Pc Hardware Power Supply Cooling and Protection - Copy

8/4/2019 Pc Hardware Power Supply Cooling and Protection - Copy

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Power Supply Cooling And ProtectionPower Supply Functions

Provide PCs lifeblood by supplying electricity to each and every

component

It takes A.C Power from the wall outlet and convert it into much lowerDC voltages needed by the system and thereby supply clean electricalpower to the system

Interacts with motherboard to perform CPU startup,system powerdown,voltage and temperature detectionand other functions

Provide the following supply voltages

+5 V : It is the basic supply voltage used for nearly all the electroniccomponents

+12V :Used by some motors used in PC,communication ports like serial

port etc


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Power Supply Cooling And Protection

-12V : Very few devices use this voltage.Typically used for some of

the serial ports that use +12V line

-5V : This voltage is no longer used by any device and is retained inorder to achieve backward compatibility with the older ISA slots .Thesame voltage was used by some of the DRAM memory chips in theoriginal PC design

+3.3 V:Introduced with ATX design from 1995

Currently most CPUs, DRAM and PCI adapter cards uses 3.3

Volts

Power Supply Working and block diagram Switch mode power supply (SMPS) is used in pcs whereas linear mode

power supply is used in the old mainframe computers


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Block diagram of SMPS and Linear modepower Supply


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Power supply working and block diagram

The first part of switching power supply directly converts AC to DC

The switcher circuit chops up the input voltage to a much higherfrequency than the input AC frequency

This makes PC immune to high voltage variations.

Any load fluctuations are fed back to the input which causes theswitcher circuit to adjust the input power to the transformer so that itcan maintain the same output voltage to the final regulator circuitwhich makes the regulator works with greater efficiency

Switching circuits are vulnerable to voltage surges and hence the PCpower supplies are equipped with additional filter circuitry on the input


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Power Supply Cooling And ProtectionPower Supply Signals

a) Power Good signal

It is denoted as PWR_OK or Pwr_Good and is a logic level on/offsignal

This signal is the power supply output signal to the CPU circuitryis used to start the CPU running

This signal will turn on until all the power supply voltages arecorrect and stabilized

b) Power-on Signal

It is the part of the ATX standard

It carries a low voltage logic level signal from the motherboard tothe power supply telling it when to turn on and when to turn off


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Power Supply Cooling And ProtectionPower Supply Signals (contd)

When the front panel on/off switch is activated through motherboard,the motherboard uses the PS-ON signal to tell the power supply topower up fully

c)+5VSB : The purpose of this supply line is to power stand by circuitrysuch as PS_ON circuit, the wake-on LAN and remote ring-on circuitry,

intrusion detection or any other circuit the designer wishes to haveactive even while in stand by mode

d)+3.3 Sense :

New signal introduced with the ATX power standard

Used to sense the actual voltage of 3.3 volt supply when it reach themotherboard. This allows the power supply to actively adjust the 3.3 Voutput to compensate the line lose between power supply and motherboard


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Power Supply Form Factors

Full sized AT and XT power supplies

Made by IBM

Available with power supply from 135 to 300 watts

Use standard P8/P9 pair of connectors (6pin) for motherboard,two to

four drive connectors with four pins each

Out of the two motherboard connectors ,each connector has 6 keyswith one key larger than the rest

Position of the single large key is different for two connectors andhence cannot swap them accidentally

The pin out arrangement of the AT/XT standard motherboard powerconnectors are as shown below


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Power Supply Cooling And Protection On power supply connectors to the drives and for the large drives two of the

corners are squared off while the other two are chamfered to allow the plug tofit only in one way

For smaller power connectors a small raised bump on the bottom of theconnector fits into the notch in the receptacle

Mini XT Form factor

Use exactly the same power connectors as full sized AT and XT

Uses a remote mounted power switch on the front panel from which powercable carrying 230v AC is connected. This can cause a serious potential shockhazard

ATX Form factor

The major features of ATX standard are as listed below

Front panel power switch carry only a harmless logic level signal

Remote power control : Wake-on-LAN whereby system administrator from anywhere on the network can turn on ,turn off or reboot the system


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Features of ATX form factor (contd)

and wake-on-ring whereby dial-in system can normally be in stand by

mode until a user dials into the systemStand-by mode

Also called sleep or suspend mode

Stand-by mode allows most of the systems to shut down except the

supply line carrying +5vsb for the circuits that need to remainenergized to wake up the system on some event

Optimum cooling : ATX power supply cooling fan is placed directly overor next to the CPU so that optimum airflow occurs over the CPU andhence eliminate the need of a separate cooling fan

+3.3 V power supply : In older mother boards +5V supply voltage isconverted to 3.3 V needed for the CPU and DRAM which generatesheat and the same is solved in ATX standard by directly allowing 3.3 Vsupply


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Features of ATX form factor (contd)

Single polarized motherboard connector : The single motherboard connectorused in ATX design is polarized in such a way that it is practically impossible toinsert it incorrectly

NLX and SFX power supplies

These are the new versions of ATX design

NLX design is intended to be low profile

It uses a custom power supply design that meets the standards

SFX standard applies to smaller implementations of the ATX standard whichincludes micro ATX (limited to 4 adapter cards) , ATX flex(limited to 3 slots )

SFX standard can include older ISA slots

5V stand by mode is optional


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Ventilation and cooling protection

Power supply fan

Power supply fan can move air by using positive pressure ventilation ornegative pressure ventilation

Positive pressure ventilation is used in PC file servers in which the aircomes in through one point entry in which it can be filtered before

reaching the system electronics but have the following disadvantages

a) Periodic maintenance is required for cleaning the filter withoutwhich the airflow may block and will cause the system to overheat

b) Filter reduces airflow

c) Power supply preheats the airflow which occur if the fan insidethe power supply exhaust air over the CPU after it had already receivedheat from the power supply

Typical PC system always use a negative pressure ventilation to move airthrough the system or single point exhaust


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Power Supply Cooling And ProtectionPower supply fan placement

Power supply fan can be placed either in the back panel or in theinterior panel of the power supply closer to the CPU

If it is placed in the back panel , the air flow pulled over the CPUtoward the power supply will not be good

Fan specifications and Temperature monitoring

The ability of the fan to move air through the system is measured inCubic Feet Per Minute (CFM)

The practical way of determining whether the power supply fan orchasis fan is sufficient to keep the system cool is to record the internal

temperatures in several areas inside the case which includes CPU, inletto the power supply, inlet to the chassis ventilation fan if exist ,videocard ,high powered graphic accelerator card etc

External ambient temperature is measured


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Fan specifications and Temperature monitoring

If the difference between intake and exhaust temperatures of the power

supply is very large say 10 degree then the power supply may be underexcessive stress

If the difference between the external temperature and the temperatureinside the case is too great ,a problem had occurred to reduce air flowthrough the case

If the inlet temperature to the fan inside the case is reasonable but the directCPU temperature is too hotCPU heat cannot be able to effectively removed

Processor Cooling

The following are the devices that help to cool the CPU

a) CPU cooling fan :From Pentium and newer processors all chips requiresome kind of fan mounted on the heat sink to allow adequate air flowthrough the cooling fans

b) CPU heat sink :All processors since 486 have needed heat sink

Heat sink is a piece of metal in contact with the chip casing and with radiatorfins exposed to the air flow. Some CPU use a fan mounted on the heat sink


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Power Supply Cooling And ProtectionProcessor Cooling(contd)

c) Chasis or power supply fan

ATX and NLX chasis designs make provisions for cooling most CPUswithout using a CPU fan by positioning the power supplyfan or chasisventilation fan directly next to CPU

d) Peltier Junction cooling elements

These are electrical heat pumps where application of electrical energycauses one side of the Ceramic sandwich to get cold and the other sideto become hot. This type of cooling can reduce the chip temperature

below ambient temperature. Hence Peltier junction cooling systemshould include some kind of thermostatic control so that CPU cannot beover cooled


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Temperature Limit

a)Repeated temperature changes : Repeated changes in temperature

(70 to 120 Fahrenheit) can cause unreliability in electrical connectorslike sockets ,slots , cables etc Older DIP chips can come out of thesocket completely after too much heating and cooling cycles and iscalled chip walking

b)Temperature above 110 fahrenheit :Many products will warm if the

temperature exceeds 110 F. The problematic components includechips, drives, connectors, plastic retaining clips used in SIMM

c) Temperature above 130F: Manufacturers suggest that any internaltemperature above 130 F is not acceptable

d) Direct contact CPU temperature: Technically most CPUs are ratedto perform at temperature as high as 170 F. The temperature of theheat sink have to be at least 10 F lower than the chip rating andcertainly not higher which may cause eratic operation of the CPU


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Power Supply Cooling And ProtectionTemperature Limit(contd)

Hot spots :Air flow focussed in some area may create hot spots especially

near the video card with AGP

Temperature Alarms : Several temperature alarms are available in themarket and it will be a good idea to incorporate these alarms in thesystem when the internal temperature go above 110 F and is usuallyplaced at the inlet of the power supply

Temperature Monitoring:

Temperature monitoring is built into the mother board with sensorsplaced near the CPU and other locations in the motherboard tomeasure internal temperature

Ambient temperature and CFM :A higher ambient externaltemperature will result in high internal temperature and can only becompensated by increasing the air flow inside the machine by fittingCPU fans/powersupply fans etc with high CFM ratings


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Backup power systems

UPS Types

a)SPS (Stand by power source )

It is a back up power source and will remain in stand by mode if theutility power stayed within an acceptable range.If the utility power isvaried outside that range the battery circuit will be switched on and the

PC system will be disconnected from the utility powerand connected tothe battery powered inverter Most of today low cost UPS are SPS

b) True Online UPS

Its cost is about two to three times the price of an SPS. These UPS

units have battery online all the time which requires moremaintenance. It provides better isolation from power surges andspikes. It usually have a pure sine wave output


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Power Supply Cooling And ProtectionUPS Types (contd)

c) Ferro resonent UPS : Older SPS designs with zero switch over time.Ithas an intermediate transformer that bridge over the time betweenswitching off of utility power and switching on of battery power

d) Hybrid online UPS :The utility power is constantly rectified to DC andthe switch over is made between rectified utility DC power and batteryDC power with both feeding the inverter which is always supplying230V of output.It is an ideal combination because the battery circuit isnot used unless needed and the utility power goes through arectification stage that eliminates any chance of surges and spikesgetting through

e) Line Interactive UPS :When the utility power is good it passes itthrough the unit to the output. Part of the AC power passes throughthe invertor circuit will charge the battery when the utility power isactive.when the utility power goes down or bad , it draw the powerfrom the battery to run the invertor


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UPS Output waveform

Square wave form : Not recommended for PC

Stepped square wave or modified square wave Not recommended for PCand would not be suitable for some power supply

Step approximated sine wave : If the Total Harmonic Distortion is


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Guidelines for choosing UPS (contd)

c) Check the surge suppression specifications .True online UPS have

almost perfect isolation from utility power and any power line surges atall times. If the built in surge suppression does not meet the minimumrequirement you can add a surge suppressor between the UPS outputand the PC power input.

d) Size the UPS according to maximum load of the units you have pluggedinto the UPS


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Storage devices communicate with the system through interface

Interface consists of all the components that lie between the storagedevice and the computer system bus which include a separate adaptercard, an adapter integrated into the motherboard, a separatecontroller and one or more cables

Primary interface types used in PCs today are the Enhanced IntegratedDrive Electronics (EIDE) and the Small Computer Systems Interface(SCSI)

Most Pcs have EIDEs integrated into motherboard

SCSI is usually implemented with an external adapter card installedinto the systems expansion bus

Pcs have a separate controller for floppy disc drives integrated into themotherboard

Mass Storage Interfaces


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IDE Interface(contd)

In IDE the communication between the controller and the drive unit donot have to be standardized and the IDE manufacturers can modifythe communications between the drive and the controller in whateverthey want because the two components are inseparable

ATA standards

For IBM AT computer that first introduced 16 bit ISA bus use the IDEinterface used by the IDE drives named as AT attachment (ATA)interface and today ATA interface is the industry standard.

Four updates have been made to the ATA standard since its inception

These standards have been designed to remain backward compatibility

ATA standards are designed by Technical committee T13 of theNational Committee on Information Technology standards (NCITS)using development process approved by the American National

standards Institute (ANSI)


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IDE Interface

Majority of PC use an interface called IDE for hard drives and other

storage devices

IDE stands for Integrated Drive Electronics

In IDE controller is built into the drive instead of being a separate unit.

This makes the communications between the controller and the drivemore efficient

Drive controller perform the function of converting the digital signalsgenerated by the computer to analog signals that can be written tomagnetic media

In IDE the entire conversion process occurs inside one unit and thecontroller can send the converted data to the drive almostinstantaneously


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ATA standards (contd)

ATA standard were in use from 1988 but was approved from 1994

when it was published as ANSI X3.221-1994 asAT attachmentInterface for disk

ATA-2 standard

ATA-2 standard was designed and published as ANSI X3.279-1996,ATAttachment Interface With Extensionsin 1996

Introduced to overcome the short comings of ATA standard which hasless speed and limited drive capabilities up to 504 Mb

ATA-2 expands the available data transfer modes to include PIO modes

3 and 4 and multiword DMA modes 1 and 2 which increases the speedof ATA-2 by almost 100 percent

ATA-2 introduces power management functions ,block mode transfers ,group read or write commands for execution with a single interrupt


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Mass Storage InterfacesATA-2 standard (contd)

ATA-2 standard introduced logical block addressing (LBA) which breaksthe 528 MB disk size barrier

This standard adds Identify Drive Instruction to the AT command set.This enables the drive to supply its own detailed information to the

system. This enables the BIOS to automatically configure itself toaccess the drive instead of entering the settings of the drive manually

FAST ATA and FAST ATA-2

FAST ATA-2 manufactured by Seagate and Quantum conform almost

the same as ATA-2 standard

FAST ATA drives are the same as FAST ATA-2 dives except that theydo not support PIO mode 4 or multiword DMA mode 2


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Mass Storage InterfacesEnhanced IDE

EIDE conforms to ATA-2 standard with two additions

a) Inclusion of two host adapters enabling the system to support up to fourdevices

b) Inclusion of ATA Packet Interface (ATAPI) which provides support for devicesother than hard disk drives on the IDE interface. This provides room for theCD-ROM drives to become standard equipment on PCs

ATA-3

Published in 1996 as ANSI X3.298-1997,AT Attachment-3 interface

The standard increases the reliability of IDE interface by introducing bus

termination at both ends of the IDE cable to reduce noise

The standard defines Self-Monitoring Analysis and ReportingTechnology (SMART) which is a system for monitoring the performance ofIDE devices and anticipating problems using a technique called predictivefailure analysis


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Mass Storage InterfacesATA-4

AT Attachment-4 with Packet Interface Extension defines the use of

additional devices such as CDROM ,cartridge and tape drives on theIDE interface

This standard introduces a new data transfer mode named Ultra DMAmod 2 running at 33 MB/sec which is also known as UDMA 33 or ultr-ATA/33 .This transfer mode doubles the theoretical maximum

throughput provided by the PIO and DMA modes

ATA-4 support Advanced Power Management (APM) standard. AllowsIDE cable to facilitate data transfers running at high speeds, supportfor compact flash adapters and BIOS enhancements that makes PCs to

support drives up to 9.4 trillion GB

ATA-5

In this standard Ultra DMA transfer rate is increased from 33 MB/sec

to 44 MB/Sec which is called Ultra DMA mode 3 and 66 MB/sec called

Ultra DMA mode 4 or UDMA/66


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Mass Storage InterfacesATA-6

Even though ATA-5 standard have not yet been ratified ,work is

proceeding to the next version of the standardAT-Attachment 6 withpacket interface

This standard will define an Ultra DMA mode 5 running at 100MB/sec

Data Transfer ModesThe three commonly used data transfer modes that increase thespeed at which the system can read data and write data to the driveare

a) Programmed Input Output (PIO)

b) Direct Memory Access (DMA)

c) Ultra DMA

The transfer modes supported by the computer and the drives thatare placed in it are the important factors that decide the overallperformance of IDE installation


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Mass Storage InterfacesData Transfer Modes ( contd )

For a system to use a particular data transfer mode both the drive andthe system must support a given transfer mode

PIO Transfer Modes

Programmed Input /Output is a protocol for transferring data that usesthe systems processor as an intermediary data path

The speed of transfer is determined by the data cycle time of aparticular mode which is the maximum amount of time in which 16 bitsare transferred to or fro the drive measure in nanoseconds

PIO transfer is available in mode 0,1,2,3,4 respectively and the datatransfer rates for different modes are as given below


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Mass Storage InterfacesTable : PIO Transfer Modes and Standards

PIO Mode MaximumTransferrate(MB/sec)

DataCycleTime (ns)

Supporting Standards

0 3.3 600 All

1 5.2 383 All

2 8.3 240 All

3 11.1 180 ATA-2,Fast ATA , Fast ATA-2,ATA-3 , ATAPI , Ultra DMA , EIDE

4 16.6 120 ATA-2,Fast ATA-2,ATA-3,ATAPI,Ultra DMA,EIDE


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Mass Storage InterfacesDMA Transfer Modes

Data is transferred directly between the drive and the system memory

without using processor as an intermediary

Processor is relieved from the burden of data transfer and therebyimproves the overall efficiency of the system

DMA channels are built into the ISA bus through which data transfer

takes place. DMA channels are implemented by a DMA controller builtinto the chipset on the computers motherboard

In the earlier PC the DMA transfer modes used are single-word DMAmodes which are now obsolete and have been removed from the ATAstandard

The other form of DMA that PC use for data transfers is calledmultiword DMA or Bus mastering DMA where the DMA controllercircuitry are built into the drive controller itself rather than on themother board


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Mass Storage InterfacesDMA Transfer Modes (contd)

IDE interface that uses bus mastering DMA modes utilizes the PCI busto provide superior data transfer rates

The following table illustrates the data transfer speed of Single word

DMA transfer modes and Bus mastering (multiword) DMA transfermodes

Single Word DMA Mode Maximum Transfer rate(MB/Sec)

0 2.1

1 4.2

2 8.3


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DMA

Mode

Maximum

Transfer Rate(MB/Sec)

Data Cycle

Time (ns)

Supporting standards

0 4.2 480 All

1 13.3 150 ATA-2, Fast ATA , Fast ATA-

2 , ATA -3,Ultra DMA ,EIDE

2 16.6 120 ATA-2 , Fast ATA-2, ATA-3,Ultra DMA , EIDE

M St I t f


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Ultra DMA Transfer Mode

Most of the drives being manufactured today support Ultra DMA mode2

If the interface of the computer does not support Ultra DMA, anexpansion card introduced by the company named PromiseTechnologythat upgrades the system BIOS and provides an interfacethat supports Ultra DMA

The data transfer rates and the supporting standards for Ultra DMA

transfer mode is as shown below


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UltraDMAMode

MaximumTransferRate(MB/Sec)

Data CycleTime (ns)

Supporting Standards

0 16.6 240 ATA-4,ATA-5,ATA-6,Ultra ATA/33,Ultra ATA/66

1 25.0 160 ATA-4,ATA-5,ATA-6,ultraATA/33,ultraATA/66

2 33.3 120 ATA-4,ATA-5,ATA-6,ultraATA/33

,ultraATA/663 44.4 90 ATA-4,ATA-5,ATA-6,ultraATA/33

,ultraATA/66

4 66.6 60 ATA-5 , ATA-6,ultraATA/66

5 100 40 ATA-6


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Mass Storage InterfacesThe SCSI Interface

The Small Computer Systems Interface is one of the most popular PC

drive interface in use today

SCSI supports more devices and device types and is expensive

It is best suited to network environments in which many users are

accessing a shared drives simultaneously

Generally a SCSI interface is not built into the PC ,the user have topurchase a SCSI host adapter card and install it into a bus slot andSCSI device to the host adapter which forms an independent bus withinthe computer. This bus enables the devices to communicate amongthemselves without involving the rest of the computer

Designed to support wide variety of devices including hard disk drivesCD-ROM drives, tape drives , Cartridge drives ,media changers ,Graphics scanners etc


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The SCSI Interface (cont d)

SCSI adapters have both internal and external connecters so that same

adapter card can be used to connect both the hard drives and theCDROM drives inside the computer. This capability is useful for networkservers that utilize high end devices such as hard drive arrays ,opticaljukeboxes and tape changers

SCSI Standards

SCSI standards appears to have three revisions and are backwardcompatible

SCSI-1

The SCSI standards are published by ANSI and was published in 1986

M St I t f


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Mass Storage InterfacesSCSI-2

The SCSI 2 standard was published in 1994 and includes thefollowing features

a) Fast SCSI :A high speed transfer protocol that doubles the speedof the SCSI bus from 5 to 10 MHz providing 10MB/Sec transfer rate

b) Wide SCSI : Doubles the width of SCSI bus to 16 bits and therebydouble the data transfer speed to 10 MB/Sec when used withoriginal 5 MHz and 20 MB/Sec when used in combination with FastSCSI

c) Command queuing : Enables the host adapter to send up to 256commands to a SCSI device which stores them in its buffer andexecute them all independently

d) New Cables : The fast SCSI and Wide SCSI requires high densitycables for high speed data transfer

e) Additional Commands: Command set expanded to supportadditional device types which includes CD-ROM and other removablemedia drives

S f


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Mass Storage Interfacesf)Active Termination : SCSI standard defines the use of active

terminators on the bus

g) Bus Parity :Parity verification is optional in SCSI-1 and is mandatory inSCSI-2

h) Terminator Power : In SCSI-2 host adapters provide the power that

terminators need to perform their function

In SCSI-2 Backward compatibility is assured

SCSI-3

It is still not officially ratified as an ANSI standard The important development in SCSI-3 is the introduction of Ultra SCSI,

a new synchronous data transfer scheme that increases the bus speed

Ultra SCSI,Ultra-2 and Ultra-3 are the standards currently available inhigh-end hard drive market and it support transfer rates up to 160MB/sec


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Mass Storage InterfacesSCSI-3(contd)

SCSI-3 transfer modes are backward compatible

Serial SCSI

Serial SCSI is also known as FireWire

FireWire uses serial communication

FireWire Connections can run at 400 MHz or more

It needs bus width of size 1



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Mass Storage InterfacesSCSI Hardware

SCSI hardware consists of a host adapter, some cables and theactual devices that need to connect to the bus

The following are the factors that have to be considered while choosingSCSI hardware

a) Performance level : SCSI devices should be selected to meet thecurrent as well as future needs

b) Connectors :Make sure that all the devices have proper connectorsto support it

c) Cables : Use proper cable for the device that is going to be used

SCSI Host Adapter

Host adapter is the brain of the SCSI bus The computer sends to the adapter all its request for access to the

SCSI device



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SCSI Host Adapter (contd)

The adapter translates the request to the appropriate SCSI commandsand transmits them to the correct device

The adapter then receives the responses to the commands from thedevices and relays the result to the computers processor

SCSI host adapters are plug into one of the computers bus slots likeexpansion cards

Adapters range from low priced cards designed to connect a singledevice to a PC to high end units that support network server featureslike RAID

The following are the criteria that should be considered whenevaluating host adapter


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Mass Storage InterfacesSCSI Host Adapter (contd)

a) PC Bus type :ISA cards can be used to connect SCSI to low speed

devices. PCI bus and a card that connect it essential to connect SCSI tohigh speed devices like hard disk.While using ISA bus the data tranferspeed is limited to 8 Mbps where as PCI bus can support Ultra SCSIprotocols that support high speeds

b) Resource Utilization : SCSI host adapters require hardware resourceslike IRQ,I/O address ,Memory address ,DMA channels etc

c) Bus Width: Host adapters are available with 8 bit narrow SCSI bus widthto 16 bit wide SCSI versions depending on the width of the SCSIdevices that are connected to it

d) Bus speeds : Host adapters are available in all bus speeds fromstandard 5 MHz SCSI to fast SCSI to Ultra SCSI,ULTRA2 and ULTRA3SCSI .Full function PCI adapters can support speeds up to ULTRA 2SCSI with few support ULTRA 3

M St I t f


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Mass Storage InterfacesSCSI Host Adapter (contd)

e) Signaling types :Signaling scheme now used commonly are single

ended SCSI and LVD. Utra 2 and faster drives use LVD to achieve theirhigh speeds

f) Connector types : Some low end SCSI adapters support a singleexternal device and so that they have only one external connector. Atypical full featured adapter card have both internal and externalconnectors

g) BIOS :SCSI adapters have their own BIOS built into the card so thatsystem can boot from a SCSI hard drive

h) Caching : Some host adapters contain on-board cache that

temporarily stores data accessed from the devices connected to it

i) Multiple Channels Dual channel SCSI adapter is available .This hastwo separate adapters on a single card that enables to run twoindependent SCSI buses



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Mass Storage InterfacesSCSI cables

SCSI standard define three types of cables

a) A Cable

a) B Cable

b) P Cable

A Cable :

Contains 50 wires

Internal cables are standard ribbon cables with multiple IDE 50 pinfemale connectors

External cables use either 50 pin Centronix connectors with wiregate latches or high density 50 pin D - Shell connectors with springclip latches called HD50s

A Cable is suited for narrow SCSI devices running at regular ,fast

or Ultra speedsB Cable :

68 Pin cable designed to provide support for the wide , fast/wideand Ultra Wide SCSI buses.

This cable was used in addition to the A cable and hence wasdropped from the SCSI-3 standard



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Mass Storage InterfacesSCSI Cables (contd)

P cable

P cable has 68 wires

It is a replacement for the B/A cable combination intended for wideSCSI variants.

External cables use a high density 68 pin male D- shell connector withthumbscrews called HD68

Internal cables use the same type of connector without additionalshielding and thumbscrews

For internal connections SCSI bus use a ribbon cable with multipleconnectors on it

Internal SCSI devices have one connector only so you must purchase aribbon cable with a sufficient number of connectors to support all the

internal devices in your system including host adapter

M St I t f


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Mass Storage InterfacesSCSI

SCSI is expensive than IDE

Along with the SCSI devices theuser have to buy SCSI host adapterand cables

Performance of SCSI devices withrespect to data transfer rate ishigher than IDE

SCSI devices are used in situationsin which speed can make differencein areas like network data server

SCSI intended to support a widevariety of devices in the initialdesign itself

Each device on a SCSI bus canqueue multiple commands and canexecute them in the most efficientpossible order

IDE

IDE is not as expensive as SCSI

Along with IDE devices the usernot needed to buy adapter andcables

The Performance of IDE deviceswith respect to data transferrate is lesser than SCSI

Not typically used in networkservers

IDE primarily focused on harddrives and then scaled tosupport different devices

IDE is single threaded and canhandle only one command at atime

M St I t f


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Mass Storage InterfacesSCSI Versus IDE (Contd)

SCSI

SCSI uses DMA or busmastering relieving theprocessor from the burden ofdata transfer

IDE

In most systems IDE interfaceoperates on PIO mode oftransfer which make use ofprocessors involvement in datatransfer

M i D i


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Magnetic storage

Consists of Hard disks , floppy disks and tape drives, cartridge drives

etc

Data storage is based on the principles of magnetism

Storage medium consists of a moving surface coated with magnetic

material capable of holding a magnetic charge

In the original state the magnetic particles in the storage medium hasa magnetic charge that are disorganized ,the magnetic field point in

random direction and are cancel each other

To record data onto the medium , a recording head passes close tothe surface

Magnetic storage Devices



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Writing Data(contd)

The head is a U shaped conductor that carries an electric charge

which turns it into an electromagnet

When the electric charge from the head encounters the magneticparticles on the recording medium, it aligns them all in the samedirection depending on the polarity of the charge running through thehead

When the particles of the medium are aligned , the individual magneticfields produce a cumulative charge called flux

The drive is capable of changing the polarity of the charge running

through the head very quickly and when it changes ,the direction ofparticle alignment also changes and thereby produce a flux transitionor flux reversal

The medium moves with respect to head creates a pattern of flux

transitions using a code to represent one bit of data

M ti t D i


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Magnetic storage DevicesWriting Data (contd)

The medium is divided into units called bit cells or transition cells eachof which contains a flux transition that represents one bit of data

Once the particles in the medium are aligned in a particular pattern ,they remain that way until the head applies another charge whichmake the magnetic medium a permanent storage solution

Reading Data The head passes over the surface of the medium while reading data

Head generates a voltage pulse when it passes over a flux transition. Atransition from positive to negative charge registers a negative voltage

and a negative to positive transition registers a positive voltage pulse

The drives controller amplifies the pulses and decodes them toreproduce the original data


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Magnetic storage DevicesReading Data (contd)

IBM developed a new type of read technology called magneto-resistive

(M R) head.

MR head functions by measuring the nonlinear resistance change asa bias current passes through the head that is travelling over thesurface of the disk

The drive circuitry monitors the current passing through the head,when the flux transitions cause increased resistance ,the voltagechanges in the same pattern as when the data are written to themedium

Magnetic Encoding Schemes

Magnetic storage device is an analog medium and hence a D/Aconverter is needed before the drive writes data

The drive performs this conversion using a device called anendec(encoder/decoder)



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Magnetic Encoding Schemes(contd)

The endec encodes the data generated by the computer into the

proper pattern for storage as flux transitions on the magnetic mediumand also decodes the pulses generated during the read operation backto binary data

Timing plays a crucial part in encoding .If the timing is not correct theflux transitions may be interpreted wrongly

To make sure that the timing is correct, encoding schemes used bymagnetic storage devices include a timing signal and clock signal

A timing signal is a special pattern of flux transitions that is placedbetween each series of transitions that represent a bit of data. Thetiming signal ensures that the head reads each bit of data separatelyeven if several consecutive bits have the same value



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Magnetic Encoding Schemes (contd)

MFM Encoding

It is a refined version of the original frequency modulation

FM Encoding is also called single density encoding used in Floppy diskdrives

MFM encoding is called double density encoding and it reduces thenumber of bit cells devoted to timing signals

MFM devices insert a clock transition only when writing two consecutive

bits with a value 0

MFM devices can store twice the amount of data than FM devices



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Magnetic Encoding Schemes (contd)

RLL Encoding ( Run Length Limited)

It can store up to 50% more data than MFM in the same space andhence RLL encoding replaced MFM for hard drives

MFM encoding works with single bit at a time whereas RLL encodingencodes several data bits and their required timing signal at once

Most hard drives used today make use of RLL 17

Hard Disk Drives

It is a nonvolatile storage used in PCs

Hard drive is faster and holds more data than any other form ofnonvolatile storage

It functions using the magnetic storage principles



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Hard Disk Drives (contd)

Magnetic media in a hard drive consist of rigid platters made of

aluminum alloy or a glass ceramic composite material

Platters are coated with magnetic material on both sides one on top ofthe other

The entire platter assembly is mounted on a central spindle that isconnected to the drives motor which spins the platters at high speed

The heads that read and write the data on the platters are mounted toan arm that moves inward and outward on the surface of the platters

There are separate heads for each platters surface all mounted to asingle arm assembly that moves the heads across the surface of theplatters in unison

The entire unit ,platters and heads is called a hard disk assembly



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Hard disk drives (contd)

Cylinders ,Tracks and sectors

All hard disks write data onto the platters in concentric circle calledtracks

A typical Hard drive has 10,000 to 20,000 tracks on each plattersurface

Each platter surface is also split into pie shaped wedges. The part ofthe track within one of the wedges is called a sector. Each sector stores512 bytes of data.

The number of tracks and sectors is the same for each surface of eachplatter in a particular hard drive

A cylinder consists of all the tracks in the same position on each plattersurface. The number of cylinders is equal to the number of tracks on asingle platter surface



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Cylinders, tracks and sectors (contd)

Each sector is numbered beginning with the number 1 while thecylinders are numbered starting with 0

Each sector has an address area and data area. The address areaidentifies the sector, the cylinder in which the sector and the head usedto access that sector

These three addresses are used by the drive to locate the data storeduniquely

The address area contains CRC information used to validate theinformation stored

The data area of the sector contains several bytes of timing signalinformation followed by the 512 bytes of actual data. Several bytes oferror checking code are also included that are used to verify thecontents of the data field. At the end of the data field is a gap thatseparates one sector from the next and provides a margin for error toaccommodate slight variances in the speed in which the drive spins



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Magnetic storage DevicesCylinders , tracks and sectors contd

Cylinder skewing : If the drive is writing data and fills an entire track a ,it must then move to the next track to continue writing. But the timetaken by the head to move from one track to the other prevents it fromcatching the first sector on the new track during the same rotation.Hence the drive must wait for the disk to rotate completely before itcan write to the first sector in the new track. Drive addresses thisproblem by using cylinder skewing in which the first sector of each

track is offset from that of the next track by one sector. When the driveis finished to writing to one complete track ,it can move the heads tothe next track and then begin writing immediately at the first sectorinstead of waiting for the disk to rotate

Zoned bit recording

Usually outer tracks are much longer than the inner tracks resulting in a lot

of storage space wasted. Zoned bit recording is a technique that variesthe number of sectors on each track The The drive cylinders are split inzones(10 to 20 zones). Each of the track in a particular zone has thesame number of sectors with the outer zone is having more sectorsthan inner zone which enables the drive to store more data on theouter tracks than on the inner ones. Each zone transfer data at

different rates



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Magnetic storage DevicesCylinders , tracks and sectors (contd)

Areal Density

It is a measurement of the overall efficiency of a magnetic storage

device

Areal density is obtained by multiplying the number of bits stored inone inch length of a single track (bits per inch BPI) by the number oftracks in one inch length measured between the center of a platter andits outer edge (Tracks per inch TPI) and is measured in Bits Per Square

Inch (BPSI)Hard drive components

a)Platters :

Platters are the magnetic storage medium in the drive and is made of

rigid materials like aluminum or glass

Earlier PCs use large platters that were up to 14 or more inches indiameter. PC drive later use platters of 5.12 inches in diameter.most ofthe hard drives used today use 3.74 inch platters. Portable systems likelaptop and notebooks use drives with smaller platters 2 inch to 1

inch platter



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Magnetic storage Devices Platters are constructed earlier using aluminum alloy but later it was

replaced with ceramic and glass and it forms the structure of theplatter called substrate

The actual magnetic medium is applied as a coating to the substrate

The technology used for applying the magnetic material to thesubstrate was to coat the platter with a viscous liquid containingmagnetic iron oxide particles and the spin the platter on its spindle.The centrifugal force produced while the platter spins causes theexcess liquid to fly off the platter leaving a thin coating of magneticmaterial. The disadvantage of the method was that the coating ofmagnetic material created by the process was too thick to support the

increase in areal density.

Drive manufacturers eventually used thin film media process whichreduce thickness of the magnetic coating on the platters



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Thin film coating make use of electroplating and Sputtering

Electroplating deposits molecules of magnetic cobalt alloy materialonto the platter while it is immersed in a chemical bath

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