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PROGRAMMABLE LOGIC CONTROLLERS AND MICROCONTROLLERS

LEARNING OBJECTIVES:

Basics of programmable Logic Controllers, their working and programming

Instructions sets like latch master control self holding relays

Timer and counter instructions sets

Electrical drives which were controlled by relays

Design , modify and troubleshoot control circuit

UNIT 1: INTRODUCTION TO PLC

1.1 What is PLC?

A programmable logic controller (PLC) or programmable controller is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes,

such as assembly lines, or robotic devices, or any activity that requires high reliability control

and ease of programming and process fault diagnosis.

They were first developed in the automobile manufacturing industry to provide flexible, ruggedized and easily programmable controllers to replace hard-wired relays, timers and

sequencers. Since then they have been widely adopted as high-reliability automation controllers

suitable for harsh environments. A PLC is an example of a "hard" real-time system since output

results must be produced in response to input conditions within a limited time, otherwise

unintended operation will result

PLCs can range from small modular devices with tens of inputs and outputs (I/O), in a housing

integral with the processor, to large rack-mounted modular devices with a count of thousands of

I/O, and which are often networked to other PLC and SCADA systems.

They can be designed for multiple arrangements of digital and analog I/O, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control

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

It was from the automotive industry in the USA that the PLC was born. Before the PLC, control,

sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. Since these could

number in the hundreds or even thousands, the process for updating such facilities for the yearly

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

individually rewire the relays to change their operational characteristics.

When digital computers became available, being general-purpose programmable devices, they were soon applied to control sequential and combinatorial logic in industrial processes. However

these early computers required specialist programmers and stringent operating environmental

control for temperature, cleanliness, and power quality. To meet these challenges the PLC was developed with several key attributes. It would tolerate the shop-floor environment, it would

support discrete (bit-form) input and output in an easily extensible manner, it would not require

years of training to use, and it would permit its operation to be monitored. Since many industrial processes have timescales easily addressed by millisecond response times, modern (fast, small,

reliable) electronics greatly facilitate building reliable controllers, and performance could be

traded off for reliability.

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1.2Invention and early development

In 1968 GM Hydramatic (the automatic transmission division of General Motors)

issued a request for proposals for an electronic replacement for hard-wired relay

systems based on a white paper written by engineer Edward R. Clark. 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 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, later acquired by German

Company AEG, and then by French Schneider Electric, the current oOne of the very

first 084 models built is now on display at Schneider Electric's facility 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.

Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder

logic to programming languages such as specially adapted dialects of BASIC and C.

Another method is state logic, a very high-level programming language designed to

program PLCs based on state transition diagrams. The majority of PLC systems

today adhere to the IEC 61131/3 control systems programming standard that defines

5 languages: Ladder Diagram (LD), Structured Text (ST), Function Block Diagram

(FBD), Instruction List (IL) and sequential function chart (SFC).

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 logic to be used, for the aforementioned reasons and because it

was a familiar format used for electro-mechanical control panels. 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 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.

Early PLCs, up to the mid-1990s, were programmed using proprietary

programming panels or special-purpose programming terminals, which often had

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dedicated function keys representing the various logical elements of PLC

programs Some proprietary programming terminals displayed the elements of

PLC programs as graphic symbols, but plain ASCIIcharacter representations of

contacts, coils, and wires were common. Programs were stored on cassette tape

cartridges. Facilities for printing and documentation were minimal due to lack of

memory capacity. The oldest PLCs used non-volatile magnetic core memory.

More recently, PLCs are programmed using application software on personal

computers, which now represent the logic in graphic form instead of character

symbols. The computer is connected to the PLC through USB, Ethernet, RS-

232, RS-485, or RS-422 cabling. The programming software allows entry and

editing of the ladder-style logic. In some software packages, it is also possible to

view and edit the program in function block diagrams, sequence flow charts

and structured text. Generally the software provides functions for debugging and

troubleshooting the PLC software, for example, by highlighting portions of the

logic to show current status during operation or via simulation. The software will

upload and download the PLC program, for backup and restoration purposes. In

some models of programmable controller, the program is transferred from a

personal computer to the PLC through a programming board which writes the

program into a removable chip such as an EPROM.

UNIT2: Timers and counters

2.1 INTRODUCTION-The main function of a timer is to keep an output on for a specific length of time.

A good example of this is a garage light, where you want power to be cut off after 2 minutes so as to

give someone time to go into the house. The three different types of timers that are commonly used

are a Delay-OFF, a Delay-ON, and a Delay-ON-Retentive. A Delay-OFF timer activates immediately when turned on, counts down from a programmed time before cutting off, and is cleared when the

enabling input is off. A Delay-ON timer is activated by input and starts accumulating time, counts up

to a programmed time before cutting off, and is cleared when the enabling input is turned off. A Delay-ON-Retentive timer is activated by input and starts accumulating time, retains the accumulated

value even if the (ladder-logic) rung goes false, and can be reset only by a RESET contact.Counters

are primarily used for counting items such as cans going into a box on an assembly line. This is important because once something is filled to its max the item needs to be moved on so something else

can be filled. Many companies use counters in PLC's to count boxes, count how many feet of

something is covered, or to count how many pallets are on a truck. There are three types of counters,

Up counters, Down counters, and Up/Down counters. Up counters count up to the preset value, turn on the CTU (CounT Up output) when the preset value is reached, and are cleared upon receiving a

reset. Down counters count down from a preset value, turns on the CTD (CounT Down output) when

0 is reached, and are cleared upon reset. Up/Down counters count up on CU, count down on CD, turn on CTUD (CounT Up/Down output) when the preset value is reached, and cleared on reset.

2.2 Programmable logic relay (PLR) -In more recent years, small products called PLRs

(programmable logic relays), and also by similar names, have become more common and accepted. These are 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 needed, 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 8

to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2 analog inputs. Size is usually about 4" wide,

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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 or RS-485 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 modular and greatly expandable, the PLRs are usually not modular or expandable, but their price can be two orders of magnitude less than a PLC, and they still offer robust design and

deterministic execution of the logics. 2.3 Scan time: A PLC program generally loops i.e. executes repeatedly, as long as the controlled

system is running. At the start of each execution loop, the status of all physical inputs are copied to

an area of memory, sometimes called the "I/O Image Table", which is accessible to the processor.

The program then runs from its first instruction rung down to the last rung. It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of

outputs.[14] Scan times of a few milliseconds may be encountered for small programs and fast

processors, but for older processors and very large programs much longer scan times (on the order of 100 ms) may be encountered. Excessively long scan times may mean the response of the PLC to

changing inputs or process conditions is too slow to be useful .As PLCs became more advanced,

methods were developed to change the sequence of ladder execution, and subroutines were

implemented. This simplified programming could be used to save scan time for high-speed processes; for example, parts of the program used only for setting up the machine could be segregated from

those parts required to operate at higher speed. Newer PLCs now have the option to run the logic

program synchronously with the IO scanning. This means that IO is updated in the background and the logic reads and writes values as it's required during the logic scanning.

UNIT 3: WORKING OF PLC

3.1Process of a scan cycle -There are 5 main steps in a scan cycle:

Reading inputs

Executing the program

Processing communication requests

Executing CPU diagnostics

Writing outputs

3.2User interface-User interface and List of human-computer interaction topics.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 man-machine

interfaces (MMIs) and graphical user interfaces (GUIs). 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 programming and monitoring software installed on a computer, with the PLC connected via a communication interface.

3.2 Communications-Many models of PLCs have built-in communications ports, using RS-232, RS-

422, RS-485, or Ethernet. Various protocols are usually included. Many of these protocols are vendor

specific.Most modern PLCs can communicate over a network to some other system, such as a

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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. Formerly, some manufacturers offered dedicated communication modules as an add-on function where the processor had no network

connection built-in.

3.3 Security-Prior to the discovery of the Stuxnet computer worm in June 2010, security of PLCs received little attention. Modern PLCs generally contain a real-time operating system such as OS-

9 or VxWorks, and exploits for these systems exist much as they do for desktop computer operating

systems such as Microsoft Windows. PLCs can also be attacked by gaining control of a computer they communicate with.

3.4 Simulation-In order to properly understand the operation of a PLC, it is necessary to spend

considerable time programming, testing, and debugging PLC programs. PLC systems are inherently

expensive, and down-time is often very costly. In addition, if a PLC is programmed incorrectly it can result in lost productivity and dangerous conditions. PLC simulation software such as PLCLogix can

save time in the design of automated control applications and can also increase the level of safety

associated with equipment since many"what if" scenarios can be tried and tested before the system is activated.

3.5 Redundancy-Some special processes need to work permanently with minimum unwanted down time.

Therefore, it is necessary to design a system which is fault-tolerant and capable of handling the process with faulty modules. In such cases to increase the system availability in the event of hardware

component failure, redundant CPU or I/O modules with the same functionality can be added to

hardware configuration for preventing total or partial process shutdown due to hardware failure

3.6 PLC Hardware-The hardware components of a PLC system are CPU, Memory, Input/Output,

Power supply unit, and programming device. Below is a diagram of the system overview of PLC.

An Overview Of Hardware Components Of A PLC System

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CPU – Keeps checking the PLC controller to avoid errors. They perform functions including logic

operations, arithmetic operations, computer interface and many more.

Memory – Fixed data is used by the CPU. System (ROM) stores the data permanently for the operating

system. RAM stores the information of the status of input and output devices, and the values of timers,

counters and other internal devices.

I/O section – Input keeps a track on field devices which includes sensors, switches.

O/P Section - Output has a control over the other devices which includes motors, pumps, lights and

solenoids. The I/O ports are based on Reduced Instruction Set Computer (RISC).

Power supply – Certain PLCs have an isolated power supply. But, most of the PLCs work at 220VAC or

24VDC.

Programming device – This device is used to feed the program into the memory of the processor. The

program is first fed to the programming device and later it is transmitted to the PLC’s memory. System Buses – Buses are the paths through which the digital signal flows internally of the PLC. The four

system buses are: Data bus is used by the CPU to transfer data among different elements.Address bus

sends the location’s addresses to access the data.

3.7Working of PLC (Programmable Logic Controller)

The Programmable logic controller functions in four steps.

Typical Block Diagram Of Programmable Logic Controller Functions

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Input scan: The state of the input is scanned which is connected externally. The inputs include switches,

pushbuttons, and proximity sensors, limit switches, pressure switches. Ideally, they are transformers and

not relays.

Program scan: The loaded program is executed to carry out the function appropriately.

Output scan: The input sources have a control over the output ports to energize or de-energize them. The

outputs include solenoids, valves, motors, actuator, and pumps. Depending on the model of PLC, these

relays can be transistors, triacs or relays.

UNIT4: PLC Applications

4.1 INTRODUCTION-The simple suitable application is a conveyor system. The requirements of the

conveyor systems are as follows:

A programmable logic controller is used to start and stop the motors of the conveyor belt.

The conveyor system has three segmented conveyor belts. Each segment is run by a motor.

To detect the position of a plate, a proximity switch is positioned at the segment’s end.

The first conveyor segment is turned ON always.

The proximity switch in the first segment detects the plate to turn ON the second conveyor segment.

The third conveyor segment is turned ON when the proximity switch detects the plate at the second

conveyor.

As the plate comes out of the detection range, the second conveyor is stopped after 20 secs.

When the proximity switch fails to detect the plate, the third conveyor is stopped after 20 secs.

4.2History

Programmable Logic Controllers were discovered by the automotive industry to substitute the re-

wiring of the machine’s control panel.

Prior to the invention of PLC, automobiles were manufactured using plenty of relays, cam timers,

and closed loop controllers. The electricians had to re-wire every part of the machine daily which

was time consuming and highly expensive on the financial front.

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Later in the year 1968, a request for an electronic device for the hard-wired relay systems was

made by GM hydramatic. Bedford Associates won the proposal and started a new company to

develop, fabricate, sell, and service this new launched product. The first PLC launched was

designated 084 as it was the eighty fourth projects of Bedford Associates. Dick Morley worked on

this project and is being considered as the Father of PLC. In the year 1977, the brand invented by

Modicon was sold to Gould Electronics. The Gould Electronics later sold it to German Company

AEG which was later taken over by French Schneider Electric.

The first 084 model of PLC was revealed in North Andover, Massachusetts at the Modicon

headquarters.

The automotive industry is one of the largest users of PLC.

4.3Advantages & Disadvantages Advantages

PLCs can be programmed easily which can be understood clearly well.

They are fabricated to survive vibrations, noise, humidity, and temperature.

The controller has the input and output for interfacing.

Disadvantages

It is a tedious job when replacing or bringing any changes to it.

Skilful work force is required to find its errors.

Lot of effort is put to connect the wires.

The hold up time is usually indefinite when any problem arises.

UNIT 5: .PLC Memory Organisation:

5.1 INTRODUCTION

The term processor memory organization refers to how certain areas of memory in a given Basis of PLC

Programming are used. Different PLC manufacturers organize their memories in different ways. Even

though they do not use the same memory make up and terminology, the principles involved are the same.

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Figure 21.67(a) shows a typical connection of a switch to the input image table through the input

module. When the switch is closed the processor detects a voltage at the input terminal and

records that information by storing a binary 1 in the proper bit location. Each connected input

has a bit in the input image table that corresponds exactly to the terminal to which the input is connected. The input image table is constantly being changed to reflect the current status of the

switch. If the input is ON (switch closed), its corresponding bit in the table is set to 1. If the input is OFF (switch open) the corresponding bit is cleared or reset to zero.

The output image table is an array of bits that controls the status of digital output devices which

are connected to output interface circuits. Figure 21.67(b) shows a typical connection of a light to the output image table through the output module.

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The status of this light (ON-OFF) is controlled by the user program and indicated by the

presence of 1 ‘ s (ON) and 0’s (OFF). Each connected output has a bit in the output image table

that corresponds exactly to the terminal to which output is connected. If the program calls for a specific output to be ON, it’s corresponding bit in the table is set to 1, if the program calls for the output to be OFF, its corresponding bit in the table is set to 0.

5.2Program Scan

During each operating cycle, the processor reads all the inputs takes their values and according to the user program energises or de-energises the outputs. This process is known as a scan. Figure. 21.68 illustrates a single PLC scan, which consists of the 1/0 scan and the program scan.

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5.3 The Basis of PLC Programming-

Scan time specification indicates how fast the controller can react to changes in inputs. Scan

time varies with program content and length. The time required to make a single scan can

vary from 1 ms to 100 ms. If a controller has to react to an input signal that changes states twice during the scan time it is possible that the PLC will never be able to detect this change.

The scan is normally a continuous and sequential process of reading the status of inputs,

evaluating the control logic and updating the outputs. Figure. 21.69 illustrates this process.

When the input device connected to address 10114 is closed, the input module circuit senses a voltage and a 1 (ON) condition is entered into the input image table bit 101-14.

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During the program scan, the processor examines bit 101-14 for a 1 (ON) condition. In this case, since input 101-14 is 1, the rung is said to be TRUE. The processor then sets the output image

table 001-04 to 1. The processor turns on output 101-14 during the next I/O scan, and the output

device (light) wired to this terminal becomes energized. This process is repeated as long as the processor is in the RUN mode. If the input device were to open, a 0 would be placed in the input

image table. As a result, the rung would be said to be False. The processor would then set the output image table bit 001-04 to 0 causing the output device to turn off.

5.4 PLC PROCESSOR MEMORY ORGANIZATION

The memory of a PLC is organized by types.

The memory space can be divided into two broad categories:

5.4.1Program and Data Memory:

Advanced ladder logic functins allow controllers to perform calculatins, make decisions and do

other complex tasks. Timers and counters are examples of ladder logic functions. They are more comples than basic inputs contacts and output coils and relay heavily upon data stored in the

memory of the PLC.

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The user program will account for most of the memory of a PLC system. Program files contain the logic controlling machine operation.

This logic consistes of instructions that are programmed in a ladder logic format.

5.4.2 DATA FILES:

The data file protion of memory stores input and output status, processor status, the status of

various bits and numerical data.

5.5 Programmable Logic Controllers (PLCs) are the major components in industrial automation and

control systems. The controlling nature of PLC is ranging from simple- push button switching to a single

motor to several complex control structures. The PLC programming is an important task of designing and implementing control application depending on customers need. A PLC program consists of a set of

instructions either in textual or graphical form, which represents the logic to be implemented for

specific industrial realtime applications.

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A dedicated PLC programming software comes from a PLC hardware of specific manufacturer that allows entry and development of user application code, which can be finally download to the PLC

hardware. This software also ensures Human Machine Interface (HMI) as a graphical representation of

variables. Once this program gets downloaded to the PLC and if the PLC is placed in Run mode, then the

PLC continuously works according to the program. Before going to the program of the PLC, let us know

the basics of the PLC programming tutorial and its basic concepts.

UNIT6: MICRO CONTROLLER SERIES (MCS-51)

6.1OVERVIEW

The Intel MCS-51 (commonly termed 8051) is a single chip microcontroller (MCU) series

developed by Intel in 1980 for use in embedded systems. The architect of the instruction set of the

Intel MCS-51 was John H. Wharton.[1][2] Intel's original versions were popular in the 1980s and

early 1990s and enhanced binary compatible derivatives remain popular today. It is an example of a complex instruction set computer, and has separate memory spaces for program instructions

and data (Harvard architecture). Intel's original MCS-51 family was developed using N-type metal-oxide-semiconductor (NMOS)

technology like its predecessor Intel MCS-48, but later versions, identified by a letter C in their name (e.g., 80C51) used complementary metal–oxide–semiconductor (CMOS) technology and

consume less power than their NMOS predecessors. This made them more suitable for battery-

powered devices.

The family was continued in 1996 with the enhanced 8-bit MCS-151 and the 8/16/32-bit MCS-251 family of binary compatible microcontrollers.[3] While Intel no longer manufactures the

MCS-51, MCS-151 and MCS-251 family, enhanced binary compatible derivatives made by

numerous vendors remain popular today. Some derivatives integrate a digital signal

processor (DSP). Beyond these physical devices, several companies also offer MCS-51 derivatives as IP cores for use in field-programmable gate array (FPGA) or application-specific

integrated circuit (ASIC) designs.

6.2 PIN DIAGRAM OF MCS-51

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Pins 1 to 8 − These pins are known as Port 1. This port doesn’t serve any other functions. It is

internally pulled up, bi-directional I/O port.

Pin 9 − It is a RESET pin, which is used to reset the microcontroller to its initial values.

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Pins 10 to 17 − These pins are known as Port 3. This port serves some functions like interrupts,

timer input, control signals, serial communication signals RxD and TxD, etc.

Pins 18 & 19 − These pins are used for interfacing an external crystal to get the system clock.

Pin 20 − This pin provides the power supply to the circuit.

Pins 21 to 28 − These pins are known as Port 2. It serves as I/O port. Higher order address bus

signals are also multiplexed using this port.

Pin 29 − This is PSEN pin which stands for Program Store Enable. It is used to read a signal

from the external program memory.

Pin 30 − This is EA pin which stands for External Access input. It is used to enable/disable the

external memory interfacing.

Pin 31 − This is ALE pin which stands for Address Latch Enable. It is used to demultiplex the

address-data signal of port.

Pins 32 to 39 − These pins are known as Port 0. It serves as I/O port. Lower order address and

data bus signals are multiplexed using this port.

Pin 40 − This pin is used to provide power supply to the circuit

UNIT 7: ASSEMBLY LANGUAGE PROGRAMMING

7.1 Assembler

An assembler program creates object code by translating combinations of mnemonics

and syntax for operations and addressing modes into their numerical equivalents. This representation typically includes an operation code ("opcode") as well as other control bits and

data. The assembler also calculates constant expressions and resolves symbolic names for

memory locations and other entities. The use of symbolic references is a key feature of

assemblers, saving tedious calculations and manual address updates after program modifications. Most assemblers also include macro facilities for performing textual substitution –

e.g., to generate common short sequences of instructions as inline, instead of called subroutines.

Some assemblers may also be able to perform some simple types of instruction set-

specific optimizations. One concrete example of this may be the ubiquitous x86 assemblers from various vendors. Most of them are able to perform jump-instruction replacements (long jumps

replaced by short or relative jumps) in any number of passes, on request. Others may even do

simple rearrangement or insertion of instructions, such as some assemblers for RISC architectures that can help optimize a sensible instruction scheduling to exploit the CPU

pipeline as efficiently as possible.

Like early programming languages such as Fortran, Algol, Cobol and Lisp, assemblers have

been available since the 1950s and the first generations of text based computer interfaces.

However, assemblers came first as they are far simpler to write than compilers for high-level

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languages. This is because each mnemonic along with the addressing modes and operands of an instruction translates rather directly into the numeric representations of that particular

instruction, without much context or analysis. There have also been several classes of translators

and semi automatic code generators with properties similar to both assembly and high level

languages, with speedcode as perhaps one of the better known examples.

There may be several assemblers with different syntax for a particular CPU or instruction set architecture. For instance, an instruction to add memory data to a register in a x86-family

processor might be add eax,[ebx] , in original Intel syntax, whereas this would be written addl

(%ebx),%eax in the AT&T syntax used by the GNU Assembler. Despite different appearances,

different syntactic forms generally generate the same numeric machine code, see further below. A

single assembler may also have different modes in order to support variations in syntactic forms

as well as their exact semantic interpretations (such as FASM-syntax, TASM-syntax, ideal mode

etc., in the special case of x86 assemblyprogramming).

7.2 Compiler

It is a computer program that transforms computer code written in one programming language (the source language) into another programming language (the target language).

Compilers are a type of translator that support digital devices, primarily computers. The

name compiler is primarily used for programs that translate source code from a high-level programming language to a lower level language (e.g., assembly language, object code,

or machine code) to create an executable program.

However, there are many different types of compilers. If the compiled program can run on a

computer whose CPU or operating systemis different from the one on which the compiler runs, the compiler is a cross-compiler. A bootstrap compiler is written in the language that it intends to

compile. A program that translates from a low-level language to a higher level one is

a decompiler. A program that translates between high-level languages is usually called a source-

to-source compiler or transpiler. A language rewriter is usually a program that translates the form of expressions without a change of language. The term compiler-compiler refers to tools

used to create parsers that perform syntax analysis.

A compiler is likely to perform many or all of the following operations: preprocessing, lexical

analysis, parsing, semantic analysis(syntax-directed translation), conversion of input programs to an intermediate representation, code optimization and code generation. Compilers implement

these operations in phases that promote efficient design and correct transformations of source

input to target output. Program faults caused by incorrect compiler behavior can be very difficult to track down and work around; therefore, compiler implementers invest significant effort to

ensure compiler correctness.

Compilers are not the only translators used to transform source programs. An interpreter is

computer software that transforms and then executes the indicated operations. The translation

process influences the design of computer languages which leads to a preference of compilation or interpretation. In practice, an interpreter can be implemented for compiled languages and

compilers can be implemented for interpreted languages.

7.3Assembly directives

Assembly directives, also called pseudo-opcodes, pseudo-operations or pseudo-ops, are

commands given to an assembler "directing it to perform operations other than assembling

instructions." Directives affect how the assembler operates and "may affect the object code, the

symbol table, the listing file, and the values of internal assembler parameters." Sometimes the

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term pseudo-opcode is reserved for directives that generate object code, such as those that

generate data.

The names of pseudo-ops often start with a dot to distinguish them from machine instructions.

Pseudo-ops can make the assembly of the program dependent on parameters input by a

programmer, so that one program can be assembled different ways, perhaps for different

applications. Or, a pseudo-op can be used to manipulate presentation of a program to make it easier to read and maintain. Another common use of pseudo-ops is to reserve storage areas for

run-time data and optionally initialize their contents to known values.

Symbolic assemblers let programmers associate arbitrary names (labels or symbols) with

memory locations and various constants. Usually, every constant and variable is given a name so instructions can reference those locations by name, thus promoting self-documenting code. In

executable code, the name of each subroutine is associated with its entry point, so any calls to a

subroutine can use its name. Inside subroutines, GOTO destinations are given labels. Some assemblers support local symbols which are lexically distinct from normal symbols (e.g., the use

of "10$" as a GOTO destination).

Some assemblers, such as NASM, provide flexible symbol management, letting programmers

manage different namespaces, automatically calculate offsets within data structures, and assign

labels that refer to literal values or the result of simple computations performed by the assembler.

Labels can also be used to initialize constants and variables with relocatable addresses.

7.4Assembly languages

Like most other computer languages, allow comments to be added to program source code that will be ignored during assembly. Judicious commenting is essential in assembly

language programs, as the meaning and purpose of a sequence of binary machine

instructions can be difficult to determine. The "raw" (uncommented) assembly language

generated by compilers or disassemblers is quite difficult to read when changes must be

made.

UNIT 8 :Applications of 8051 microcontroller

1.1Applications of 8051 Microcontroller:

The microcontroller 8051 applications include large amount of machines, principally

because it is simple to incorporate in a project or to assemble a machine around it. The

following are the key spots of spotlight:

Energy Management: Competent measuring device systems aid in calculating energy

consumption in domestic and industrialized applications. These meter systems are prepared competent by integrating microcontrollers.

Touch screens: A high degree of microcontroller suppliers integrate touch sensing abilities

in their designs. Transportable devices such as media players, gaming devices & cell phones

are some illustrations of micro-controller integrated with touch sensing screens.

Automobiles: The microcontroller 8051 discovers broad recognition in supplying automobile

solutions. They are extensively utilized in hybrid motor vehicles to control engine variations.

In addition, works such as cruise power and anti-brake mechanism has created it more capable with the amalgamation of micro-controllers.

Medical Devices: Handy medicinal gadgets such as glucose & blood pressure monitors

bring into play micro-controllers, to put on view the measurements, as a result, offering

higher dependability in giving correct medical results.

MULTIPLE CHOICE QUESTIONS

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Q1. The PLC was invented by. a) Bill Gates b) Dick Morley c) Bill Landis d) Tod Cunningham

Q2. The first company to build PLCs was .

a) General Motors b) Allen Bradley c) Square D d) Modicon

Q3. Classify the following as automatic control, manual control, discrete control or continuous control.

a) A sensor is used to turn on and off the lights of a room.

b) A temperature sensor is used to adjust the room temperature and maintain it at a set point.

c) A user starts the bottle filling machine, places the bottle on the conveyor belt to fill it with the required

liquid.

d) The luggage system at the airport.

e) A factory that makes red pens and has no human employees.

Q4. The part that monitors the inputs and makes decisions in a PLC is the CPU.

a. True b. False c. None of the above

Q5. One of the following is an input device

a. Motor b. Light c. Valve d. Sensor

Q6. Which one of the following is not a PLC manufacturer

a. Siemens b. Mitsubishi c. Microsoft d. ABB

Q7. Solenoids, lamps, motors are connected to:

a. Analog output b. Digital output c. Analog input d. Digital input

Q8. In a PLC “I” is used for output and “Q” is used for input

a. True b. False c. None of the above

Q9. PLC stands for programmable logo controller

a. True b. False c. None of the above

Q10. To increase the number of inputs and outputs of the PLC, one can use expansion modules.

a. True b. False c. None of the above

Q11. An example of discrete (digital) control is:

a. Varying the volume of a music system

b. Turning a lamp ON or OFF

c. Varying the brightness of a lamp

d. Controlling the speed of a fan

Q12. A solenoid is an example of an output device.

a. True b. False c. None of the above

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Q13. Which of the following statements is not correct? a) The PLC rung output [-( )-] is a discrete output instruction or bit in memory.

b) Each rung of the ladder logic represents a logical statement executed in software – inputs on the right

and outputs on the left.

c) Input and output instructions in ladder logic do not directly represent the switches and actuators.

d) PLC input instructions are logical symbols associated with voltage at the input module terminals.

Q14. Which of the following statements is correct?

a) Ladder logic is a PLC graphical programming technique introduced in the last 10 years.

b) A ladder logic program is hard to analyze because it is totally different when compared with the

equivalent relay logic solution.

c) The number of ladder logic virtual relays and input and output instructions is limited only by memory

size.

d) The number of contacts for a mechanical relay is limited to number of coils on the relay.

Q15. Which of the following statements is NOT correct?

a) The status of each input can be checked from one location and outputs can be forced on and off.

b) All symbols in the RLL represent actual components and contacts present in the control system.

c) PLCs are not as reliable as electromechanical relays in RLL.

d) Input (-| |-) and output (- ( ) -) instruction symbols in the ladder logic represent only data values stored

in PLC memory.

Q16. Which of the following statements is NOT correct?

a) If a problem in a PLC module occurs, the module can be changed in a matter of minutes without any

changes in wiring.

b) Outputs can be paralleled on the same rung.

c) The physical wires between the input and output field devices and the PLC input and output modules

are the only signal wires required in the PLC system.

d) The cost and size of PLCs have increased significantly in the last 10 years.

Q17. Which of the following statements about a single pole double throw relay is NOT true?

a) It is called an SPDT type of relay.

b) It has one common contact.

c) It has two positions (NC and NO).

d) It has a center off position.

Q18. Which of the following statements about a single pole double throw relay is true?

a) Insulators are used in the armature to isolate the electrical switching contacts from the rest of the

relay components.

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b) The NC contact and the pole are in contact when the relay is off.

c) It has just one coil.

d) All of the above.

Q19. Which of the following statements about RLL is NOT true?

a) NO contact symbol has two parallel lines to indicate an open contact.

b) RLL stands for Relay Ladder Logic.

c) NC contact symbol has the same two parallel lines with a line across them to indicate closed contacts.

d) The right power rail is positive or the high side of the source, and the left power rail is the power

return or ground.

Q20. The _____ is moved toward the relay electromagnet when the relay is on.

a) Armature b) Coil c) NO contact d) NC contact

Q21. When a relay is NOT energized:

a) There is an electrical path through the NO contacts

b) There is an electrical path through the NC contacts

c) Neither the NO or the NC contacts have an electrical path

d) Both the NO and the NC contacts have an electrical path

Q22. Which of the following RLL applications is not normally performed in early automation systems?

a) On/off control of field devices

b) Logical control of discrete devices

c) On/off control of motor starters

d) Proportional control of field devices

Q23. Current flows into the _____

a) Input terminal of a sinking DC input module

b) Input terminal of a sinking output field device

c) Output terminal of a sinking input field device

d) All of the above

Q24. In a current sinking DC input module _____

a) The current flows out of the input field device

b) Requires that a AC sources be used with mechanical switches

c) The current flows out of the input module

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d) Currents can flow in either direction at the input module

Q25. AC output field devices can interface to _____

Short Questions & Answers

Q1. What is PLC?

Q2. Role of PLC in Automation ?

Q3. Difference Between Fixed and Modular PLCs ?

Q4. What is meaning of scan time in PLC?

Q5. What is redundancy ?

Q6. What are components of redundant PLC system ?

Q.7 Manufacturers of PLC?

Long Questions & Answers

Q1. What is ladder diagram ?

Q.2. Difference between PLC & Relay ?

Q.3 Memory Organization of PLC??

Q.4 What is role of I/O modules?

Q.5 Scan cycle of PLC?

Q.6 Architecture of PLC?

Q7. Working of PLC and its applications?

Q8.Pin diagram of MCS8051?