Railways NTPC (Technical Ability) Micro Processor and Computer Networks

Micro Processor and Computer Networks

 

MICROCOMPUTER

A microcomputer has three basic blocks: a central processing unit (CPU), a memory unit, and an input/output unit.

The CPU executes all the instructions and performs arithmetic and logic operations on data. The CPU of the microcomputer is called the "microprocessor." The microprocessor is typically a single VLSI (Very Large-Scale Integration) chip that contains all the registers, control unit, and arithmetic/ logic circuits of the microcomputer.

A memory unit stores both data and instructions. The memory section typically contains ROM and RAM chips. The ROM can only be read and is nonvolatile, that is, it retains its contents when the power is turned off. A ROM is typically used to store instructions and data that do not change. For example, it might store a table of codes for outputting data to a display external to the microcomputer for turning on a digit from 0 to 9.

One can read from and write into a RAM. The RAM is volatile; that is, it does not retain its contents when the power is turned off. ARAM is used to store programs and data that are temporary and might change during the course of executing a program. An I/O (Input/Output) unit transfers data between the microcomputer and the external devices via I/O ports (registers). The transfer involves data, status, and control signals.

In a single-chip microcomputer, these three elements are on one chip, whereas with a single-chip microprocessor, separate chips for memory and 1/0 are required. Microcontrollers evolved from single-chip microcomputers. The microcontrollers are typically used for dedicated applications such as automotive systems, home appliances, and home entertainment systems. Typical microcontrollers, therefore, include on-chip timers and AID (analog to digital) and D/A (digital to analog) converters. Two popular microcontrollers are the Intel 8751 (8 bit)/8096 (16 bit) and the Motorola HC 11 (8bit)/HC16 (16bit). The 16-bit microcontrollers include more on-chip ROM, RAM, and I/O than the 8-bit microcontrollers. Figure above shows the basic blocks of a microcomputer. The System bus (comprised of several wires) connects these blocks.

 

MICROPROCESSOR

• It is basically a programmable integrated device which has the capacity of computing & decision making & functions as the CPU of a computer.
• It operates & communicates with the system attached to it in binary numbers 0 & 1 called bits.
• Every microprocessor has a fixed set of instruction in the form of binary patterns known as machine language.
• These binary instructions have been given the abbreviated names called mnemonics which forms the assembly language for a given microprocessor.

 

Architecture of 8085 Microprocessor

• Its hardware model has two major segments:

One includes ALU (arithmetic/logic unit) & an 8-bit regal (accumulator), instruction decoder & flags.

The other one includes 8-bit & 16-bit registers.

• An internal bus connects both the segments with various internal connections.
• 8085 uses 3 buses: a 16-bit unidirectional address bus to send out memory addresses.an 8-bit bidirectional data bus to transfer data & a corn bus for timing signals.
• 8085 hardware model is given in the fig.

 

 

Fig. Flag register

 

Registers

The 8085 programming model fig. includes six registers. accumulator, and one flag register, as shown in Figure. The 3 has six general-purpose registers to store 8 –bit data; these identified as B, C, D, E. H, and L as shown in the fig. They can combined as register pairs - BC, DE, and HL - to perform some bit operations. The programmer can use these registers to store or copy data into the registers.

 

Flags

The AL 17 includes five flip-flops, which are set or reset after an operation according to data conditions of the result in the accumulator and other registers. They are called Zero (Z), Carry Sign (S), Parity (P), and Auxiliary Carry (AC) flags; The most commonly used flags are Zero, Carry, and Sign. The microprocessor uses these flags to test data conditions.

For example, after an addition of two numbers, if the sum in the accumulator is larger than eight bits, the flip-flop uses to indicate a carry — called the Carry flag (CY) - is set to one. When an arithmetic operation results in zero, the flip-flop called the Zero (Z) flag is set to one. The flags are stored in the 8-bit register so that the programmer can examine these flags (data conditions) by accessing the register through an instruction.

 

Program Counter (PC)

This 16 – bit register deals with sequencing the execution of instrutions. This register is a memory pointer. Memory locations have 16 – bit addresses, and that is why this is a 16 – bit register.

The microprocessor uses this register to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte (machine code) is being fetched, the program counter is incremented by one to point to the next memory location.

 

Stack Pointer (SP)

The stack pointer is also a 16 – bit register used as a memory Pointer, It points to a memory location in R/'W memory, called the stock. The beginning of the stack is defined by loading 16 – bit address in the stack pointer.

Address bus is basically a group of 16 lines named from \[{{A}_{0}}\,\,to\,\,{{A}_{15}}\]is unidirectional in nature and helps in identifying a peripheral or memory location. Its structure is given in the fig.

 

Fig. 8085 Bus structure

The address lines are split into two segments: \[{{A}_{15}}-{{A}_{8}}\And A{{D}_{7}}-A{{D}_{0}}\] are unidirectional & are used for higher order address & latter are bidirectional & are used for lower order address bus as well as data bus.

 

8085 Addressing Modes

The various formats for specifying operands are called the

ADDRESSING MODES For 8085, they are of various types are:

1. Immediate addressing: Data is present in the instruction. Load the immediate data to the destination provided.

Example: MVI R, data

2. Register addressing: Data is provided through the registers.

Example: MOV Rd. Rs

3. Direct addressing: Used to accept data from outside devices to store in the accumulator or send the data stored in the accumulator to the outside device. Accept the data from the port 00H and store into the accumulator or Send the data from the accumulator to the port 01H,

Example: IN 00H or OUT 01H

4. Indirect Addressing: This means that the Effective Address is calculated by the processor. And the contents of the address (and the one following) is used to form a second address. The second address is where the data is stored. Note that this requires several memory accesses; two accesses to retrieve the 16-bit address and a further access (or accesses) to retrieve the data which is to be loaded into the register.

 

Instruction Word Size

The 8085 instruction set is classified into the following three groups according to word size:

1. One-word or 1 -byte instructions
2. Two-word or 2-byte instructions
3. Three-word or 3-byte instructions

 

Memory interfacing:

While executing an instruction, there is a necessity for the microprocessor to access memory frequently for reading various instruction codes and data stored in the memory. The interfacing circuit aids in accessing the memory.

Memory requires some signals to read from and write to registers. Similarly the microprocessor transmits some signals for reading or writing a data.

The interfacing process involves matching the memory requirements with the microprocessor signals. The interfacing circuit therefore should be designed in such a way that it matches the memory signal requirements with the signals of the microprocessor. For example for carrying out a READ process, the microprocessor should initiate a read signal which the memory requires to read a data. In simple words, the primary function of a memory interfacing circuit is to aid the microprocessor in reading and writing a data to the given register of a memory chip.

 

I/O Interfacing:

We know that keyboard and Displays are used as communication channel with outside world. So it is necessary that we interface keyboard and displays with the microprocessor. This is called I/O interfacing. In this type of interfacing we use latches and buffers for interfacing the keyboards and displays with the microprocessor.

But the main disadvantage with this interfacing is that the microprocessor can perform only one function. It functions as an input device if it is connected to buffer and as an output device if it is connected to latch. Thus the capability is very limited in this type of interfacing.

 

DATATRANSFER SCHEMES IN A

MICROPROCESSOR

 

Data transfer schemes

are dependent on on-line or off-line processing, type of I/O device (capable of parallel or serial data transfer, synchronous or asynchronous) and the particular application-Data transfer schemes may toe categorized into parallel data transfer and serial data transfer.

 

Parallel “ucp” data transfer

 

In parallel data transfer, a group of bits (for example 8–bits) are transmitted from one device to another at any time. To achieve parallel data transfer, a group of data lines will be conducting the processor and peripheral devices. Normally in microprocessor based system the parallel data transfer schemes are adopted to transfer data between various devices inside the system. Parallel data transfer is impractical over long distances because of the prohibitive cost of installing a large number of lines.

 

Serial Data Transfer

In serial data transfer, each bit of the word is sent in succession, one at a time over a single pair of wires. A parallel to serial converter is used to convert the incoming parallel data to a serial form and then the data is sent out with the least significant bit DO first and the most significant bit D7 last.

 

Addressing Modes:

The different ways in which the location of an operand is specified in an instruction are referred to as addressing modes.

Types of Addressing Modes

• Implied Mode: In this mode the operands are specified implicitly in the definition of an instruction.
• Immediate Mode: In this mode the operand is specified in the instruction itself or we can say that, an immediate mode instruction has an operand rather than an address.
• Register Mode: In this mode, the operands are in registers.
• Direct Address Mode: It this mode, the address of the memory location that holds the operand is included in the instruction. The effective address is the address part of the instruction.
• Indirect Address Mode: In this mode the address field of the instruction gives the address where the effective address is stored in memory.
• Relative Address Mode: In this mode the content of program counter is added to the address part of the instruction to calculate the effective address.
• Indexed Address Mode: In this mode, the effective address will be calculated as the addition of the content of index register and the address part of the instruction.

 

Types of Instructions

 

• Data Transfer Instructions: Data transfer instructions cause transfer of data from one location to another without changing the information content. The common transfers may be between memory and processor registers, between processor registers and input/output.

Typical Data Transfer Instructions

Name	Mnemonic
Load	LD
Store	ST
Move	MOV
Exchange	XCH
Input	IN
Push	OUT
Pop	PUSH
POP	POP

 

• Data Manipulation Instructions: Data manipulation instructions perform operations on data and provide the computational capabilities for the computer. There are three types of data manipulation instructions:

(i)  Typical Arithmetic Instructions Ex: INC, DEC, ADD, SUB, MUL, DIV, ADDC, SUBB, NEG

(ii) Typical Logical and Bit Manipulation Instructions Ex: CLR. COM, AND, OR, XOR, CLRC, SETC, COMC, El, DI.

(iii) Typical Shift Instructions Ex: SHR. SHL. ROR, ROL.

 

Program Control Instructions

Program control instructions specify conditions for altering the content of the program counter, while data transfer and manipulation instructions specify conditions for data processing operations. The change in value of a program counter as a result of the execution of a program control instruction causes a break in the sequence of instruction execution.

 

Typical Program Control Instructions

Name	Mnemonic
BRANCH	BR
JUMP	JMP
SKIP	SKP
CALL	CALL
RETURN	RET
COMPARE	CMP
TEST	TST

 

Program Interrupt  

The program interrupts are used to handle a variety of problems that arise out of normal program sequence.                 

• Program interrupts are used to transfer the program control from a currently running program to another service program as a result of an external or internal generated request. Control returns to the original program after the service program is executed.                              

 

Types of Interrupts

There are three major types of interrupts

1. External interrupt: External interrupts come from Input-

Output (I/O) devices or from a timing device.

2. Internal interrupt: Internal interrupts arise from illegal or erroneous use of an instruction or data. External and internal interrupts from signals that occur in the hardware of the

CPU.

3. Software interrupt: A Software interrupt is initiated by executing an instruction.

 

I/O INTERFACE (INTERRUPT AND DMA MODE)

 

Peripheral Devices

• The I/O system provides an efficient mode of communication between the central system and the outside environment.
• Programs and data must be entered into computer memory for processing end results obtained from computations must be displayed for the user. The most familiar means of entering information into a computer is through a type writer-like keyboard. On the other hand the central processing unit is an extremely fast device capable of performing operations at very high speed.
• To use a computer efficiently, a large amount of programs and data must be prepared in advance and transmitted into a storage medium such as magnetic tapes or disks. The information in the disk is then transferred into a high-speed storage, such as disks.
• Input or output devices attached to the computer are called the peripheral devices. The most common peripherals are keyboards, display units and printers. Peripherals that provide auxiliary storage for the system are magnetic disks and tapes.

 

Input-Output Interface

• Input-Output interface provides a method for transferring information between internal storage and external I/O devices.
• Peripherals are connected to the central processing unit with a special communication links (I/O bus).
• The I/O bus from processor is attached to all peripheral interfaces.

 

I/O communication

• There is a need to I/O bus for communication between CPU and peripheral devices because of many reasons

(a) Data formats of internal memory of CPU and the peripheral devices (I/O devices) are different.

(b) Data transfer rates CPU and the I/O devices are different.

 

Asynchronous Data Transfer

• The two units such as CPU and I/O interface, are designed independently of each other. If the registers in the interface does not have a common clock (global clock) with the CPU registers, then the transfer between the two units is said to be asynchronous.
• The asynchronous data transfer requires the control signals that are being transmitted between the communicating units to indicate the time at which data is being transmitted.

 

Handshaking

• The basic approach of handshaking is as follows. In handshaking method, there are two control signals unlike strobe control method. One control signal is in the same direction as the data flow in the bus from the source to the destination. This signal is used to inform the destination unit whether there are valid data in the bus. The second control signal is in the other direction from the destination to the source. It is used to inform the source whether it can accept .data.

 

Synchronous Data Transfer

In synchronous data transfer a global or shared clock is provided to both sender and receiver. The sender and receiver works simultaneously.

Modes of Transfer

The information from external device is stored in memory. Information transferred from the central computer into an external device via memory unit. Hence, this data transfer between the central computer and I/O devices is handled in various modes.

1. Programmed I/O
2. Interrupt- initiated I/O
3. Direct Memory Access (DMA)

Programmed I/O: In this mode, each data item is transferred by an instruction in the program. The CPU issues a command then waits for I/O operations to be complete. As the CPU is faster than the I/O module, the problem with programmed I/O is that the CPU has to wait a long time for the

I/O module of concern to be ready for either reception or transmission of data.

Programmed I/O basically works in these ways:

• CPU requests I/O operation
• I/O module performs operation
• I/O module sets status bits
• CPU checks status bits periodically
• I/O module does not inform CPU directly
• I/O module does not interrupt CPU
• CPU may wait or come back later

The CPU, while waiting, must repeatedly check the status of the I/O module, and this process is known as Polling. As a result, the level of the performance of the entire system is severely degraded.

Interrupt-initiated I/O: This mode removes the drawback of the programmed I/O mode. The CPU issues commands to the I/O module then proceeds with its normal work until interrupted by I/ O device on completion of its work.

For input, the device interrupts the CPU when new data has arrived and is ready to be retrieved by the system processor. The actual actions to perform depend on whether the device uses I/O ports, memory mapping.

For output, the device delivers an interrupt either when it is ready to accept new data or to acknowledge a successful data transfer.

Memory-mapped and DMA-capable devices usually generate interrupts to tell the system they are done with the buffer.

Although Interrupt relieves the CPU of having to wait for the devices, but it is still inefficient in data transfer of large amount because the CPU has to transfer the data word by word between I/O module and memory.

Below are the basic operations of Interrupt:

• CPU issues read command
• I/O module gets data from peripheral whilst CPU does other work
• I/O module interrupts CPU
• CPU requests data
• I/O module transfers data

Direct Memory Access (DMA)

Direct Memory Access (DMA) means CPU grants I/O module authority to read from or write to memory without involvement.

DMA module controls exchange of data between main memory and the I/O device. Because of DMA device can transfer data directly to and from memory, rather than using the CPU as an intermediary, and can thus relieve congestion on the bus. CPU is only involved at the beginning and end of the transfer and interrupted only after entire block has been transferred.

The CPU initiates the transfer by supplying the interface with the starting address and the number of words needed to be transferred and then proceeds to execute other tasks. When the transfer is made, the DMA requests memory cycles through the memory bus. When the request is granted by the memory controller, the DMA transfers the data directly into memory.

The Bus Request (BR) input is used by the DMA controller to request the CPU to get the control of buses. When this input is active, the CPU terminates the execution of the current instruction and places the address bus and the data bus. The CPU activates the Bus Grant (BG) output to inform the external DMA that the buses are available. The DMA now takes the control of the buses to conduct the memory transfer. When DMA terminates the transfer, it disables the bus request line. The CPU disables the bus grant, takes the control of the buses.

 

 

DATA COMMUNICATION

It refers to the exchange of data between a source and a receiver. Data communication is said to be local if communicating devices are in the same building or a similarly restricted geographical area.

Datum mean the facts information statistics or the like derived by calculation or experimentation. The facts and information so gathered are processed in accordance with defined systems of procedure. Data can exist in a variety of forms such as numbers, text, bits and bytes. The Figure is an illustration of a simple data communication system.

 

 

A data communication system may collect data from remote locations through data transmission circuits, and then outputs processed results to remote locations. Figure provides a broader view of data communication networks. The different data communication techniques which are presently in widespread use evolved gradually either to improve the data communication techniques already existing or to replace the same with better options and features. Then, mere are data communication jargons to contend with such as baud rate, modems, routers, LAN, WAN, TCP/IP. ISDN, during the selection of communication systems.

Hence, it becomes necessary to review and understand these terms and gradual development of data communication methods.

 

ISO/OSI MODEL IN COIVSMUNSCATION NETWORKS

 

The ISO-OSI model is a seven layer architecture. It defines seven layers or levels in a complete communication system.

Functions of Different Layers

 

Layer 1: The Physical Layer:

1. It is the lowest layer of the OSI Model.
2. It activates, maintains and deactivates the physical connection.
3. It is responsible for transmission and reception of the unstructured raw data over network.

 

Layer 2: Data Link Layer:

1. Data link layer synchronizes the information which is to be transmitted over the physical layer.
2. The main function of this layer is to make sure data transfer is error free from one node to another, over the physical layer.
3. Transmitting and receiving data frames sequentially is managed by this layer.

 

Layer 3: The Network Layer:

 

1. It routes the signal through different channels from one node to other.
2. It acts as a network controller. It manages the Subnet traffic.
3. It decides by which route data should take.

 

Layer 4: Transport Layer:

 

1. It decides if data transmission should be on parallel path or single path.
2. Functions such as Multiplexing, Segmenting or Splitting on the data are done by this layer
3. It receives messages from the Session layer above it, convert the message into smaller units and passes it on to the Network layer.

 

Layer 5: The Session Layer;

1. Session layer manages and synchronize the conversation between two different applications.
2. Transfer of data from source to destination session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

 

Layer 6: The Presentation Layer:

1. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data.
2. While receiving the data, presentation layer transforms the data to be ready for the application layer.
3. Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role of translator.
4. It perfroms Data compression, Data encryption. Data conversion etc.

 

Layer 7: Application Layer:

1. It is the topmost layer.
2. Transferring of files disturbing the results to the user is also done in this layer. Mail services, directory services, network resource etc are services provided by application layer.
3. This layer mainly holds application programs to act upon the received and to be sent data.

 

MODEM

Modem is abbreviation for Modulator - Demodulator. Modems are used for data transfer from one computer network to another computer network through telephone lines. The computer networks in digital mode, while analog technology is used for carrying massages across phone lines.

The transmission medium between the two modems can be dedicated circuit or a switched telephone circuit. If a switched telephone circuit is used, then the modems are connected to the local telephone exchanges. Whenever data transmission is required connection between the modems is established through telephone exchanges.

 

TYPES OF MODEMS

• Modems can be of several types and they can be categorized is a number of ways.
• Categorization is usually based on the following basic modem features:

1. Directional capacity: half duplex modem and full duplex modem.
2. Connection to the line: wire modem and 4-wire modem.
3. Transmission mode: asynchronous modem and synchronous modem.

 

Asynchronous & Synchronous Modems

 

Asynchronous Modem

• Asynchronous modems can handle data bytes with start and stop bits.
• There is no separate timing signal or clock between the modem and the DTE.
• The internal timing pulses are synchronized repeatedly to the leading edge of the start pulse.

 

Synchronous Modem

• Synchronous modems can handle a continuous stream of data bits but requires a clock signal.
• The data bits are always synchronized to the clock signal.
• There are separate clocks for the data bits being transmitted and received.

 

Modulation techniques used for Modem:

 

The basic modulation techniques used by a modem "to convert digital data to analog signals are:

• Amplitude shin keying (ASK).
• Frequency shift keying (FSK).
• Phase shift keying (PSK).
• Differential PSK (DPSK).

These techniques are known as the binary continuous wave (CW) modulation.

Modems are always used in pairs. Any system whether simplex, half duplex or full duplex requires a modem at the transmitting as well as the receiving end.

Thus a modem acts as the electronic bridge between two worlds - the world of purely digital signals and the established analog world.

 

TCP/IP

Short for Transmission Control Protocol/Internet Protocol, TCP/IP is a set of rules (protocols) governing communications among all computers on the Internet. More specifically. TCP/IP dictates how information should be packaged (turned into bundles of information called packets), sent, and received, as well as how to get to its destination.

Three of the most common TCP/IP protocols

• HTTP - Used between a web client and a web server, for non-secure data transmissions. A web client (i.e. Internet browser on a computer) sends a request to a web server to view a web page. The web server receives that request and sends the web page information back to the web client.
• HTTPS - Used between a web client and a web server, for secure data transmissions. Often used for sending credit card transaction data or other private data from a web client (i.e. Internet browser on a computer) to a web server.
• FTP - Used between two or more computers. One computer sends data to or receives data from another computer directly.

Domain names and TCP/IP addresses

The TCP/IP address for a website or web server is typically not easy to remember. To remedy this issue, a domain name is used instead. For example, 45.79.151.23 is the IP address for the Computer Hope website and computerhope.com is the domain name.

Using this method, instead of a set of numbers, makes it much easier for users to remember Computer Hope's web address.

 

ISDN

ISDN is a circuit-switched telephone network system, that also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than- an analog phone. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 Kbit/s.

 

ISDN Interfaces

There are several kinds of access interfaces to the ISDN dermed:

Basic Rate Interface (BR1)

Primary Rate Interface (PR1)

Broadband-ISDN (B-ISDN)

 

ISDN Services

The purpose of the ISDN is to provide fully integrated digital services to users. These services fall into categories- better services, teleservices and supplementary services.

1. Bearer Services: Bearer services provide the means to transfer (voice, data and video) between users without the network manipulating the content of that information. The network does not need to process the information and therefore does not change the content.

Bearer services belong to the first three layers of the OSI model and are well defined in the ISDN standard. They can be provided using circuit-switched, packet-switched, frame switched, or cell-switched networks.

2. Teleservices: In teleservices, the network may change or process the contents of the data. These services correspond to layers 4-7 of the OSI model. Teleservices relay on the facilities of the bearer services and are designed to accommodate complex user needs, without the user having to be aware of the details of the process. Teleservices include telephony, teletex, telefax, videotex, telex and teleconferencing. Although the ISDN defines these services by name, they have not yet become standards.
3. Supplementary Service: Supplementary services are those services that provide additional functionality to the bearer services and teleservices. Examples of these services are reverse charging, call waiting, and message handling, all familiar from today's telephone company services.

 

LAN (LOCAL AREA NETWORK)

LAN looks like an acronym that a board of directors spent a lot of money and time trying to create, but it actually stands for any generic local area network. A network is a group of and other devices connected together so they can pass back and forth.

• The local area network (LAN) is a network which is designed to operate over a small physical area such as an office, factor. or a group of buildings. LANs are very widely used in a variety of applications.

The data rates for LAN range from 4 to 16 Mbps with the maximum of 100 Mbps.

The components used by LANs can be divided into cabling standards, hardware, and protocols. Various LAN protocols are

Ethernet, Token Ring: TCP/IP, 5MB, NetBIOS and Net Beui, IPX SPX, Fiber Distributed Data Interchange (FDDI) and Asynchronous Transfer Mode (ATM).  

                

LAN topologies           

Various topologies are possible for the broadcast LANs such as bus topology or ring topology.        

     

                  

Bus topology                       

 

• Bus topology is shown in above Fig. In this topology at an instant only one computer acts as master and it is allowed transmit (broadcast). The others are supposed to listen, f
• If two or more machines want to transmit simultaneous then an arbitration mechanism has to be used for resolving the conflict.                             
• It is possible to have a centralized or distributed type arbitration mechanism.
• The most popular example of bus topology is Ethernet (IEEJJ 802.3). It has a decentralized control and it operates at 10 or 100 Mbps.