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-maki
ng process when a new hard disk was to be bought or repaired.  The first hard d
isks 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 thi
s article I will try and explain to you just what are these drives that drive u
s crazy.  -  ESDI, IDE, and SCSI, when and why.

ST506/412 (MFM & RLL)
 Original the ST506 drives used an encoding method know as Modified Frequency M
odulation (MFM).  As the need for bigger drives evolved a new encoding method w
as 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 ea
ch 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 high
er grade of media and timing is more critical.  Prices for media have dropped i
n the last 3 years and RLL drives have just about wiped MFM drives from the mar
ket 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 ca
pacity of 5MB - 100MB.  ST506 RLL has a data transfer rate of 937K bytes per se
cond 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 com
munication 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 cont
rolled and is suitable for single tasking environments.  ESDI has a data transf
er rate of 1-3M bytes per second and a storage capacity of 80MB - 2GB.  One con
troller 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 typ
ically it is built directly into the motherboard.  If the interface is not buil
t into the motherboard a simple paddle-board is used and because so little elec
tronics 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 ES
DI.
 The IDE drive is not a device level interface and has the ability to lie to th
e 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 driv
e without specific utilities for that drive.
 IDE uses a single 40 pin cable.  It is limited to a 2 foot cable and 2 address
es 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 seco
nd and a storage capacity of 20MB-500MB.


SCSI
 SCSI (Small Computer Systems Interface), pronounced "scuzzy", is a more genera
l version of the IDE interface.  SCSI hard disks boast the fastest transfer rat
es 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 w
idth.  SCSI also implements other performance features, including controller ba
sed RAM caching and tag command queuing.  By queuing commands the SCSI controll
er 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 involvemen
t.
 SCSI uses a single 50 pin cable with devices daisy chained together and termin
ated on both ends.  Seven devices can be installed per controller with up to fo
ur 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 byte
s 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 fo
r 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.
