83 lines
4.9 KiB
Plaintext
83 lines
4.9 KiB
Plaintext
DRIVING US CRAZY, BUT FOR A REASON
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by Alan Brenden
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In the early days of the PC, there wasn't much involved with the decision-maki
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ng process when a new hard disk was to be bought or repaired. The first hard d
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isks used Seagate's ST506 technology and that was your choice.
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Times and technology have changed and today's high-performance systems make it
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neccessary to match the needs of the system to the storage technology. In thi
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s article I will try and explain to you just what are these drives that drive u
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s crazy. - ESDI, IDE, and SCSI, when and why.
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ST506/412 (MFM & RLL)
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Original the ST506 drives used an encoding method know as Modified Frequency M
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odulation (MFM). As the need for bigger drives evolved a new encoding method w
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as developed to pack data tighter together. This method is know as Run Length
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Limited (RLL). This method involves looking at groups of 16bits rather then ea
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ch individual bit. This achieves a kind of compression of the data that allows
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roughly 50% more on a disk then MFM. The trade off was that you needed a high
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er grade of media and timing is more critical. Prices for media have dropped i
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n the last 3 years and RLL drives have just about wiped MFM drives from the mar
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ket place. ESDI, SCSI, and IDE also use a type of RLL encoding.
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ST506/412 drives have 2 cables, a 34 pin control cable and a 20 pin data cable
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. ST506 MFM has a data transfer rate of 625K bytes per second and a storage ca
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pacity of 5MB - 100MB. ST506 RLL has a data transfer rate of 937K bytes per se
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cond and a storage capacity of 30MB-200MB.
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ESDI
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ESDI (Enhanced Small-Device Interface) was developed to allow faster transfer
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rates and high disk capacities. Greater intelligence reduces the amount of com
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munication between the drive and the controller. The transfer of data between
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the drive and the controller uses a pulse code that isn't required to return to
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zero between pulses, as does ST506. This is know as Non Return to Zero (NRZ)
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and increases data transfer.
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ESDI uses the same cables as the ST506 but can never be mixed. It is CPU cont
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rolled and is suitable for single tasking environments. ESDI has a data transf
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er rate of 1-3M bytes per second and a storage capacity of 80MB - 2GB. One con
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troller can handle up to 2 drives with multiple controllers possible.
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IDE
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As the name implies, IDE (Integrated Drive Electronics) combines both the disk
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and the controller on the same unit. Only a simple interface is needed and typ
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ically it is built directly into the motherboard. If the interface is not buil
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t into the motherboard a simple paddle-board is used and because so little elec
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tronics are needed an additional serial and parallel port is sometime included.
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It transfers only data and doesn't need to send format and sector information
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as does ESDI. Therefore the data transfer rate can be 3-4 times faster then ES
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DI.
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The IDE drive is not a device level interface and has the ability to lie to th
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e BIOS and give the logical appearance of a know device type, while physically
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it may be totally different. You won't see bad tracks on an IDE drive because
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the drive hides them. Because of this you can not low level format an IDE driv
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e without specific utilities for that drive.
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IDE uses a single 40 pin cable. It is limited to a 2 foot cable and 2 address
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es with no termination needed. The first drive is configured as the master and
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the second as the slave. IDE has a transfer rate of .625MB-2MB bytes per seco
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nd and a storage capacity of 20MB-500MB.
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SCSI
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SCSI (Small Computer Systems Interface), pronounced "scuzzy", is a more genera
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l version of the IDE interface. SCSI hard disks boast the fastest transfer rat
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es of all the discussed technologies, with SCSI 2 having a transfer rate up to
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40M bytes per second. SCSI implements 2 ways of boosting transfer rates, fast
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and wide. FAST SCSI doubles the clock speed, and WIDE SCSI increases the bus w
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idth. SCSI also implements other performance features, including controller ba
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sed RAM caching and tag command queuing. By queuing commands the SCSI controll
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er can free up the CPU to do other tasks while it finishes its task. SCSI also
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has the ability to transfer data to another SCSI device without CPU involvemen
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t.
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SCSI uses a single 50 pin cable with devices daisy chained together and termin
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ated on both ends. Seven devices can be installed per controller with up to fo
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ur controllers. SCSI 1 has a data transfer rate of 1-5M bytes per second and a
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storage capacity of 20MB-1.5GB. SCSI 2 has a data transfer rate of 1-40M byte
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s per second and a storage capacity of 40MB-3GB.
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WHICH IS BEST?
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Performance isn't without price. Many applications don't need the performance
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of SCSI, it is by far the most expensive. IDE or ESDI will usually suffice fo
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r most applications. IDE being the cheapest of the three. SCSI has the added
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advantage of the greatest expandability, so if you need SCSI the money is well
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spent.
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