemens

The Simatic S5 PLC is an automation system based on Programmable

Logic Controllers. It is manufactured and sold by Siemens AG. Such

automation systems control process equipment and machinery used in

manufacturing. This product line is considered obsolete, as the

manufacturer has since replaced it with their newer Simatic S7 PLC.

However, the S5 PLC still has a huge installation base in factories

around the world and a number of companies such as Direct-

Industrial.com continue to provide support for users of the S5 range

Hardware

The S5 line comes in the 90U, 95U,101U,100U,115U, 135U, and

155U chassis styles.Higher the number, the more sophisticated and

more expensive the system. Within each chassis style, several CPUs

are available, with varying speed, memory, and capabilities. Some

systems provide redundant CPU operation for ultra-high-reliability

control, as used in the pharmaceutical manufacturing, for example.

Each chassis consists of a power supply, and a backplane with slots for

the addition of various option boards. Available options include serial

and Ethernet communications, digital input and output cards, analog

signal processing boards, counter cards, and other specialized interface

and function modules.

Software

The S5 product line is usually programmed with a PC based software

programming tool called Step 5. Step 5 is used for programming,

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testing, and commissioning, and for documentation of programs for S5

PLCs.

The original Step 5 versions ran on the CPM operating system. Later

versions ran on MS-DOS, and then versions of Windows through

Windows XP. The final version of Step 5 is version 7.2. No further

development of this product line has occurred since that time, due to its

announced obsolescence.

In addition to Step5, Siemens offerred a proprietary State logic

programming package called Graph5. Graph5 is a sequential

programming language intended for use on machines that normally run

through a series of discrete steps. It simulates a State machine on the

S5 platform.

Several third-party programming environments have been released

for the S5. Most closely emulate Step5, some adding macros and

other minor enhancements, others functioning drastically differently

than Step5. One allows Step5 programs to be cross-compiled to and

from the C-programming language and BASIC.

Structured text

STEP 5 allows the creation of structured or unstructured

programming, from simple AND/OR operations up to complex

subroutines. A STEP 5 program may, therefore, contain thousands of

statements.

To maintain maximum transparency, STEP 5 offers a number of

structuring facilities:

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Block technique

A linear operation sequence is divided into sections and packed into

individual blocks.

Segments

Within blocks, fine structuring is possible by programming subtasks in

individual segments.

Comments - Both a complete program as well as individual blocks or

segments and individual statements can be directly provided with

comments. m,k

Methods of representation

STEP 5 programs can be represented in three different ways:

Statement List (STL) - The program consists of a sequence of mnemonic

codes of the commands executed one after another by the PLC.

Ladder Diagram (LAD) - Graphical representation of the automation task

with symbols of the circuit diagram

Function Block Diagram (FBD) - Graphical representation of the

automation task with symbols to DIN 40700/ DIN 40719.

Absolute or symbolic designations can be used for operands with all

three methods of representation.

In LAD and FBD complex functions and function block calls can be

entered via function keys. They are displayed on the screen as graphical

symbols.

Additional functions

Saving user-specific project settings

Symbol editor

Automatic generation and updating of cross-reference lists

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Comparison of user programs

Transferring blocks to EPROM and EEPROM memory modules for

programmable controllers

Rewiring inputs, outputs, flags, timers and counters

Testing and service functions for startup and maintenance

2.9.2 GE Fanuc

GE Fanuc's Computer Numerical Control business provides machine

tool controls and solutions, servo drives and motors, computer

numerical control productivity solutions and industrial CO2 lasers. In

Europe Fanuc GE CNC Europe is responsible for the CNC business.

Logix5000 controllers are true 32-bit controllers, meaning the

memory words are 32-bits wide. No matter what, a tag always

reserves 32 bits of memory even if it is a Boolean or integer data

type. For this reason, it is best to use a DINT when dealing with

integers. Furthermore, a Logix5000 controller typically compares or

manipulates values as 32-bit values (DINTs or REALs).

A Logix5000 controller lets you divide your application into multiple

programs, each with its own data. The Scope of the tag defines if a

tag is global (controller tags) and therefore available to all programs

or local (program tags) to a select program group. Pay careful

attention to this field as creating it in the wrong area may lead to

some confusion later on as to its location.’

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RSLogix 5000 Tips and Tricks

Everybody enjoys nifty little tips and tricks to get their work done

faster. This listing is for Allen Bradley's RSLogix 5000 software. Feel

free to add your own tips and tricks using the 'add comment' link.

General

To access Release Notes for this version of software, choose Release

Notes from the Help menu.

The Quick View Pane, located below the Controller Organizer,

provides "thumbnail" information for the selected component.

The Watch Pane, located below the language editor window, provides

monitoring for all tags referenced in the active routine window.

The Controller Organizer is dockable. That is, you can drag it to the left

or right side of the screen, or float it somewhere in between.

Hide/show the Controller Organizer via a toolbar button to make more

display area for editors.

RSLogix 5000 supports Cut/Copy/Paste/Drag/Drop of components

within the Controller Organizer as well as to other instances of

RSLogix 5000.

Double-clicking on error messages displayed in the Error Window will

navigate you to where the error was encountered.F4 and Shift-F4 can be

used to move between errors.

You can reorder the columns in the tag editor by clicking on the title

and dragging it to a new position.

To simultaneously display logic in multiple routines, select Window ->

New Window and then arrange the windows manually. Or select

Window -> Tile Horizontal.

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To remove a yellow triangle warning symbol on a device, first check

the connection status. If the status is "Connection is not scheduled", re-

open the RSNetWorx software. Return to RSLogix 5000 software and

the yellow triangle should be gone.

On one computer, you can install and simultaneously launch (run)

multiple translated versions of RSLogix 5000 software.

Once you do a partial import of rungs, add-on instructions, or user-

defined data types, you can't undo the import. If the import didn't work

as expected, close the project without saving.

When you select a partial import, make sure to select the correct rung or

trend file. Both files have L5X extensions and the software doesn't

prevent you from selecting the wrong file. If you try to import a rung

where a trend is expected, or vice versa, the software does display an

error that the import failed.

Partial import of rungs works in all ladder routines, including Add-On

Instructions.

I/O Configuration

Module icons in the I/O Configuration folder change to indicate the

module has faulted or the connection to the module has been

interrupted.

To remove a yellow triangle warning symbol, first check the connection

status. If the status is "Connection is not scheduled", re-open the

RSNetWorx software. Return to RSLogix 5000 software and the yellow

triangle should be gone.

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To easily find a module in the Select Module Type dialog, simply start

typing any part of the module name or description. When you start

typing, the Find Module dialog is launched automatically.

Use rack optimized communication formats for digital I/O modules to

minimize amount of controller memory and communications overhead

associated with these modules.

RSLogix 5000 automatically creates controller tags when you create an

input or output module. You can reference these tags directly in your

logic.

Use alias tags to assign names to specific input/output data and/or to

provide a short alternative to lengthy structure member names.

When you configure an analog I/O module, hold the shift key as you

move the slider to increment HH, H, L, and LL values in whole

numbers.

Copy I/O data to a User-Defined Type (UDT) so you can synchronize

I/O data with program scan. The UDT also enables easier mapping of

physical I/O.

Basic instructions

Be aware that specific nomenclature and operational details vary widely

between PLC manufacturers, and often implementation details evolve

from generation to generation.

Often the hardest part, especially for a beginning PLC programmer, is

practicing the mental ju-jitsu necessary to keep the nomenclature

straight from manufacturer to manufacturer.

Positive Logic (most PLCs follow this convention)

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True = logic 1 = input energized.

False = logic 0 = input NOT energized.

Negative Logic

True = logic 0 = input NOT energized

False = logic 1 = input energized.

Normally Open

(XIC) - examine If Closed.

This instruction is true (logic 1) when the hardware input (or internal

relay equivalent) is energized.

Normally Closed

(XIO) - examine If Open.

This instruction is true (logic 1) when the hardware input (or internal

relay equivalent) is NOT energized.

Output Enable

(OTE) - Output Enable.

This instruction mimics the action of a conventional relay coil.

On Timer

(TON) - Timer ON.

Generally, ON timers begin timing when the input (enable) line goes

true, and reset if the enable line goes false before setpoint has been

reached. If enabled until set point is reached then the timer output goes

true, and stays true until the input (enable) line goes false.

Off Timer

(TOF) - Timer OFf.

Generally, OFF timers begin timing on a true-to-false transition, and

continue timing as long as the preceding logic remains false. When the

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accumulated time equals set point the TOF output goes on, and stays on

until the rung goes true.

Retentive Timer

(RTO) - Retentive Timer On.

This type of timer does NOT reset the accumulated time when the input

condition goes false.

Rather, it keeps the last accumulated time in memory, and (if/when the

input goes true again) continues timing from that point. In the Allen-

Bradley construction this instruction goes true once setpoint (preset)

time has been reached, and stays true until a RES (Reset) instruction is

made true to clear it.

Latching Relays

(OTL) - Output Latch.

(OTU) - Output Unlatch.

Generally, the unlatch operator takes precedence. That is, if the unlatch

instruction is true then the relay output is false even though the latch

instruction may also be true. In Allen-Bradley ladder logic (and others)

latch and unlatch relays are separate operators.

Edit and Monitor Tags

To edit existing tags select the Logic > Edit Tags menu item. A spread

sheet like view lets you create and edit tags.

 

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Table 1:- Tags & Monitor

Clicking the + sign next to a tag reveals its structure. For a DINT tag this is

the 32 individual bits that make up the tag which will not be of interest if

you are using the tag as a number rather then individual bits. If you do

wish to use the individual bits then you can address them in this way with

the tag name followed by a period and then the bit position (e.g. MyTag.5).

Shown below is the expanded structure for a TIMER. Notice it is made of

two DINTs and three BOOLs. In this case, the Booleans are packed into

one DINT and therefore a timer uses three DINTs of memory.

 

An Easier Way to Create Tags

The easiest way to create tags is on the fly while programming. When

an instruction is first used a “?” will indicated the need for a tag. There

are three options at this point:

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1. Double click on the “?” and select an existing tag from the drop down

box.

2. Right click on the “?” and select new tag.

3. Double click on the “?” and type in the tag name. If it does not all ready

exist, then right click on the tag name and select Create

“NewTagName”. Be careful with this method not to use spaces or

special characters.

The nice thing about all these methods is that RSLogix5000 will

automatically fill in the correct data type according to the instruction

used.

Another quick method is to drag and drop an existing tag to a new

instruction. Make sure to click on the tag name rather then the

instruction.

 

Fig 2.9 :- Ladder diagram

Conclusion

These are the basics of tags. The advantages are:

1. Tags, if done right, create a level of documentation that is stored in the

PLC.

2. The software does an automatic housekeeping of memory locations.

There’s no more worrying about physical addressing and memory

conflicts.

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3. Structures can be more easily put together based on function rather then

data type.

Advance subjects include arrays, user defined data types (UDT) and

Add-On Instructions. Hopefully, you will continue to learn more about

the power of tags. There is no doubt that if you grasp the principles

presented here you will be well on your way to using and

troubleshooting any Logix5000 controller.

3.0 PLC Applications in Process Industries

PLCs are widely used in process industries; mainly there are three

types of applications:

DCS- PLC combination

PLC- Blind controller

PLC- substitute of DCS

4.0 Advantages

PLCs have gained popularity in industry as they offer specific

advantages, which are given below:

Faster scan time

Intelligent I/O

High-speed counters

Supervisory control capability

Reliability in operation

Flexibility in programming and reprogramming in the plant

Cost effective for controlling complex systems

Flexibility in control techniques

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Minimum maintenance

Small physical size

Superior computational capabilities to execute complex control

techniques

ASCII message handling capability

Ability to communicate with computer systems in the plant

Programmable troubleshooting aids which reduce down time

5. CONCLUSION

In this article The Programmable logic controller is Used in Industries &

also for the substitute of D.C.S. It can also be used as a part of D.C.S

While configured with D.C.S. The Various Manufacturers PLCs are used

according to the application. The presentation contains the application

where Traffic Light Control & Interlocking of the Motor has been done.

6.BIBILIOGRAPHY

[1] Cox, Richard D.technician”s guide to programmable controllers

NY:Delmar,1995

[2] Filer, Robert. Programmable controller Using Allen Bardley SLC-500

& control logix.Prentice Hall,2002

[3] Morris, Brian C. programmable logic controller NJ: Prentice Hall,2000.

[4] Programmable Logic controllers,Principle & applications. John W.

Webb&Ronald Reis Nj: 2007 .

[5] Stevenson, Jon. Fundamental of Programmable Logic Controllers, 2nd ed.

NY : Glencoe, 1998

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