Re: Difference between IDE and SCSI ??

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.




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Cheers
John

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