Difference between IDE and SCSI ??

[John Summerfield]

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?  
> 

SCSI, ATA (sometimes retrospectively PATA) and SATA are technical terms 
that refer to specific technologies, much like christianity, islam and 
hinduism refer to different belief systems.

SCSI has evolved most incompatibly over time; originally it used a 
50-pin connector and copper wires, then 68-pin, then there were optical 
versions. Basically they have a fairly decent controller that can drive 
several devices (disk, tape, some printers) concurrently, with little 
loss of perfomance, at least until the bus gets fairly busy. It's often 
configured in hotplug setups, and used in servers and (expensive) 
workstations.

ATA, sometimes called IDE, started from IBM's PC/AT (from whence the AT) 
with 40-pin connectors, and more recently 40 pins and 80 wires, with 
every second wire grounded. The last incantation is ATA-6.
A serious limitation is that one device floods the bus; driving a second 
disk at the same time incurs a serious performance penalty - the 
combined performance is scarcely more than the performance of either 
one. that and the fact that (mostly) each interface can only drive one 
device.
SATA, aka ATA-7, uses smaller data and power cables and uses a serial 
interface. It seems strange (or did to me when serial interfaces 
appeared on mainframes in the late 80s/early 90s), that serial 
interfaces can go faster. I think this is because there's not a lot of 
(long) signal needing to be coordinated, and there's less risk of crosstalk.


> 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? 

IDE==Integrated Drive Electronics. It's used to refer to ATA drives, but 
in reality SCSI (and other) drives also have integrated electronics.

> 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?
yes
> Or, to a specific driver design?
> 
> Can you have an IDE device without SCSI?
By common usage, yes. They are different.

> Or, can you have a SCSI device without it being IDE?
By common usage, yes. They are different


> 
> 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).

ATAPI is a sort of cross between SCSI and ATA, It used ATA wiring and 
electronics, and some SCSI commands. Mostly used for optical drives, but 
also (I think) for tape drives.




-- 

Cheers
John

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