Difference between IDE and SCSI ??

Les hlhowell at pacbell.net
Sun Feb 3 02:12:32 UTC 2008

On Sat, 2008-02-02 at 15:13 -0600, Mikkel L. Ellertson wrote:
> Les wrote:
> > 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.
> > (http://pcworld.about.com/news/Sep132006id127105.htm)
> > 
> > 	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.
> > (http://pcworld.about.com/news/Sep132006id127105.htm)
> > 
> > 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
> > size.  
> > 
> > 1986: The official SCSI spec is released; Apple Computer's Mac Plus is
> > one of the first computers to use it.
> > (http://pcworld.about.com/news/Sep132006id127105.htm)
> > 
> > 	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).
> > 
> > Regards,
> > Les H
> > 
> Are you sure about your time line? I can remember using SCSI drives 
> while PC's were still using MFM and RLL drives, long before IDE 
> drives showed up. I could have sworn that SCSI drives pre-dated the 
> Mikkel
> -- 
All these standards evolved almost simultaneously, but the patents and
use by the public was what is documented by PC World Magazine.  I had
subscribed to Byte (when it was folded and stapled with no
advertisements), and Dr Dobbs Journal when it was still labeled Dr.
Dobbs Journal of Computer Calisthenics and Orthodontia, and watched the
evolution of many things, including C, PASCAL, ADA and other languages.
But the time line I used was from PC World.  That is why I noted the
URL.  Check it out.

Les H

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