Sunday, 8 April 2012

Five weighing blocks and 121kgs....

Using five weighing blocks how can we weigh any weight up to 121 kgs? What shall be the five weights that have to be considered?
            Because we use a common balance with two compartments it is possible to add as well as reduce weights. If we have 1 kg block 1kg can be weighed. We don’t need a 2kg block because we can measure2kgs by using 3kgs block on one compartment and 1kg weight on the other. It is evident that for weighing 3, 4 kgs we have 1 and 3 kg blocks. Let us see what we will be our next block, blocks 5,6,7,8 are not required, As we add 9 kg block with 1&3  kgs blocks up to 13 kgs can be weighed .As we begin using 1 kg block  1kg is weighed. Here is our little calculation for next block of weight, just 1 more than the two, 1*2=2,2+1=3.Using these two blocks up to 4kgs can be weighed. Our next block is 1 more than the double of, 9kgs.Using this we can measure up to 13kgs.Next block one more than the double of, 13*2=26, 26+1=27.Let us put this in a series form 1, 3,9,27 .We find all of them a multiple of 3.Next number will be 81, very next will be 243.Thus if we have 1,3,9,27,81,243 designations .What is the amount up to which we can make a measurement? To find out, multiply 243 with 3 and reduce 1, ie, 243*3=729,729-1=728.Find the half of the result728/2=364.It can be found that we can measure various weights up to 364 kgs.If we consider 1,3,9,27,81 blocks, 81*3=243,243-1=242,242/2=121.
            We can conclude that a measurement up to 121 kgs can be made with the blocks of 1, 3,9,27 and 81 kgs blocks.

Sunday, 4 March 2012

A puzzle about discrete distribution

One is carrying one thousand gold coins. These coins are packed in separate small bags. It is supposed to give away coins whatever the discrete amount is demanded without unpacking the bags. How is it going to be packed in ten bags?

Here is the solution

Number of coins in first bag = 1
Number of coins in second bag = 2
Number of coins in third bag = (2+2) = 4
Number of coins in fourth bag = (4+4) = 8
Number of coins in fifth bag = (8+8) = 16
Number of coins in sixth bag = (16+16) = 32
Number of coins in seventh bag = (32+32) = 64
Number of coins in eighth bag = (64+64) = 128
Number of coins in ninth bag = (128+128) = 256

Sum of the coins in all nine bags = 1+2+4+8+16+32+64+128+256 = 511

Number of coins in the tenth bag = 1000-511= 489

For example, if one demands 300 coins 256+32+8+4 = 300 i.e., 9th bag+ 6th bag +4th bag+3rd bag are given away. Likewise any number of coins can be given selecting appropriate bags from 1 to 9 and also by adding them, so that the sum is any number between 1 and 1000.

Thursday, 23 February 2012

Work and time

12 people finish a work in 10 days then how long it takes for 15 people to finish the work. This problem can be solved using equations
People * days = different number of people * days required for them.
Deriving, people * days / diff number of people = days required 
From our previous example 12 * 10 = 15 * x,
12 * 10 / 15 = 8
Take the other problem if work is done by A in 12 days B in 4 days and C in 6 days, then together all of them how many days it take to finish the work?. Consider unit amount of work done by each and find the total
1/12 + 1/6 + ¼ = (1 + 2 + 3)/ 12 = 6/12 = ½
It is found that all of them together it takes 2 days to finish this work.
The other problem is solved by subtraction of unit work done by each. Take A& B together finishes a work in 4 days. This work can be done by A alone in 12days.Then how long it takes for B alone to finish this work?
            Both finish unit amount of work in ¼ days then B takes 1/4 – 1/12 = (3-1) / 12 =
2/12 = 1/6 i.e., 6days to finish this work.
And another problem, 5 people works for 7 hours to finish a work in 4 days, then a man if work for 10 hours, how long it takes to finish the work?
Again it is a game of equations
(5 * 7 * 4) / (1* 10) = 140/10 = 14
Our friend takes 14 days to finish the work!

Thursday, 5 January 2012

Miniature circuit breaker

Miniature circuit breaker

            In earth leakage circuit breaker system, the parts of an installation which are sought to be protected are connected to an earth electrode through the coil of an earth leakage circuit breaker, which controls the supply to that part of the installation. In addition to the equipment earth. It provides the means of disconnecting the faulty circuit at a current level (about 30 mille amperes) which is only a fraction of the normal full load current, and is the most effective method of protection
            The trip circuit breaker may be provided with time-lag feature to allow the operation of the protective devices protecting individual circuits.
            MCB body is made of flame retardant high strength plastic with high melting point and high dielectric strength. The contacts are of silver, silver alloy, or of silver graphite with wiping action to ensure maintenance-free long life. The arc produced on separation of the contacts is rapidly moved under the influence of a magnetic field, under the arc chute stack where it is broken down in to partial arcs, resulting in rapid extinction of arc. Most MCBs are equipped with thermal and/or magnetic and current limiting protection for normal overload inverse time current characteristic protection. Thermal operation is achieved with a bimetal strip which deflects when heated by the overload current flowing through it. In doing so, it moves the trip lever releasing the latch mechanism and causing the contacts to open under spring action, when short circuit fault occurs, the rising fault current energizes the solenoid of hammer trip mechanism and the trip lever causes immediate opening of the contacts.
            “L”series circuit breakers are recommended for installations with low inductive loads like heaters,ovens,geysers,GLS lamp etc.They have pickup current five times more than the normal current.
            “G”series or motor duty MCBs are recommended for installations with high inrush current peaks and which, at the same time require closer overload protection such as motors, air conditioners, fluorescent lamps, sodium vapor lamps, machine tools etc.These MCBs have eight times the normal rated current as magnetic pickup current.
            Back up protection-If the prospective short circuit current is expected to be more than 9000Amps, MCBs will require suitable back up protection for example HRC fuse or higher capacity circuit breakers.

Wednesday, 30 November 2011

Connect laptop to desktop pc!

A network is a group of two or more computers that intelligently share hardware or software devices with each other. A network can be small and simple as two computers that share a printer and DVD drive attached to one of them or as large as the world’s largest network: the internet. Virtually any storage or output device can be shared over a network, but the most common devices include the following,printers,disk drives, optical drives modems, fax machines, scanners etc.In addition to reducing hardware costs by sharing expensive printers and other peripherals among multiple users, networks provide additional benefits to users, multiple users can share access to software and data files, email is sent, contribute to a single document, remote controlled programs are executed to demonstrate a task, simply internet is shared etc.
            LAN is the smallest network formed from computers in a single office or building. LAN in deferent locations can be connected by high speed fiber optic, satellite or leased phone lines to form a wide area network (WAN).Intranets and extranets use firewalls and other security tools to keep its content private.
            Each network requires one or more cables or wireless hardware, communication rules known as protocols, software commonly associated with operating systems such as windows and Linux, resources that can be shared such as printers and diskdrives, software such as network client, which provided as part of common operating systems.
            Even though early forms of wireless networks were much slower than wired networks. Newest wireless networks offer speeds that are that can outpace fast Ethernet. There are two basic types of LANs-client/server networks and peer-to-peer networks. On a client/server network, a server is designed to share its resources among client computers on the network, servers are located in secured areas such as data centres.Dedicated server computer typically has faster processors, more memory and more storage space than a client because it might have to service dozens or even hundreds of users at the same time. High performance servers typically use from two to eight processors, have several giga bytes of memory installed, use server network operating system such as windows, Linux etc,In a group of servers each server can specialize in particular task that is file server print server, fax server, email  server etc.Several servers can act as single unit through parallel processing.
            Every computer on peer to peer network can function as both a server and client, any computer on peer to peer network is considered a server if it shares a printer, a folder, a drive or some other resource with the rest of the network. Peer to peer network ranges from two to a hundred systems network. As the size increases security become an issue. Microsoft imposes a 10 station limit on computers running XP professional. Windows XP home can not use passwords, groups or users to control file sharing, nor can windows Professional if simple file sharing is enabled. Fast Ethernet with a speed of 100mbps maximum of 1025 workstations which uses cat5 cable is the most popular wired networking standard.10G Ethernet runs at 10000 mbps, and is used primarily in enterprise data centers and server blades.
            Both the most popular form of fast Ethernet(100base-TX)and  10 base-T standard Ethernet use two of the four wire pairs found in unshielded twisted pair(UTP)category 5 cable .Unlike hubs, which broadcast data packets to all computers connected to them, switches create a direct connection between the sending and receiving computers, thus switches produce faster performance than hubs; most switches also support full-duplex operation, doubling the rated speed of the network  when full-duplex network cards are used. Twisted pair cable carries insulated wires with in a protective casing with a specified number of twists per foot. Twisting the wires reduces the effect of electromagnetic interference on the signals being transmitted.
            You should build your own twisted-pair cables if you plan to perform a lot of networking, need cable lengths longer than the lengths you can buy preassembled at typical computer departments. Want to create both standard and cross over cables, want choose your own cable color, want maximum control over cable length, want to save money, have the time necessary to build cables.
            If you want to create twisted pair cables yourself, be sure your cable pairs match the color coding. Because there are eight wires in TP cables many incorrect combinations are possible. Refer the following color coding for cross over cabling

Making your own UTP cables requires a few tools that are not commonly found in a typical toolbox. You will need the following tools to build your own Ethernet cables UTP cable (cat 5 or better), RJ 45 connecters, wire stripper, RJ45 crimping tool.
Arrange the wires according to EIA568B standard. Trim the wire edges so the eight wires ar5e even with one another and are slightly less than 1/2” past the end of the jacket. If the wires are too long crosstalk can result. If the wires are too short they cannot make a good connection with the RJ45 plug. Push the cables into place and use crimping tool to squeeze the RJ45 plug on to the cable. The end of the cable should be tight enough to resist being removed by hand.
For IP address configuration in windows XP ,click on start and then control panel-> network connections->local area connection->right click and select properties set an IP.Give IP as static ,give a private IP for example give OK.Enable a printer sharing also.Check whether computer is in domain network or work group network, for that go to my computer, right click select properties select computer name, use change button to change the work group name .you can change computer name as well. Click O.K. check the IP configuration, go to command prompt type ipconfig command to list IP configuration.
            Go to the other machine and repeat the procedures, be careful that different IP is given in the other machine. To share files and printer go to my computer->tools->folder options->view button unselect ,use simple file sharing, right click the drive to select file sharing and security, check share this folder give share name then give OK.Share folder will appear with the hand signal. This covers peer to peer networking procedures.

Sunday, 2 October 2011

Telecom landline resistance facts

The telecommunications industry has experienced tremendous changes during the last 25 years on both legislative and technical fronts. Landline is near extinction and mobile telecommunications dominate the system. Telecommunications is communication over a distance. It encompasses all forms of communications and includes communications by voice; video or data that we wish to transmit from one point to another. People often separate the communication of voice and data into two catagories, using the term telecommunications to denote the transmission of voice signals and data communication to refer transmission of data signals.
            In public switched telephone network (PSTN), voice signals emerge from a telephone as analog signals. The word analog is related to analogous, electrical signals caused by a voice wave hitting the diaphragm of a telephone set’s transmitter are analogous (similar) to the shape of the voice wave. All telephones connect to a local central office, which have an automated switching system that connects callers to their desired destination. The local central office contains a line circuit for every telephone connected to it. The line circuit in turn contains a device called codec (coder/decoder) that converts analog voice signals received from the telephone into digital signals. These digital voice signals are then carried over the digital circuits that comprise the PSTN.
            Data is converted into digital codes when it is stored inside computers. The transmission of these digital codes results in a digital signal. Digital data can be transmitted over the PSTN by using modems(MOdulator-DEModulator)or can be transmitted over packet data network(also known as public data network-PDN).The PDN is a wide area network which uses facilities in the PSTN that have been reserved for data transmission. All circuits in PSTN are digital circuits and can carry digitized voice data and video. It is appropriate today, therefore, to consider telecom as including voice data and video.
           Though telecommunications began with telegraph in 1837, In 1876 Gray and Graham bell filed papers with patent office for an invention called telephone. Modern telecoms have VLSI devices to convert analog signals to digital and carry them between central offices. Many signals from many different conversations or data, inputs into one very signal stream. These devices use a technique called multiplexing to place many signals over one transmission medium. The multiplexing devices used to place many signals over glass fiber have developed to a point where they change the light signals used transmit signals over fiber at a rate of almost ten billion times a second.
            Most large businesses use a private voice switching system called a private branch exchange (PBX).The use of a PC to serve as aPBX also has the benefit of allowing the computer to provide information about the call on the screen. Customer information can be kept in a data base file on the PC, and the PC can use that database to get information that helps with handling the call. Several software programs are available that enhance the PCs ability to interact with incoming call according to information associated with the call.
            For data applications (internet) all computers in a building are connected together by a LAN, these LANs will be connected together with a Metropolitan Area Network (MAN) or Wide Area Network (WAN).LANs use a technology called Ethernet where PCs are connected via two twisted pair copper cables with RJ45 connecters at both ends.
            The voltage for the transmitter in telephone instrument and the dual tone multi frequency (DTMF) circuit of the touch tone dial is supplied from the central office. The voltage at central office battery is 52Volt DC.If the phone is on hook, no electric current will flow and we would also measure 52Volt at the telephone. When the telephone is taken off-hook to make or answer a call, electric current will flow. As the electric energy travels over the wire pair between the telephone and the central office, some of the voltage is lost as heat in the wire pair. We refer to the electric energy as a voltage drop ,The amount of voltage drop in the wires depend on the resistance of the wire and amount of current flowing in the wire pair. The resistance of the wire depends on the size (gauge) and length of wire used. Resistance (R) of a conductor, twisted copper wire of length’l’ cross sectional area ’a’ and resistivity ’ρ’ is given as R= (ρ*l)/a resistivity of copper (ρ) is 17.2*10^-9 ohm/meter. And that of silver and gold are 16.29*10^-9 and 24.4*10^-9 respectively.
            The length is variable and depend on the distance the phone is at from the central office(telephone exchange).Every wire pair used to connect telephone to a central office will connect through an electronic circuit(called the line circuit)to the central office battery. The amount of voltage used up by the line circuit and wire pair will leave a voltage between 8 and 24 volts for the telephone. The variance in voltage at the phone is because longer wire pair will have more voltage drop (use up more voltage) than shorter wire pairs. For the longest wire pair, the telephone will get about 8 volts. On a very short wire pair the phone will get about 24 volts.
            A 19 gauge wire 1000ft long has a resistance of 8.051ohms and a 16 gauge wire of same length has a resistance of 4.016ohms.Thus a difference in gauge size of 3 results in a resistance change by a factor of 2.In telecommunications the gauges of wire used are 26,24,22,and 19 guages,with 26 gauge predominating. As the gauge number increases the cross sectional area dicreaseesThe cables that connect our telephone to the local telephone exchange switching system must not contain mire than1800ohms of resistance, when the local switching system is computer controlled switch. With older electromechanical switching system, the resistance of wires connecting a telephone to a switch was limited to 1200ohms.
Phone requires 0.02 amp of current flow through the instrument to provide adequate power for the transmitter and touch tone dialing pad. Computer controlled switching system use a DC power supply of 52volts.All telephones attach to switching system one pair of wires. At the switching system this pair of wire connect to a device called a line circuit the job of the line circuit is to interface a telephone to a switching system. The switching system connects the negative terminal of a 52volts battery through the line circuit to one of the wires connected to a telephone. This wire is called the ring .The positive terminal of 52volts battery is connected through the line circuit to the second wire connected to a telephone. This wire is called the tip. The line circuit has the resistance of 200ohms between the -52v DC and the wire going to the telephone. The line circuit also has the resistance of 200ohms between the positive terminal of the battery and the other wire going to the telephone. The positive terminal of the battery is connected to the earth ground, thus the positive terminal is grounded and the negative terminal is -52 v with respect to ground.
  Since telephone needs at least 20 mile amps (0.02amps) of current flowing through it to operate properly, the resistance of the wires attaching a telephone to the central switch must be selected to ensure this criterion is met. Notice in the circuit of battery supply, that the resistance of each wire is 900ohms.The resistance of the telephone is400ohms and resistance of each side of the line circuit is 200ohms, for each connection to the wire pair. Ohms law dictates that with 52volt applied to the circuit, 20mA of current will flow through the telephone (and every where in the series circuit) I=V/R=52/ (200+900+400+900+200) =52/2600=0.02A
            Now that we know 1800Ω is the maximum resistance allowed in wires connecting a telephone to a local switch. We can select the gauge of wire needed to ensure that this value is not exceeded. Resistance per 1000ft of 26 gauge wire is 40.81 Ω.We need two wires to connect a telephone to the local switch. Electric current will flow out over one wire from the switch to telephone and will return to the switch over the second wire. Thus the ring wire, the telephone, and the tip wire are electrically in series. The resistance for 2000ft of wire is 81-62 Ω.We refer to the two wires as a loop because they form a loop [in connecting a telephone to a central switch. The resistance per 1000ft loop using 26 gauge wire is 81.62 Ω.If we divide 1800 by 81.62 we will find that maximum allowable loop using 26 gauge wire is 22,000ft or a little more than4 miles. If the telephone is located more than 4mi from the central switching system, a larger diameter wire will be needed. The gauge of wire to use in the will be governed by the length of the loop needed to connect a telephone to a switch (the distance from the switch to telephone)
            Since we know that the resistance can be substituted for actual components in a laboratory environment to perform test or measurements, let’s do that for a telephone. We stated earlier that telephones have a resistance of 400 Ω.This is referred to as the nominal resistance value of the telephone. Its actual resistance varies depending on the voltage applied to it. The resistance of a telephone cannot be measured accurately with an ohmmeter because the ohmmeter does not place a high voltage on the telephone. The ohmmeter will provide us with the resistance of a telephone under a very low voltage condition. We can calculate resistance of a telephone in a line circuit indirectly by measuring voltage and current in a line circuit and using Ohm’s law to find resistance. Using Ohm’s law V=IR we can calculate the various volatages dropped across each components of the circuit
1.For one half of the line relay in the central switch:E=0.02*200=4V
2.For one wire connecting the telephone to the switch:E=0.02*900=18V
3.For the telephone:E=0.02*400=8V
4.For the return wire telephone to the switch:E=0.02*900=18V
5.For one half of the line relay in the central switch:E=0.02*200=4V

Total of all voltage drops:52V

Kirchoff's voltage law is satisfied. The sum of the voltages dropped in the circuit equals the sum of the voltage applied to the circuit, we can also see that the voltage dropped across the telephone in the circuit is 8V.

Tuesday, 30 August 2011

Hard disk drive a dialectical dialogue.

Hard disk drive a dialectical dialogue

Hard disk contains rigid, disk shaped platters usually constructed of aluminum or glass. It’s a sealed unit that PC uses for  nonvolatile data storage.Hard drive is used to store crucial programming and data, As a result when the hard disk fails, the consequences are usually very serious. The platters of hard disk are hard in texture hence the name harddisk.Moreover the platters are not removable.
            3 ½”, SATA drives are most commonplace in desktop systems. The basic physical construction of a hard disk drive consists of spinning disks with heads that move over the disks and store data in tracks and sectors. The heads read and write data in concentric rings called tracks, which are divided in to segments called sectors, which typically store 512 bytes each.

            Most drives have two or three platters, resulting in four or six sides, but some disks have from 1 to 12 platters and 24 sides with 24 heads to read them. The heads fitted on actuator move radially across the disk in unison. Most standard-issue drives found in PCs today spin at 7,200 rpm, with high performance models spinning at 10,000 rpm mostly being servers. High rotational speeds combined with fast head-positioning mechanism and more sectors per track are what make one hard disk faster overall than another.

            The heads do rest on platters when powered off. When powered on heads move to the inner most cylinder, where they land on platter surface, slide on platter as they spin up, until a very thin cushion of air builds up between the heads and the platter surface, causing the heads to lift of and  remain suspended a short distance above or below the platter. Because few companies repair the hard disk assemblies (HDAs).Repair or replacement of the parts inside a sealed HAD can be expensive. Every hard disk ever made eventually fails. The only questions are when the failure will occur and whether your data is backed up.Hard disks are one of the most fragile components of a PC.Manufacturers void guaranty if the seal is broken. Most of the modern hard disks are sold preformatted and advertise only the formatted capacity. That means each sector on disk typically occupies up to 571 bytes of the disk, of which only 512 bytes available for the storage of the user data .That is the usable space for data on each track is about 15% less than its total unformatted capacity.
            Preparing a hard disk driver for data storage involves three steps namely low level formatting, which is done at factory. Disk partitioning for any multiple operating and/or file systems on each directory and finally high level formatting. All modern hard disks use zoned recording to enable the capacity of disk to 20-50% more.
            Every hard disk must have at least one partition on it and can have up to four partitions. Each of which can support NTFS or different file systems. During high level format, the operating system writes the structures necessary for managing files and data on the disk. High level formatting is not really the physical formatting of the drive, but rather the creation of table of contents for the disk. In low level formatting which is the real physical formatting process, tracks and sectors are written on the disk, A FORMAT DOS command performs high level formatting on the disk.
            The basic components of a typical hard disk drive are disk platters, read/write heads, head actuator mechanism, spindle motor (inside platter hub).These are situated in a sealed assembly called Hard disk assembly (HDA).Other parts are external to the drives HAD such as the logic boards, bezel, and other configuration or mounting hardware, can be disassembled form the drive.
            Virtually all the modern drives use glass or glass ceramic platters instead of aluminum/magnesium alloy platters. Platters are covered with a thin layer of a magnetically retentive substance, called medium, on which magnetic information is stored. Old days since 1955 oxide media remained popular. Modern platters are sputtered with thin film medium, Anti ferromagnetic alloy coupled (AFC) media being the most recent development. All hard disk drives being manufactured today use voice coil actuators to achieve the accuracy necessary. In typical hard disk drive’s voice coil system, the electromagnetic coil is attached to end of head track and placed near a stationary magnet. No physical contact occurs between the coils and the magnet, instead the coil moves by pure magnetic force, Use a feedback signal from the drive to accurately determine the head positions and adjust them if necessary. The positioning system often called a closed loop feedback mechanism or servo controlled sytem.Virtully all voice coil sytems today have rotary actuator systems. All servo mechanism relies on special information that is written to the disk when it is manufactured. This information is usually in the form of special code called gray code, a special binary notational system.
            Most drive today use an embedded servo to control the positioning system. Embedded servo design writes the servo information before the start of each sector. Hard disk assemblies are manufactured in clean room environment. The equipment for clean room environment is expensive which makes the repair/opening up of hard disk for repair companies costlier. There is no program written to perk or retract heads, as was necessary with early hard disk designs. In the event of a power outage, the heads park themselves automatically.
            The HDA in hard disk drive is sealed but not air tight. The HDA is vented through a barometric or breather filter element that enables pressure equalization (breathing) between inside and outside of the drive. For this reason most hard drives are rated by the drives manufacturer to run in a specific range of altitudes, usually form 1,000 feet’s below to 10,000 feet’s above sea level.Infact some hard drives are not rated to exceed 7,000 feet while operating because the air pressure would be too low inside the drive to float the heads properly. You must place the hard disk drive that has been placed in colder than normal environment into its normal operating environment for a specified amount of time to allow it to acclimate before you power it on.
            The logic boards contain the electronics that control the drives spindle and head actuator systems and present data to the controller in some agreed upon form. Many disk drive failures occur in the logic board, not in the mechanical assembly. Most of the time the boards are fairly easy to change with nothing more than a screw driver. Merely removing and reinstalling a few screws as well as unplugging and reconnecting a cable or two is all that is required to remove and replace a typical logic board.
            The power is supplied by the larger 4-pin peripheral power connector found on all PC power supplies. Most hard disks have both 5 and 12 volts power. In most cases the 12 volts power run the spindle motor and head actuator, and 5volts power run the circuitary.Other than power connector hard disk contain interface connector and ground connector to the system’s chassis.
            If the motherboard ROM BIOS dates before 1998 and is limited to8.4GB or dates before 2002 and is limited to 137GB, you have to install a larger drive. First contact your motherboard manufacturer to see whether an update is available. Virtually all motherboards incorporate a flash Bromwich allows easy updates via a utility program.
            By comparison the interface rate merely indicates how quickly data can move between the motherboard and the buffer on the drive. The rotational speed of the drive has the biggest effect on the drives true transfer speed, be sure to check the true media transfer rates of any drive you are comparing.
            Average seek time usually measured in milliseconds is the average amount of time it takes to move the heads from one cylinder to another a random distance away. Latency is the average time it takes for the sector to be available after the heads have reached a track. A measurement of drives average access time is the sum of its average seeks time plus latency.
            When an application program wants to read data from a hard drive, A cash program such as SMARTDRV (DOS) and VCACHE (windows) intercepts the read request, passes the read request to the hard drive controller in the usual way, saves the data read form the disk in it’s cache memory buffer, and the passes the data back to the application program.Nowdays some 31/2” hard disk drives can have up to 16MB of cache memory built right in!
            When you shop for a drive you might notice a statistics called the mean time between failures (MTBF) described in the drives specifications.MTBF figures usually range from 3, 00,000 to 10, 00,000 hours or more. Self monitoring, analysis and reporting technology (S.M.A.R.T) is an industry standard providing failure prediction for hard disk drives. When S.M.A.R.T is enabled for a given drive, the drive monitors predetermined attributes that are susceptible to or indicative of drive degradation. Based on changes in the monitored attributes, a failure prediction can be made. If a failure deemed likely to occur S.M.A.R.Tmakes a status report available so that the system BIOS or driver software can notify the user of the impending problems perhaps enabling the user to  backup the data on the drive before any real problem occur. If you connect the new and existing (failing) drive to the same system. You might be able to copy the entire contents of the existing drive to new one. Saving you from having to install or reload all the applications and data from your backup. Because standard copy commands or drag-and-drop methods don’t copy system files, hidden files or the files that are open, to copy an entire drive successfully and have the destination copy remain bootable you need a special application such as Symantec Norton ghost or acronis true image