Difference between IDE and SCSI ??

Tim ignored_mailbox at yahoo.com.au
Sun Feb 3 03:57:30 UTC 2008

On Sat, 2008-02-02 at 13:39 -0500, William Case wrote:
> Can someone briefly explain

On this list?  ;-)

> to me the difference between an IDE (ATA) and a SCSI device.

They're different systems.  At the simplest description, they use
different connectors, and you can't use a IDE device on a SCSI bus, nor
vice versa.  This is square peg and round hole territory.

Getting more technical, it's also about the methodology of talking to
the device.  So it's not just a case of having the same signals in
different connectors.

If you can remember back to when there were at least four or five
different types of CD-ROM connections, none compatible with each other,
and some hard to tell which they were, we finally got a mostly
standardised ATAPI system.  And what's that?  An IDE style of connection
to a drive that's pretending to be SCSI (it uses the commands that SCSI
does, and acts like a SCSI drive, but using the IDE connection).  And
how did Linux handle that?  Sometimes you talked to your CD-burner as if
it were an IDE (ATAPI) device, other times as if it were SCSI.

Then the high speed UDMA IDE drives started coming out (the ones that
use the 80-wire connection), and we got computers with new interfaces to
handle them.  They, mostly, show up on a system as SCSI devices.

Then we had things like IDE hard drives in an external box, that
connects to the computer using a USB connector, and appearing to the
computer as if it were a SCSI device.

Now we have SATA, which is a move away from the parallel ATA to a
serialised ATA system, and again, the drive is acting in a SCSI manner.

You didn't expect things to get less confusing, did you?  :-\

> 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)?

I'd say, to explain it simply, all of the above.  You use an IDE drive
on an IDE drive interface, using IDE leads, etc.

> Does SCSI refer to a set of protocols used when designing the device?
> Or, to a specific driver design?

As a simple explanation, there's two aspects to SCSI.  The commands used
to use the drive, and the bus used to connect it.  They go together,
mostly.  Though, now, there's more and more devices that talk SCSI, but
are connected in different manner than traditional SCSI devices.

> Can you have an IDE device without SCSI?
> Or, can you have a SCSI device without it being IDE?

Yes, they're two different systems.  They're not interchangeable, and
one doesn't depend on the other being present.  And, at least in the
olden days, it wasn't too common to find a personal computer that had
both types at the same time.

Generally speaking, if someone talks about a SCSI drive, they're talking
about a SCSI drive that connects via a SCSI bus.  Likewise, for IDE.
Unless they're odd, they won't be meaning a SATA drive, or any other
drive, simply because of the commands it might use.  Most people call
the drive for how you connect it up.

Just to be confusing, there's two ways to connect an IDE drive, to a 40
wire IDE port (whether using a 40 or 80 wire cable), or the 44 pin
connector inside laptops (it has power, as well as data in the lead).
Just one way to connect a USB drive, to a USB connector (though I don't
ever recall seeing a genuine USB harddrive, they're supposed to exist).
Just one way to connect firewire drives.  There's two ways to connect a
SATA drive, internal and external SATA connectors are physically
different.  And there's several ways of connecting SCSI drives -
internally, a 50-wire ribbon is usual, but externally there's the 25 &
27 pin D connectors, small 50 pin connectors, large 50 pin connectors,
and several other connector types.

I would have though that this, would have answered some of your
questions about SCSI:

> 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.

It's a physical and technical standard.

(This computer runs FC7, my others run FC4, FC5 & FC6, in case that's
 important to the thread.)

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