Bus Standards and Networking

Dept of IT | III YEAR | V SEMESTER IT E51 | COMPUTER HARDWARE AND TROUBLESHOOTING | UNIT 2
1 |Prepared By : Mr. PRABU.U/AP |Dept. of Computer Science and Engineering | SKCET |
UNIT II
Bus Standards and Networking: ISA – PCI – SCSI – IDE – USB – comparative study and
characteristics – Network Interface Cards – Cables and connectors - MODEM –
AT command set.
BUS STANDARDS
 Buses carry signal between the components.
 A bus is a common pathway in which data can travel within a computer.
 This pathway used for communication to travel between components inside or
outside of a computer
 A bus connects all the internal computer components to the CPU and Main
memory.
 Every bus has a clock speed measured in MHz.
 A fast bus allows data to be transferred faster, which makes applications run
faster.
 On PCs, the old ISA bus is being replaced by faster buses such as PCI.
Different types of Bus
1. ISA - Industry Standard Architecture
2. EISA - Extended ISA
3. PCI - Peripheral Component Interconnect
4. USB - Universal Serial Bus
2.1 ISA (INDUSTRY STANDARD ARCHITECTURE)
 History
 Originally introduced in the IBM PC (1981) as an 8 bit expansion slot
 Runs at 8.3 MHz with data rate of 7.9 Mbytes/s
 16-bit version introduced with the IBM PC/AT
 Runs at 15.9 MHz with data rate of 15.9 Mbytes/s (?)
 Sometimes just called the “AT bus”
 Today, all ISA slots are 16 bit
 Is used with sound cards, disk drives or most network and video cards.
Figure 2.1: 8-bit and 16-bit ISA Bus

1. ISA Bus Architecture
 The BUS is divided into two sides.
 The first side pins are named A1 to A31 and it is the components side.
 It consists of the address and data buses.
 The second side pins are named B1 to B31 and it is the solder side.
 This side contents the power pins and the signals related to interrupts and DMA
transfers.
Figure 2.2 ISA Bus Architecture
2. EISA Bus
 The Extended Industry Standard Architecture is a bus standard for IBM
compatible computers.
 EISA buses are 32 bits wide and support multiprocessing.
3. Uses
 Just about any peripheral (sound cards, disk drives, etc.)
 PnP ISA
 In 1993, Intel and Microsoft introduced “PnP ISA”, for plug-and-play ISA
 Allows the operating system to configure expansion boards automatically
 Form factor
 Large connector in two segments
 Smaller segment is the 8-bit interface (36 signals)
 Larger segment is for the 16-bit expansion (62 signals)
 8-bit cards only use the smaller segment
4. Advancements
 EISA
 Extended ISA
 Design by nine IBM competitors (AST, Compaq, Epson, HP, NEC, Olivetti,
Tandy, WYSE, Zenith)
 Intended to compete with IBM’s MCA
 EISA is hardware compatible with ISA
 MCA
 Micro Channel Architecture
 Introduced by IBM in 1987 as a replacement for the AT/ISA bus
 EISA and MCA have not been successful!

2.2 PCI (PERIPHERAL COMPONENT INTERCONNECT)
 A computer bus is used to transfer data from one location or device on the
motherboard to the central processing unit where all calculations take place.
 Two different parts of a Bus
 Address bus-transfers information about where the data should go
 Data bus-transfers the actual data
1. History
 PCI(Peripheral Component Interconnect) bus is based on ISA (Industry
Standard Architecture) Bus and VL (VESA Local) Bus.
 Introduced by Intel in 1992
 Revised twice into version 2.1 which is the 64-bit standard that it is today.
 PCI provides direct access to system memory for the devices that are
connected to the bus which is then connected through a bridge that connects
to the front side bus.
 This configuration allowed for higher performance without slowing down the
processor
 Most modern PCs include a PCI bus
 PCI is an interconnection system between a microprocessor and attached
devices in which expansion slots are spaced closely for high speed operation.
 PCI transmits 32 bits at a time in a 124-pin connection (the extra pins are for
power supply and grounding) and 64 bits in a 188-pin connection in an
expanded implementation.
 The advantage of a bus is that it makes parts more interchangeable.
 The 32-bit PCI bus has a maximum speed of 33 MHz, which allows a
maximum of 133 MB of data to pass through the bus per second
 In servers you have a special 64-bit 66MHz PCI slots that can accept special
high-speed cards, from 512 MB to 1 GB of data per second.
2. PCI System Bus Performance
 What makes the PCI bus one of the fastest I/O bus used today?
 Three features make this possible:
 Burst Mode: allows multiple sets of data to be sent
 Full Bus Mastering: the ability of devices on the PCI bus to perform
transfers directly
 High Bandwidth Options: allows for increased speed of the PCI
 “Plug and Play” utilized the PCI bus concept.
3. Plug and Play
 Requirements for full implementation:
 Plug and Play BIOS
 Extended System Configuration Data (ESCD)
 Plug and Play operating system
 Tasks it automates:
 Interrupt Requests (IRQ)
 Direct Memory Access (DMA)
 Memory Addresses
 Input/Output (I/O) Configuration

4. How PCI Works: Installing a New Device
Once a new device has been inserted into a PCI slot on the motherboard
1. Operating System Basic Input/Output System (BIOS) initiates Plug and Play (PnP)
BIOS.
2. PnP BIOS scans the PCI bus for any new hardware connected to the bus. If new
hardware is found, it will ask for identification.
3. The device will respond with its identification and send its device ID to the BIOS
through the bus.
4. PnP checks the Extended System Configuration Data (ESCD) to make sure the
configuration data already exists for the card. (If the card is new, then there will
be no data for it.)
5. PnP will assign an Interrupt Request Line, Direct Memory Access, memory address
and Input/Output settings to the card, then stores the information in the ESCD.
6. When the Windows software loads, it will check the PCI bus and the ESCD to see if
there is new hardware. Windows will alert the user that new hardware has been
found if there is new hardware installed and will also identify the hardware.
7. Windows will determine the device and attempt to install its driver. The operating
system may ask the user to insert a disk containing the driver or direct it to where
the driver is located. In the event that Windows is unable to determine what the
device is, it will provide a dialog window so the user can identify the hardware and
load its driver.
2.3 SCSI (SMALL COMPUTER SYSTEM INTERFACE)
 SCSI is a computer bus interface standard for attaching different kinds of
peripherals to interface with the host computer by means of a SCSI-bus cable.
 SCSI is used to interface to a number of different device types, such as Hard Drives,
Tape Backups, CD-ROM drives, Printers, Scanners, etc.
 Developed by the American National Standards Institute (ANSI)
 SCSI is Pronounced "scuzzy"
 Supported by all major operating systems.
1. Version:
 The first version (SCSI-1), adopted by ANSI in 1986, was an 8-bit version with a
5 MBps transfer speed that allowed up to eight devices to be connected with a
maximum cable length of six meters.
 SCSI-2: 8-bit bus, six meter cable length, 5-10 MBps; connects 8 or 16 devices.
50-pin connector.
 SCSI-3: The latest version, 16-bit Ultra-640 (Fast-320) SCSI, was introduced in
2003 and has a 640 MBps transfer speed, connecting up to 16 devices with a 12
meter cable length.
The SCSI Clock can be 5 , 10, 20, or 40 MHz
SCSI bus clock 8-bit (50 pin) 16-bit (68 pin)
5 MHz (SCSI 1) 2-5 MBytes/s N/A
10 MHz (Fast SCSI) 10 MByte/s 20 MByte/s
20 MHz (Fast-20, Ultra SCSI) 20 MByte/s 40 MByte/s
40 MHz (Fast-40, Ultra-2) 40 MByte/s 80 MByte/s

2. SCSI 10 Control Signals:
1. RST—the reset signal activates to bring all devices with SCSI controller to start
up state.
2. SEL—when bus is used for selecting an SCSI target the SCSI controller examines
SCSI ID and always select the highest ID device
3. BSY (for Busy)—when SCSI bus is not available to other device (controller)
4. ATN—activates when a device draws attention before sending a message to a
target
5. REQ— requests a data transfer by a device with the SCSI controller
6. ACK—acknowledgement from the SCSI controller that data transfer completes
7. MSG—when a message or information is being transferred through the SCSI bus
8. C/D—1 or 0 when a control (command or status) or data word is being
transferred, respectively
9. I/O—1 or 0 when a data is input to SCSI controller or output from the controller,
respectively
10. Parity — check bit, checking whether data has been lost or written
3. Sequences of SCSI Signals
 Sequence 0: Arbitration when Busy is not active
 Sequence 1: Selection using SEL—the selection of signals is done
 Sequence 2: Command or status or data transfer
 Sequence 3: Reselection
4. SCSI controller arbitration policy
 Access to highest priority device first
 Advantage that it is easy to decide which device gets access to the bus
5. Disadvantage
 Starvation problem
 Starvation when a device with a high SCSI ID can prevent a device with a lower
ID from ever getting a chance to use the bus by making repeated requests for the
bus
2.4 IDE (INTEGRATED DRIVE ELECTRONICS)
 IDE also stands for integrated development environment.
 IDE is a standard electronic interface used between a computer motherboard's
data paths or bus and the computer's disk storage devices.
 The IDE interface is based on the IBM PC Industry Standard Architecture (ISA)
16-bit bus standard.
 IDE was adopted as a standard by American National Standards Institute (ANSI)
in November, 1990. The ANSI name for IDE is Advanced Technology Attachment
(ATA).
 The drive can connect directly to the motherboard or controller.
 The original IDE had a 16-bit interface that connected two devices to a single-
ribbon cable.
 IDE’s development increased data transfer rate (DTR) speed and reduced
storage device and controller issues.
 The primary interface used to connect a hard disk drive to a PC
 transmit and receive data to and from the drive

 IDE allows one master and one slave for each IDE connector on the motherboard.
 IDE costs less than SCSI.
Motherboard
Primary
Slave
Secondary
Master
IDE Drives
Secondary
Slave
Primary
Master
Figure 2.3: IDE Drives
1. IDE Cable Structure:
Figure 2.4: IDE Cable Structure
 Programmed Input/Output (PIO) a method of transferring data between two
devices
 ATA uses PIO and defines the speed of the data transfer in terms of the PIO
mode.
PIO Mode Data transfer
(MBps)
Standard
0 3.3 ATA
1 5.2 ATA
2 8.3 ATA
3 11.1 ATA - 2
4 16.6 ATA - 2
2. Version:
 There are several versions of ATA:
 ATA: supports one or two hard drives, a 16-bit interface and PIO modes 0, 1& 2.
 ATA-2: Supports faster PIO modes (3 and 4) and
 ATA-3: Minor revision to ATA-2.
 Ultra-ATA: running at 33 MBps.
 ATA/66: throughput to 66 MBps.
 ATA/100: Data transfer rates to 100 MBps.

3. PIN details
PIN NO PIN DETAILS
1 Reset
2,19,22,24,26,30,40 Ground
3 TO 18 DATA 1 TO 15
23 I/O write
25 I/O read
28 Cable select
33 Addr 1
35 Addr 0
36 Addr 2
2.5 USB (UNIVERSAL SERIAL BUS)
 Universal Serial Bus, USB is a standard that was introduced in 1995 by Intel,
Compaq, Microsoft and other computer companies.
 Universal Serial Bus (USB) connects between a computer and peripheral devices
including a mobile phone and a desktop computer.
 The connection is made by a cable that has a connector at either end.
 Data transferring application
 Two-way communication
 USB cables also carry an electric charge that can be used to power peripherals
(such as USB mice or keyboards), and many mobile phones can be charged
through their USB port.
 USB 1.x is an external bus standard that supports data transfer rates of 12 Mbps
 USB is capable of supporting up to 127 peripheral devices.
 USB 2.0, also known as hi-speed USB, was developed by Compaq, Intel,
Microsoft, Philips was introduced in 2001.
 It is capable of supporting a transfer rate of up to 480 Mbps.
 In 2012, USB 3.0 also known as Super Speed USB is the latest version of the USB
protocol.
 Most new computers feature USB 3.0 ports built-in, offering data transfer speeds
of up to five gigabits per second.
 USB 3.0 revision for improved performance and speed, including USB thumb
drives, digital cameras, external hard drives, MP3 players, and other devices.
1. USB devices
Today, there are millions of different USB devices that can be connected to your
computer.
 Camera, External drive, iPod or other MP3 player, Keyboard, Microphone, Mouse,
Printer, Scanner, Smartphone, Tablet, Webcams

2. USB connector variations
 USB connectors come in many shapes and sizes as there are many different
devices that utilize them.
 Every version of USB connector including standard, Mini, and Micro.
3. USB Device Basics – The Protocol
 USB protocol defines a set of standards that any device can follow
 No need to write a driver for a device that is in a predefined class and
follows that standard,
 Predefined classes: storage devices, keyboards, mice, joysticks, network
devices, and modems
 No defined standard for video devices and USB-to-serial devices
 Providing plug and play capabilities
4. Endpoints
 The most basic form of USB communication is through an endpoint
 Carries data in one direction
 From the host to device (OUT endpoint)
 From the device to the host (IN endpoint)
5. Benefits for Users
 Ease of Use
Ease of use was a major design goal for USB, and the result is an interface that’s a
pleasure to use for many reasons
 One interface for many devices
USB is versatile enough to be usable with many kinds of peripherals. Instead of
having a different connector type and supporting hardware for each peripheral,
one interface serves many.
 Automatic configuration
When a user connects a USB peripheral to a computer, its OS automatically
detects the peripheral and loads the appropriate software driver.
 Hot pluggable
We can connect and disconnect a peripheral whenever you want, whether or not
the system and peripheral are powered, without damaging the PC or peripheral.
The operating system detects when a device is attached and readies it for use.
2.6 COMPARATIVE STUDY AND CHARACTERISTICS
1) ISA (Industry Standard Architecture) bus: ISA bus was created by IBM in 1981.
ISA bus can transfer 8 or 16 bits at one time. ISA 8 bit bus can run at 4.77 MHz and 16
bit at 8.33 MHz.

2) PCI (Peripheral Component Interconnect) bus: PCI bus was created by Intel in
1993. PCI bus can transfer 32 or 64 bits at one time. PCI bus can run at 33 Mhz.
3) IDE (Integrated Drive Electronics) bus: IDE bus is used for connecting disks and
CDROMs to the computer.
4) USB (Universal Serial Bus): It is used for connecting keyboard and mouse, and
other USB devices to the computer. A USB bus has a connector with four wires. Two
wires are used for supplying electrical power to the USB devices. USB 1.0 has a data rate
of 1.5 MB/s and USB 2.0 which is a high speed one has a data rate of 35 MB/s.
5) SCSI (Small Computer System Interface) bus: It is a high performance bus which is
used for fast disks, scanners, and for devices which require high bandwidth. It has a data
rate of 160 MB/s.
Comparative Study
 SCSI is better than IDE
 IDE is cheaper than SCSI
 SCSI is faster than IDE
 Slow IDE devices also slow down other devices
 How to calculate your bus "saturation"
 SCSI is multithreaded, IDE is not
 SCSI needs only single IRQ, IDE needs two
2.7 NIC (NETWORK INTERFACE CARDS)
 A network interface card (NIC) is a computer circuit board or card that is
installed in a computer so that it can be connected to a network.
 It is a full-time connection to a network.
 To exchange information with other computers and the Internet.
 The NIC contains the electronic circuitry required to communicate using a wired
connection (e.g., Ethernet) or a wireless connection (e.g., WiFi).
 You need it to use e-mail, access information on the Internet, share documents
within a corporate network.
1. History of NIC
 In 1973 Robert Metcalfe needed something that was fast, could connect
hundreds of computers.
 To solve this problem, Metcalfe developed Ethernet.
 The original Ethernet sent roughly a paragraph of data over thick coaxial cable
and could handle a distance of one kilometer.
2. Ethernet Connection
 Ethernet is really a standard for computer network technologies that describes
both the hardware and the communication protocols.
 Ethernet was commercially introduced in 1980 and has largely replaced other
wired network technologies.
 Since Ethernet is so widely used, most modern computers have a NIC built into
the motherboard.

 A separate network card is not required, unless some other type of network is
used.
 An Ethernet connection uses a standard interface known as a RJ45 connector. RJ
stands for registered jack. The figure below shows an Ethernet cable with a RJ45
connector.
 Smalls LED lights built into the connection will show that a connection is active
and whether data is being transferred.
 All NICs feature a speed rating such as 11 Mbps, 54 Mbps or 100 Mbps that
suggest the general performance of the unit.
 These cards typically use an Ethernet connection and are available in 10, 100,
and 1000 Base-T configurations. A 100 Base-T card can transfer data at 100
Mbps.
3. Twisted Pair Cable
 Connect to networking devices such as network interface cards and switches
using RJ45 connectors.
 One end must connect to a host, the other to a networking device such as a
switch.
Figure 2.5: Twisted Pair Cable
4. Network Cards Convert Data from Parallel to Serial, and vice versa
 Most computers use parallel data lines internally to send data between the CPU
and the adapter cards. This is called a Bus.
 Most networking media transmit data in a single line, called serial transmission.
 The NIC translates parallel into serial for outgoing messages and serial into
parallel for incoming messages.
 Prior to the invention of NICs, data was sent via serial ports on the computer.
5. How the NIC transfers data
 The app you are using generates the data you would like to send to another
computer.
 Your NIC accepts the data from your motherboard and transfers it to a small
buffer on the card.
 The NIC adds its address (set by the manufacturer) plus the destination address
and the type of data to the buffer.
 Your NIC calculates the checksum, or CRC, for the data in the buffer.
 The data is arranged into a packet and sent over the network.
 If there are no errors, the receiving station acknowledges the received data.

2.8 CABLES AND CONNECTORS
1. VGA (Video Graphics Array)
 In 1980. VGA cable used to connect a computer to a monitor.
 VGA connections can be identified by 15 pins arranged in 3 rows with 5 on
each row.
 Each row corresponds to the 3 different color channels used in display: red,
green, and blue.
2. DVI (Digital Visual Interface)
 DVI became successor to VGA as technology moved away from analog
towards digital.
 DVI connectors come in 3 varieties.
 DVI-A can transmit analog signals, (useful for CRT monitors and LCDs of
lower quality).
 DVI-D can transmit the newer digital signals.
 DVI-I is capable of both analog and digital.
3. HDMI (High Definition Multimedia Interface)
 High-definition broadcasts means high quality
 HDMI sends both video and audio signals together. HDMI connectors come in
4 types:
 Type A is the most popular. It is 19 pins on the male head. Type A is
compatible with single-link DVI-D connections.
 Type B is larger than Type A, coming in at 29 pins on the male head. Type B
is compatible with dual-link DVI-D connections.
 Type C is a 19-pin connector that’s most often used with portable devices,
like camcorders and digital cameras.
 Type D looks similar to a micro-USB cord. It also has 19 pins.
4. USB (Universal Serial Bus)
 Micro USB, mini USB, type B standard USB, and type A standard USB
 keyboards, mice, headsets, flash drives, wireless adapters, etc. — can be
connected to your computer through a USB port.
 USB 1.0/1.1 can transmit data at speeds up to 12 Mbps.
 USB 2.0 can transmit data at speeds up to 480 Mbps.
 USB 3.0 can transmit data at speeds up to 4.8 Gbps.
5. IDE (Integrated Drive Electronics)
 IDE cables were used to connect storage devices to a motherboard.
 Cable that looks like a ribbon with more than 2 plugs.
 IDE cable have 40 pins
6. SATA (Serial Advanced Technology Attachment)
 Newer hard drives will likely use SATA ports over IDE ports.
 SATA provides higher data transfer speeds.
 SATA cable can be identified by two connectors, each having 7 pins

7. eSATA (External Serial Advanced Technology Attachment)
 extension of SATA cable
 allows connections to devices like external hard drives and optical drives.
 offers speeds much faster
8. FireWire
 FireWire is similar to that of USB
 high speed data transfer
 High bandwidth devices, like printers and scanners, will benefit from
FireWire.
 FireWire cables come in two forms:
 1394a (which has a transfer speed of 400 Mbps) and
 1394b (which has a transfer speed of 800 Mbps).
9. Ethernet
 Ethernet cables are used to set up local area networks. three varieties
 Cat 5 cables are the most basic type and provide speeds of either 10 Mbps or
100 Mbps.
 Cat 5e, which means Cat 5 Enhanced, allows for faster data transmission. It
caps at 1,000 Mbps.
 Cat 6 is the latest and offers the best performance of the three. It’s capable of
supporting 10 Gbps speeds.
10.Video Connectors: RCA
 The RCA connector is used for video and audio signals. yellow, white, red.
11.Power Cables
 An assembly of two or more electrical conductors usually held together with
an overall sheath. The assembly is used for transmission of electrical power.
2.9 MODEM
 Modem (modulator-demodulator)
 It is a device that modulates signals to encode digital information and
demodulates signals to decode the transmitted information.
 A modem is a device or program that enables a computer to transmit data over,
for example, telephone or cable lines.
 Computer information is stored digitally, whereas information transmitted over
telephone lines is transmitted in the form of analog waves

 One computer's modem converts its digital signals (which cannot be sent
efficiently over phone lines) into analog signals (which can be).
 The other computer's modem reconverts the analog signals (that the computer
cannot understand) into digital signals (that it can).
 Conversion of one type of signals to another is called modulation; their
reconversion to the original type is called demodulation.
 Bits per second (bps) used for measurement
 Modern work at 56 thousand bits per second (Kbps)
 The Modem is a hardware device that enables a computer to send and receive
information over telephone lines by converting the digital data used by your
computer into an analog signal used on phone lines and then converting it back
once received on the other end.
 There is one standard interface for connecting external modems to computers
called RS-232.
 Modems are referred to as an asynchronous device
 Transmits data in an intermittent stream of small packets.
 In asynchronous communication, 1 byte (8 bits) is transferred within 1 packet,
which is equivalent to one character.
 For the computer to receive this information, each packet must contain a Start
and a Stop bit;
1. Types of computer modems
 Internal modem that connects to a PCI slot inside a newer desktop computer or
ISA slot on an older computer. The Internal Modem shown at the beginning of
this document is an example of a PCI modem.
 External modem is located within a box and is hooked up externally to the
computer, usually the Serial Ports or USB port.
 Removable modem that is used with older laptops PCMCIA slot and is removed
when you need the PCMCIA slot for another device, but are not planning on using
the modem.
2. Mode of Transmission
Transmission mode means transferring of data between two devices. the types
are
(i) Data flow
(ii) Physical connection
(iii) Timing
(i) Data Flow
a. Simplex transmission
 Data can be send only through one direction. We cannot send a message back to
the sender.
 Eg: loudspeaker, Television broadcasting, television and remote , keyboard
and monitor.

Figure 2.6: Simplex transmission
b. Half-duplex (HDX) transmission
 We can send data in both direction, but it is done one at a time
 Eg: walkie-talkie
Figure 2.7: Half-duplex transmission
c. Full-duplex (FDX) transmission
 We can send data in both directions as it is bidirectional. Data can be sending in
both directions simultaneously.
 Eg: telephone network. Communication between two person in which both
can talk and listen at the same time.
Figure 2.8: Full-duplex transmission
3. Physical Connection
 Parallel transmission
 Fast
 Simple
 Line cost
 Serial transmission
 Complicated transmitter and receive
 Decomposing and reconstructing
4. Timing
 Asynchronous transmission
 does not need clock signal between the sender and the receiver
 slower data transfer rate
 Simple, inexpensive, slow speed transmission
 Synchronous transmission
 supports high data transfer rate
 needs clock signal between the sender and the receiver
 requires master/slave configuration

2.10 AT COMMAND SET
 AT commands are used to control MODEMs.
 AT is the abbreviation for Attention.
 These commands come from Hayes commands.
 Developed by Dennis Hayes in 1980
 The dial up and wireless MODEMs need AT commands to interact with a
computer.
 Commands used for operations such as dialing, hanging up, and changing the
parameters of the connection.
 Many of the commands that are used to control wired dial-up modems.
 Such as ATD (Dial), ATA (Answer), ATH (Hook control) and ATO (Return to
online data state),
 Also supported by GSM/GPRS modems and mobile phones.
 AT commands with a GSM/GPRS MODEM or mobile phone can be used to access
following information and services:
1. Information and configuration related to mobile device or MODEM and SIM
card.
2. SMS services.
3. MMS services.
4. Fax services.
5. Data and Voice link over mobile network.
 Get basic information about the mobile phone or GSM/GPRS modem.
 For example, name of manufacturer (AT+CGMI), model number (AT+CGMM),
IMEI number (International Mobile Equipment Identity) (AT+CGSN) and
software version (AT+CGMR).
 Note that mobile phone manufacturers usually do not implement all AT
commands
1. Rules
 A command string should start with "AT" or "at", except for the commands "A/"
and "+++".
 Several commands can be given in one command string.
 The commands can be given in upper or lower case.
 A command string should contain less than 40 characters
 1 2 3 4 5 6 7 8 9 * = , ; # + >
 1 or 0 start bits, 7 or 8 data bits, no, odd or even parity and 1 or 2 stop bits.
2. Execution of commands:
 Put the modem into command mode
 The user enters three +++ symbols and the modem responds with "OK".
 The user then enters ATCH and presses return.
 The modem responds back by displaying the current channel "C".
 The user then enters ATCHD to change the channel to D.
 The modem responds with "OK" after the channel is then set to D.

 The user can then verify the channel change by typing ATCH and pressing return.
Figure 2.9: Execution of commands
3. Two Modes
 Data mode in which the modem sends the data to the remote modem.
 A modem in data mode treats everything it receives from the computer as data
and sends it across the phone line.
 Command mode in which data is interpreted as commands to the local modem
 Commands that the local modem should execute.
4. Hayes Command divided into four groups
 Basic Command Set - capital character followed by a digit.
 Extended Command Set - “&” (ampersand) and a capital character followed by
a digit.
 Proprietary Command Set - Usually started by either a backslash (“”), or a
percent sign (“%”)
 Register Commands - Sr=n where r is the number of the register to be changed,
and n is the new value that is assigned.
5. Some commands
 AT - Tells the modem that must begin each line of commands.
 Z - Resets the modem to its default state.
 , (a comma) - Makes the software pause for a second. For example, ",,,," tells the
software to pause four seconds.
 ^M - Sends the terminating Carriage Return character to the modem.
 ; (a semi-colon) - Return to command mode immediately after dialing.
 W - wait for dial tone.
 ! - flash hook. put quickly the modem on/off hook.

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