Difference between IDE and SCSI ??

Aaron Konstam akonstam at sbcglobal.net
Sat Feb 2 21:36:36 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 
> IBM PC.
> 
> Mikkel
The iBM PC waas released in 1982 with dissk pased PC's shortly after
using IDE interface. SCSI was considerably later.



--
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I can resist anything but temptation.
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Aaron Konstam telephone: (210) 656-0355 e-mail: akonstam at sbcglobal.net




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