DRIVING US CRAZY, BUT FOR A REASON
by Alan Brenden

	In the early days of the PC, there wasn't much involved with the decision-making process when a new hard disk was to be bought or repaired.  The first hard disks used Seagate's ST506 technology and that was your choice.

	Times and technology have changed and today's high-performance systems make it neccessary to match the needs of the system to the storage technology.  In this article I will try and explain to you just what are these drives that drive us crazy.  -  ESDI, IDE, and SCSI, when and why.

ST506/412 (MFM & RLL)
	Original the ST506 drives used an encoding method know as Modified Frequency Modulation (MFM).  As the need for bigger drives evolved a new encoding method was developed to pack data tighter together.  This method is know as Run Length Limited (RLL).  This method involves looking at groups of 16bits rather then each individual bit.  This achieves a kind of compression of the data that allows roughly 50% more on a disk then MFM.  The trade off was that you needed a higher grade of media and timing is more critical.  Prices for media have dropped in the last 3 years and RLL drives have just about wiped MFM drives from the market place.  ESDI, SCSI, and IDE also use a type of RLL encoding.
	ST506/412 drives have 2 cables, a 34 pin control cable and a 20 pin data cable.  ST506 MFM has a data transfer rate of 625K bytes per second and a storage capacity of 5MB - 100MB.  ST506 RLL has a data transfer rate of 937K bytes per second and a storage capacity of 30MB-200MB.

ESDI
	ESDI (Enhanced Small-Device Interface) was developed to allow faster transfer rates and high disk capacities.  Greater intelligence reduces the amount of communication between the drive and the controller.  The transfer of data between the drive and the controller uses a pulse code that isn't required to return to zero between pulses, as does ST506.  This is know as Non Return to Zero (NRZ) and increases data transfer. 
	ESDI uses the same cables as the ST506 but can never be mixed.  It is CPU controlled and is suitable for single tasking environments.  ESDI has a data transfer rate of 1-3M bytes per second and a storage capacity of 80MB - 2GB.  One controller can handle up to 2 drives with multiple controllers possible.

IDE
As the name implies, IDE (Integrated Drive Electronics) combines both the disk and the controller on the same unit.  Only a simple interface is needed and typically it is built directly into the motherboard.  If the interface is not built into the motherboard a simple paddle-board is used and because so little electronics are needed an additional serial and parallel port is sometime included.  It transfers only data and doesn't need to send format and sector information as does ESDI. Therefore the data transfer rate can be 3-4 times faster then ESDI. 
	The IDE drive is not a device level interface and has the ability to lie to the BIOS and give the logical appearance of a know device type, while physically it may be totally different.  You won't see bad tracks on an IDE drive because the drive hides them.  Because of this you can not low level format an IDE drive without specific utilities for that drive.
	IDE uses a single 40 pin cable.  It is limited to a 2 foot cable and 2 addresses with no termination needed.  The first drive is configured as the master and the second as the slave.  IDE has a transfer rate of .625MB-2MB bytes per second and a storage capacity of 20MB-500MB.


SCSI
	SCSI (Small Computer Systems Interface), pronounced "scuzzy", is a more general version of the IDE interface.  SCSI hard disks boast the fastest transfer rates of all the discussed technologies, with SCSI 2 having a transfer rate up to 40M bytes per second.  SCSI implements 2 ways of boosting transfer rates, fast and wide.  FAST SCSI doubles the clock speed, and WIDE SCSI increases the bus width.  SCSI also implements other performance features, including controller based RAM caching and tag command queuing.  By queuing commands the SCSI controller can free up the CPU to do other tasks while it finishes its task.  SCSI also has the ability to transfer data to another SCSI device without CPU involvement.  
	SCSI uses a single 50 pin cable with devices daisy chained together and terminated on both ends.  Seven devices can be installed per controller with up to four controllers.  SCSI 1 has a data transfer rate of 1-5M bytes per second and a storage capacity of 20MB-1.5GB.  SCSI 2 has a data transfer rate of 1-40M bytes per second and a storage capacity of 40MB-3GB.

WHICH IS BEST?
	Performance isn't without price.  Many applications don't need the performance of SCSI, it is by far the most expensive.  IDE or ESDI will usually suffice for most applications.  IDE being the cheapest of the three.  SCSI has the added advantage of the greatest expandability, so if you need SCSI the  money is well spent.