Difference between IDE and SCSI ??

Les hlhowell at pacbell.net
Sat Feb 2 19:57:00 UTC 2008

On Sat, 2008-02-02 at 13:39 -0500, William Case wrote:
> Hi;
> Can someone briefly explain to me the difference between an IDE (ATA)
> and a SCSI device.  After having done due diligence with google searches
> etc., I am still in a quandary. Nothing I read seems to be consistent.
> Every time I think I have it figured out, I read a reference that calls
> for or lists IDE devices that I think should be a SCSI reference and
> vice versa.  Even going to the various standards sites doesn't clarify
> it for me.  In fact it makes it more confusing.
> Therefore, can someone explain, in plain language, how I should use the
> terms IDE or PATA, and SCSI correctly with regards to a current
> computer?  What specific attribute of a device or bus does each term
> apply to?  
> Given below are some questions that spring to mind.  They may be
> mis-formed questions and therefore need not be answered, but they may
> demonstrate where my confusion and misunderstanding are coming into
> play.
> e.g.
> Does IDE refer to the physical device? 
> Or, specifically just to the bus used? 
> Or, to the driver for the device?
> Or, the type of interface (plug)?
> Does SCSI refer to a set of protocols used when designing the device?
> Or, to a specific driver design?
> Can you have an IDE device without SCSI?
> Or, can you have a SCSI device without it being IDE?
> Below, I have listed a few of the sites I have visited with the
> definitions given to show I have found the history and some attempts at
> an explanation.  I long ago learnt that any manual's reference to IDE or
> SCSI usually simply meant some reference to my hard drive.  I am aware
> it could also mean my CD or a DVD, but usually it is a reference to a
> HD.
> http://en.wikipedia.org/wiki/IDE
> http://en.wikipedia.org/wiki/Integrated_Drive_Electronics
> http://en.wikipedia.org/wiki/SCSI
> Integrated Drive Electronics, a computer hardware bus used primarily for
> hard drives and optical drives (e.g. CD, DVD)
> Advanced Technology Attachment (ATA) is a standard interface for
> connecting storage devices such as hard disks and CD-ROM drives inside
> personal computers.
> The standard is maintained by X3/INCITS committee T13. Many synonyms and
> near-synonyms for ATA exist, including abbreviations such as IDE and
> ATAPI. Also, with the market introduction of Serial ATA in 2003, the
> original ATA was retroactively renamed Parallel ATA (PATA).
> SCSI (Small Computer System Interface) is a set of standards for
> physically connecting and transferring data between computers and
> peripheral devices. The SCSI standards define commands, protocols, and
> electrical and optical interfaces. SCSI is most commonly used for hard
> disks and tape drives, but it can connect a wide range of other devices,
> including scanners and CD drives. The SCSI standard defines command sets
> for specific peripheral device types; the presence of "unknown" as one
> of these types means that in theory it can be used as an interface to
> almost any device, but the standard is highly pragmatic and addressed
> toward commercial requirements.
> -- 
> Regards Bill
Hi, Bill,
	IDE was one of the early standards.  It evolved way early in the
progress of computers:
1985: Control Data, Compaq Computer, and Western Digital collaborate to
develop the 40-pin IDE interface. IDE stands for Intelligent Drive
Electronics, more commonly known as Integrated Drive Electronics.

	This was the first attempt to standardize the interface between mass
storage and computer systems.  Actually, though, a similar interface was
developed by several different companies around the 1978 timeframe.  I
owned a Northstar single density hard-sectored 5.25" disk system for my
Altair 8080B that used a similar connector and controller around 1979.
I still have it by the way.  

	Basically the disks had little electronics on them.  But they needed to
move the head to different tracks, keep track of the disk position,
write data to the disk, read data from the disk, change the data from a
serial stream to a parallel word, and pass that word back to the
computer.  The IDE standard established the number of bits required to
perform these functions, a means to establish which disk to boot from
and a method to perform the dat transfer, along with all the stuff
needed to control disk speed, sector count, and buffer the data.

There was a half step between IDE and SCSI called ESDI.  The design of
the original IDE had some size limitation that prevented disks from
growing to meet demands.  I don't know too much about it any more, but I
did work on some systems that had it:

1985: Western Digital produces the first ESDI (Enhanced Small Device
Interface) controller board, which allows larger capacity and faster
hard drives to be used in PCs.

SCSI was developed to address shortcomings in ESDI and add multiple
drive capability.  I don't know for sure the drive count limitation on
SCSI, but I believe it was 7 or 15 originally, due to addressing bit

1986: The official SCSI spec is released; Apple Computer's Mac Plus is
one of the first computers to use it.

	SCSI is a serial system, or at least it can be.  It allows virtually
unlimited storage size, and has been implemented as a mother board in
several systems, where the drives plug directly into the frame.  In
these cases the mother board also forms the means to permit hotplug, by
establishing the mechanical order of contact and the buffering of the
pins from spiking.

	SCSI was originally deployed by apple as noted in the article, but also
in commercial applications.  Sun Systems almost exclusively used SCSI
due to speed and capacity needed for their workstations.  Also the SCSI
bus system was ideal for server systems where large quantities of data
had to be stored and quickly retrieved.  

The standards for both are posted in the IEEE and ACM websites, along
with lots of good papers on the processes.  Look between the years 1979
and 1985 if you are interested in the evolutionary history of the two
systems (and ESDI).

Les H

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