The I/O interface supports a method by which data is transferred between internal storage and external I/O devices.

All the peripherals connected to a computer require special communication connections for interfacing them with
the CPU.

I/O Bus and Interface Modules

The I/O bus is the route used for peripheral devices to interact with the computer processor. A typical connection of
the I/O bus to I/O devices is shown in the figure.

The I/O bus includes data lines, address lines, and control lines. In any general-purpose computer, the magnetic
disk, printer, and keyboard, and display terminal are commonly employed. Each peripheral unit has an interface unit
associated with it. Each interface decodes the control and address received from the I/O bus.
It can describe the address and control received from the peripheral and supports signals for the peripheral
controller. It also conducts the transfer of information between peripheral and processor and also integrates the data
flow.
The I/O bus is linked to all peripheral interfaces from the processor. The processor locates a device address on the
address line to interact with a specific device. Each interface contains an address decoder attached to the I/O bus
that monitors the address lines.
When the address is recognized by the interface, it activates the direction between the bus lines and the device that
it controls. The interface disables the peripherals whose address does not equivalent to the address in the bus.
An interface receives any of the following four commands −

 Control − A command control is given to activate the peripheral and to inform its next task. This control
command depends on the peripheral, and each peripheral receives its sequence of control commands,
depending on its mode of operation.
 Status − A status command can test multiple test conditions in the interface and the peripheral.
 Data Output − A data output command creates the interface counter to the command by sending data from
the bus to one of its registers.
 Data Input − The data input command is opposite to the data output command. In data input, the interface
gets an element of data from the peripheral and places it in its buffer register.
PCI:
It could be a standard information transport that was common in computers from 1993 to 2007 or so. It was for a
long time the standard transport for extension cards in computers, like sound cards, network cards, etc. It was a
parallel transport, that, in its most common shape, had a clock speed of 66 MHz, and can either be 32 or 64 bits
wide. It has since been replaced by PCI Express, which could be a serial transport as contradicted to PCI. A PCI
port, or, more precisely, PCI opening, is essentially the connector that’s utilized to put through the card to the
transport. When purge, it basically sits there and does nothing.
Types of PCI:
These are various types of PCI:

 PCI 32 bits have a transport speed of 33 MHz and work at 132 MBps.
 PCI 64 bits have a transport speed of 33 MHz and work at 264 MBps.
 PCI 32 bits have a transport speed of 66 MHz and work at 512 MBps.
 PCI 64 bits have a transport speed of 66 MHz and work at 1 GBps.
Function of PCI:
PCI slots are utilized to install sound cards, Ethernet and remote cards and presently strong state drives utilizing
NVMe innovation to supply SSD drive speeds that are numerous times speedier than SATA SSD speeds. PCI
openings too permit discrete design cards to be included to a computer as well.
PCI openings (and their variations) permit you to include expansion cards to a motherboard. The extension cards
increment the machines capabilities past what the motherboard may create alone, such as: upgraded illustrations,
extended sound, expanded USB and difficult drive controller, and extra arrange interface options, to title a couple
of.
Advantages of PCI :
 You’ll interface a greatest of five components to the PCI and you’ll be able moreover supplant each of them
by settled gadgets on the motherboard.
 You have different PCI buses on the same computer.
 The PCI transport will improve the speed of the exchanges from 33MHz to 133 MHz with a transfer rate of 1
gigabyte per second.
 The PCI can handle gadgets employing a greatest of 5 volts and the pins utilized can exchange more than one
flag through one stick.
Disadvantages of PCI :
 PCI Graphics Card cannot get to system memory.
 PCI does not support pipeline.
SCSI Bus Interface
The small computer system interface operates locally as an input and output (I/O) bus that uses a common command
set to transfer controls and data to all devices. The main purpose of this interface, called the SCSI bus, is to provide
host computer systems with connections to a variety of peripheral devices, including disk subsystems, tape
subsystems, printers, scanners, optical devices, communication devices, and libraries.

The SCSI bus design for the library provides a peer-to-peer, I/O interface that supports up to 16 devices and
accommodates multiple hosts.

Peer-to-peer interface communication can be from:

 Host to host
 Host to peripheral device
 Peripheral device to peripheral device

SCSI terms defining communication between devices on the SCSI bus include:

 Initiator is the device that requests an operation.

 Target is the device that performs the operation requested.

Some targets are control units that can access one or more physical or virtual peripheral devices addressable through
the control unit. These peripheral devices are called logical units and are assigned specific addresses or logical unit
numbers (LUNs).

The library supports SCSI-3 commands.

The library and the tape drives have separate connections for attachment to the SCSI bus. Daisy-chain cables are
available to interconnect devices on the SCSI bus but keep the total cable length to a minimum. The following figure
shows an example of a library and four tape drives that are daisy-chained to two initiators (or hosts). It is
recommended that the drives be connected to a separate SCSI bus from the library.
Figure 1-1 Example of a Library Configuration on the SCSI Bus

Benefits
A small computer system interface also provides these benefits:

 Low overhead
 High transfer rates
 A high-performance buffered interface
 Conformance to industry standards
 Plug compatibility for easy integration
  Error recovery, parity, and sequence checking provides high reliability
 Provisions in the command set for vendor-unique fields
 Standard or common command sets with an intelligent interface that provides device independence

Implementation
Implementation of the SCSI bus for the library supports:

 8-bit wide transfers, asynchronous; 16-bit wide selection

 Disconnect and reselect
 Multiple initiator
 Hard resets
 Single-ended LVD
 SCSI-3, 68-pin P-cable

Implementation for the library does not support:

 Soft resets
 Command queuing
 Command linking
 Asynchronous event notification
 Extended contingent allegiance

What is USB?
USB represents Universal Serial Bus. It is an industry-standard developed in the mid-1990s that represents the
cables, connectors, and communication protocols used in a bus for connection, communication, and power supply
among computer and electronic devices.
USB operates at 12 Mbps with particular consideration for low-cost peripherals. It provides up to 127 devices with
both synchronous and asynchronous data transfers. A USB cable has a rectangular “TYPE A” plug at the computer
end and a square “TYPE B” plug at the peripheral end.
A Universal Serial Bus (USB) is a common interface that allows the connection between devices and host
controllers including a personal computer (PC). It connects peripheral devices including digital cameras, mice,
keyboards, printers, scanners, media devices, external hard drives, and flash drives.
Hot swapping is one of the major advantages of USB, which enables a device to be eliminated from the system
without the requirement of rebooting the system. Because previous ports required that a PC be reboot while
inserting or deleting a new device with them. By rebooting the system, the device will be reconfigured and can
avoid electrostatic discharge.

Advantages of USB
There are various advantages of USB which are as follows −
Single Interface for multiple devices− The versatile feature of USB eliminates the complexity of several
connector types and hardware needed for each peripheral.
Compact Size − USB sockets are small in dimensions as compared to RS232 or parallel ports. There is no external
power required. The USB interface was established from the first day to double duty as a DC power supply. Several
host devices through its USB port can supply 5V DC delivering 500mA (USB 1.0 and 2.0) to 900 mA (USB 3.0) to
the peripheral.
Speed − USB offers multiple speed modes which create it more effective and swift compared to RS232 and parallel
ports. It provides speed ranges from 1.5Mbit/s to 5Gbit/s. With the introduction of USB 3.1 in 2013, the speed has
been raised to 10Gbit/s.
Reliability − The USB protocol can hook errors during data transfer and inform the transmitter to retransmit the
information. The generic USB driver and unique driver software provide error-free data communication.

Disadvantages of USB
There are various disadvantages of USB which are as follows −
Peer to Peer Communication − According to the USB standard, the connection takes place between the host and
the peripheral. Two hosts cannot connect directly with each other. The same is the case for a peripheral. Similarly,
interfaces like FireWire provides peripheral to peripheral communication. For overcoming this drawback, the USB
introduced the term OTG (On the Go). The OTG device usually functions as a peripheral, but it can also function as
a host with some small capability when needed.
Distance − As per the USB standards, the connecting cable can be seeing 5 meters, further which, USB hubs
require to be used for enlarging the connectivity.
Broadcasting − Universal Serial Bus does not support the broadcasting nature, only single messages can be
connected between the host and peripheral.

List of USB devices

In modern times, to connect with the computer, there are many different USB devices. Some common are as
follows:

o Keyboard
o Smartphone
o Tablet
o Webcams
o Keypad
o Microphone
o Mouse
o Joystick
o Jumpdrive aka Thumb drive
o Scanner
o Printer
o External drive
o iPod or other MP3 players
o Digital Camera

Where are the USB ports?

In modern times, all computers contain at least one USB port in different locations. Below, a list is given that
contains USB port locations on the devices that may help you out to find them.

Laptop computer: A laptop computer may contain one to four ports on the left or right side, and some laptops have
on the behind of the laptop computer.

Desktop computer: Usually, a desktop computer has 2 to 4 USB ports in the front and 2 to 8 ports on the backside.

Tablet computer: On the tablet, a USB connection is situated in the charging port and is sometimes USB-C and
usually micro USB.

Smartphone: In the form of micro USB or USB-C, a USB port is used for both data transfer and charging, similar
to tablets on smartphones.

USB connector types

There are different shapes and sizes available for the USB connector. Also, there are numerous versions of USB
connectors, such as Mini USB, Micro USB, etc.

1. Mini -USB: Mini USB is used with digital cameras and computer peripherals and divided into A-type, B-
type and AB-type. It is also known as mini-B and is the most common type of interface. On the latest
devices, Micro-USB and USB-C cables have largely replaced the mini-USB. It transfers data and power
between two devices as it is made of coaxial cable. Also, it is applied to MP3 players, digital cameras, and
mobile hard drives. In a mini USB cable, one-end is a much smaller quadrilateral hub, and the other end is
a standard flat-head USB hub. Thus, it is easily plugged into mobile devices. The mini USB can also be
used to transfer data between computers with at least one USB port but is mainly used for charging devices.
It includes two advantages: Waterproofness and Portability.
2. Micro-USB: It is a reduced version of the USB (Universal Serial Bus). It was announced in 2007 and
designed to replace mini-USB and developed for connecting compact and mobile devices such as digital
cameras, smartphones, GPS devices, Mp3 players and photo printers.
Micro A, micro B and micro USB 3 are the three varieties of Micro-USB. The type Micro-A and Micro-B
have a connector size of 6.85 x 1.8 mm, although the Micro-A connector has a greater maximum overmild
size. USB 3 micro is more similar to micro B, but it has better speed as compared to micro B because it
includes an additional collection of pins on the side for twice the wires. Micro versions are hot-swappable,
and plug-and-play like standard USB and micro-USB is still widely used with electronic devices.
 3. USB Type-C: On most modern newer Android smartphones and other USB-connected devices, a USB
Type-C cable is a relatively new type of connector. It is used for delivering data and power to computing
devices. As compared to other forms of USB connections, USB-C cables are reversible; they can be
plugged either way in the devices, whether they are upside down.

USB transfer speeds

o USB 1.0 is capable of supporting up to 127 peripheral devices and also able to support data transfer rates of
12 Mbps as it is an external bus standard.
o In 2001, USB 2.0 was developed by Phillips, Lucent, Microsoft, Hewlett Packard, Intel, NEC, and Compaq
that is also known as hi-speed USB. It has the ability to support a transfer rate of 60 megabytes per second
or up to 480 Mbps (megabits per second).
o In November 2009, USB 3.0 was available for the first time by Buffalo Technology, which is also called as
SuperSpeed USB. But until January 2010, the first certified devices were not available. The performance
and increased speed in USB 3.0, also helped to improve upon the USB 2.0 technology, power management
and increased bandwidth capability.
It includes a feature to offer two unidirectional data paths to send and receive data in one go. It can support
transfer rates of 640 megabytes per second, or up to 5.0 gigabits per second (Gbps). Its name was changed
to USB 3.1 Gen1 for making purposes after releasing the USB 3.1. The first certified devices were
designed with motherboards of Gigabyte and ASUS Technology. In April 2011, Dell started to introduce
USB 3.0 ports with their Dell XPS and Inspiron series of computers.
o USB 3.1 is the latest version of the USB protocol that was made available till 31 July 2013, which is also
known as SuperSpeed It can support transfer rates of up to 10 Gbps. Nowadays, USB 3.0 and 3.1 revisions
are used by various devices to improve speed and performance.

Serial and Parallel Transmission

The process of sending data between two or more digital devices is known as data transmission. Data is transmitted
between digital devices using one of the two methods − serial transmission or parallel transmission.
In serial transmission, data bits are sent one after the other across a single channel. Parallel data transmission
distributes numerous data bits through various channels at the same time.

What is Serial Transmission?

A serial transmission transfers data one bit at a time, consecutively, via a communication channel or computer bus
in telecommunication and data transmission. On the other hand, parallel communication delivers multiple bits as a
single unit through a network with many similar channels.
 8-bits are conveyed at a time in serial transmission, with a start bit and a stop bit.
 All long-distance communication and most computer networks employ serial communication.
 Serial computer buses are becoming more common, even across shorter distances, since newer serial
technologies' greater signal integrity and transmission speeds have begun to outperform the parallel bus's
simplicity advantage.
 The majority of communication systems use serial mode. Serial networks may be extended over vast
distances for far less money since fewer physical wires are required.
What is Parallel Transmission?
Parallel communication is a means of transmitting multiple binary digits (bits) simultaneously in data transmission.
It differs from serial communication, which sends only one bit at a time; this distinction is one method to classify a
communication channel.
 A parallel interface comprises parallel wires that individually contain data and other cables that allow the
transmitter and receiver to communicate. Therefore, the wires for a similar transmission system are put in a
single physical thread to simplify installation and troubleshooting.
 A large amount of data must be delivered across connection lines at high speeds that match the underlying
hardware.
 The data stream must be transmitted through "n" communication lines, which necessitates using many wires.
This is an expensive mode of transportation; hence it is usually limited to shorter distances.
Difference between Synchronous and Asynchronous Transmission
Before starting the topic difference between synchronous and asynchronous transmission, you must know about
the transmission. The action of transferring data or anything from one place to other is referred to as transmission. It
is a method of sharing data between two devices linked by a network, also known as communication mode.
Synchronous and asynchronous transmissions are the two main types of transmission used in computer networking.

In both synchronous and asynchronous transmission, data is sent between the transmitter and the receiver based on a
clock pulse utilized for synchronization. These serial data transmission techniques are both known as synchronous
transmission.

In this article, you will learn about the difference between Synchronous and Asynchronous transmission. But
before discussing the differences, you must know about Synchronous and Asynchronous transmission with their
advantages and disadvantages.

What is Synchronous Transmission?

Synchronous transmission is an effective and dependable method of sending huge amounts of data. The data travels
in a full-duplex method in the type of frames or blocks in Synchronous Transmission. The transmitter and receiver
must be synced so that the sender knows where to start the new byte. As a result, every data block is marked with
synchronization characters, and the receiving device obtains the data until a certain ending character is found. lay
Video

It also allows connected devices to interact in real time. Synchronous transmission can be seen in chat rooms, video
conferencing, telephonic talks, and face-to-face interactions. It utilizes the broad-band and voice band channels
because they enable quicker speeds of up to 1200 bps and meet the objective of high data transfer speed.

Advantages and Disadvantages of Synchronous Transmission

There are various advantages and disadvantages of synchronous transmission. Some advantages and disadvantages
of synchronous transmission are as follows:

Advantages
 1. It aids the user in transferring a huge amount of data.
2. Every byte is sent without a pause before the next.
3. It also helps to reduce timing errors.
4. It allows connected devices to communicate in real-time.

Disadvantages

1. The sender and receiver must operate at the same clock frequency simultaneously.
2. The accuracy of the received data is determined by the receiver's capacity to count the received bits
precisely.

What is Asynchronous Transmission?

Asynchronous transmission is also referred to as start and stop transmission. It sends data from the transmitter to
the receiver using the flow control approach and synchronizes data between the source and destination without
utilizing a clock.

This transmission technique sends 8 bits or one letter at a time. In this system, each character transmits the start bit
before the transmission process begins, and it also transmits the stop bit when the character is sent. The total number
of bits is 10, including the character, start, and stop bits.

It employs character-based synchronization for the receiving terminal to synchronize with receiving data on a
character. It is easy, quick, and inexpensive and doesn't need two-way communication. Asynchronous transmission
is demonstrated via letters, televisions, emails, forums, and radios.

Asynchronous transmission makes use of voice-band channels that are narrow and operate at a slower speed. In this
case, the transmitting device operates manually or intermittently.

Advantages and Disadvantages of Asynchronous Transmission

There are various advantages and disadvantages of Asynchronous transmission. Some advantages and disadvantages
of Asynchronous transmission are as follows:

Advantages

1. It doesn't require synchronizing the receiver and transmitter.

2. It is a very flexible technique of data transmission.
3. This kind of transmission is simple to implement.
4. It allows users to send signals from sources with varying bit rates.
 5. When the data byte transmission is complete, the data transmission may be resumed.

Disadvantages

1. The timing errors may occur because synchronization is difficult to determine.

2. These bits could be mistakenly recognized due to the noise on the channel.
3. The start and stop bits are extra bits that must be utilized in asynchronous transmission.
4. It transmits information at a slower rate.

What is DMA?
DMA represents Direct Memory Access. It is a hardware-controlled data transfer technique. An external device is
used to control data transfer. The external device generates address and control signals that are required to control
data transfer. External devices also allow peripheral devices to directly access memory. The external device which
controls the data transfer is called the DMA controller.

DMA Idle Cycle

When the system is turned on, the switches are in position A. The processor starts implementing the program until it
requires to read a block of information from the disk. The disk processor transfers a sequence of commands to the
disk controller to search and read the desired block of information from the disk.
When the disk controller is ready to transmit the information from the disk, it transfers a DMA request (DRQ)
signal to the DMA controller. Thus the DMA controller sends a HOLD signal to the processor HOLD input.
The processor reply to this signal by suspending the buses and transferring an HLDA acknowledgment signal.
When the DMA controller gets the HLDA signal, it transfers a control signal to modify the switch position from A
to B.

DMA Active Cycle

When the DMA controller receives control of the buses, it transfers the memory address where the first byte of
information from the disk is to be written. It also transfers a DMA to acknowledge (DACK) signal to the disk
controller device signaling it to get ready to transfer the output byte.

Cycle Stealing Mode

In this data transfer mode, the device can make only one transfer (byte or word). After each transfer, DMAC gives
the control of all buses to the processor. This is a single transfer mode with the process as follows −

 I/O device asserts DRQ line when it is ready to transfer data.

 The DMAC asserts the HLDA line to request the use of the buses from the processor.
 The processor asserts HLDA, granting them control of buses to the DMAC.
 The DMAC asserts DACK to the requesting I/O device and executes the DMA bus cycle, resulting in data
transfer.
 I/O device deasserts its DRQ after data transfer of one byte or word.
 DMA deasserts DACK line.
 The word/byte transfer count is decremented and the memory address is incremented.
 The HOLD line is deasserted to give control of all buses back to the processor.
  HOLD signal is reasserted to request the use of buses when the I/O device is ready to transfer another byte or
word. The same process is then repeated until the last transfer.
 When the transfer count is exhausted, the terminal count is generated to indicate the end of the transfer.
Input/Output Processor
An input-output processor (IOP) is a processor with direct memory access capability. In this, the computer system is
divided into a memory unit and number of processors.

Each IOP controls and manage the input-output tasks. The IOP is similar to CPU except that it handles only the
details of I/O processing. The IOP can fetch and execute its own instructions. These IOP instructions are designed to
manage I/O transfers only.

Block Diagram Of I/O Processor

Below is a block diagram of a computer along with various I/O Processors. The memory unit occupies the central
position and can communicate with each processor.

The CPU processes the data required for solving the computational tasks. The IOP provides a path for transfer of
data between peripherals and memory. The CPU assigns the task of initiating the I/O program.

The IOP operates independent from CPU and transfer data between peripherals and memory.

The communication between the IOP and the devices is similar to the program control method of transfer. And the
communication with the memory is similar to the direct memory access method.

In large scale computers, each processor is independent of other processors and any processor can initiate the
operation.

The CPU can act as master and the IOP act as slave processor. The CPU assigns the task of initiating operations but
it is the IOP, who executes the instructions, and not the CPU. CPU instructions provide operations to start an I/O
transfer. The IOP asks for CPU through interrupt.

Instructions that are read from memory by an IOP are also called commands to distinguish them from instructions
that are read by CPU. Commands are prepared by programmers and are stored in memory. Command words make
the program for IOP. CPU informs the IOP where to find the commands in memory.