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Old 02-03-2008, 02:24 AM
Tim
 
Default Difference between IDE and SCSI ??

On Sat, 2008-02-02 at 11:57 -0800, Les wrote:
> SCSI is a serial system, or at least it can be.

Pardon? Usually, when one has a data bus for several parallel data
lines at once, one refers to it as parallel.

Serial - one data line, that sends bits sequentially.
Parallel - several data lines, that send bits simultaneously.

The most usual way of finding SCSI used with drives was as a parallel
bus, anywhere from 25 to 50 pins per connector.

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Old 02-03-2008, 02:57 AM
Tim
 
Default Difference between IDE and SCSI ??

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.

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Old 02-03-2008, 03:08 AM
Les Mikesell
 
Default Difference between IDE and SCSI ??

William Case wrote:


My original question was not to have someone else do my investigation
for me, but to either give me reassurance that all three, IDE, PATA and
SCSI are exactly the same thing and can be used interchangeably on a
current system, or to point out where I have missed the difference and
when I should use one term over the other.


From a system/filesytem/user perspective they are interchangeable. The
kernel only recently started identifying IDE's as /dev/sdX devices
(related to merging some of the SATA handling, I believe), so that part
is a little confusing. There are physical differences that you would
have to keep straight if you had both when connecting cables and telling
your bios which to boot.


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Old 02-03-2008, 03:13 AM
Les Mikesell
 
Default Difference between IDE and SCSI ??

Tim wrote:


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? :-


And SAS (Serial attached SCSI) which is oddly enough, interface
compatible with SATA and some controllers can handle both - confusing
and converging at the same time.


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Old 02-03-2008, 06:58 AM
Peter Larsen
 
Default Difference between IDE and SCSI ??

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Tim wrote:
> On Sat, 2008-02-02 at 11:57 -0800, Les wrote:
>> SCSI is a serial system, or at least it can be.
>
> Pardon? Usually, when one has a data bus for several parallel data
> lines at once, one refers to it as parallel.
>
> Serial - one data line, that sends bits sequentially.
> Parallel - several data lines, that send bits simultaneously.
>
> The most usual way of finding SCSI used with drives was as a parallel
> bus, anywhere from 25 to 50 pins per connector.

No longer. SAS = Serial SCSI. I can barely get any high-end servers
delivered without SAS anymore. The hardware is cheaper and simpler to
produce than parallel. Also, you have each device on their own line -
just like with SATA. A cable error no longer renders the whole 15
devices useless.

- --
Peter Larsen
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Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org

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Old 02-03-2008, 07:16 AM
Bob Kinney
 
Default Difference between IDE and SCSI ??

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?

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)?

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

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

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.


I like to use automobiles for analogies to computers. The data storage
subsystem can be compared to any subsystem in a car, whether it be fuel,
ignition, electrical, or seating.

For simplicity's sake, let's just focus on the data storage subsystem:
the hard disk drive. IDE and SCSI are two different, and incompatible
data storage subsystems, and the differences can be compare to VHS vs Beta.
Similarly, (IMO), the technically superior system has lost out.
I still cannot fathom why modern computers still do not use SCSI.

In the old days, computers needed a controller card to manage how the
information it needed to store was physically stored on the hard disk drive.
This controller actually told the drive how to physically manage the placement
of the data on the drive.

SCSI changed that in some respects, in that the connected drives themselves
had some "smarts" built into them, and managed the data associated with it
by itself. The controller card simply passed commands to the drives that
told it what to store or what to retrieve. SCSI offered the ability to hook
up a maximum of seven hard disk drives in a daisy chain fashion.

But then came AOL, cheap computers, and droves of dumbshits who found it
difficult to handle the ability to keep track of the requisite drive numbering,
and this, combined with most users' lack of a need for seven hard drives
provided the incentive to create a simpler, and possibly cheaper, way of
connecting disk drives to your system.

The result was IDE (Integrated Drive Electronics), a system that allowed your
computer to interface directly with the drives, by putting the "smarts" right
on the drive itself. This system provided the ability to connect up to two
drives on a channel, designated "master" and "slave." The IDE system later
was renamed "ATA," and I cannot tell you what the acronym stands for, but for
all intents and purposes IDE = ATA.

With one exception, both SCSI and IDE/ATA are electronically parallel systems.
SCSI is NOT serial, except for fiber channel.

So, to answer your main questions:

>Does IDE refer to the physical device?
>Or, specifically just to the bus used?

IDE, or ATA, or in most historic cases, PATA, refers to a physical and/or
electrical bus that provides connectivity for drives that are designed for
that bus.

>Or, to the driver for the device?

Possibly, in certain contexts. But strictly speaking, no. It is a physical
and electrical specification.

Or, the type of interface (plug)?
Yes.

In terms of the operating system, the driver must know how to "speak" IDE
in order to save/retrieve data on an IDE device, and will use a different
driver
to communicate with a SCSI controller, just as it uses another driver to
communicate with your video, chipset, processor, and other subsystems.

I can understand your confusion, because Fedora has begun labeling *all*
internal storage devices as "sd", as in "SCSI device", whether or not it
actually is on a SCSI or IDE chain.

Hope this helps.

--bobcat





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Old 02-03-2008, 01:21 PM
John Summerfield
 
Default 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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Old 02-03-2008, 01:27 PM
John Summerfield
 
Default Difference between IDE and SCSI ??

Les wrote:


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)


There was also EIDE.



SCSI is a serial system, or at least it can be. It allows virtually

With 50 and 68 pins it wasn't serial;-)


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.


It's even made its way to the IBM mainframes; folk attach them to their
zSeries systems.


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John

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Old 02-03-2008, 01:46 PM
John Summerfield
 
Default Difference between IDE and SCSI ??

Aaron Konstam wrote:

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.



the AT Attachment interface didn't arrive until the PC/AT.

The PC didn't (originally) have disk at all, the PC/XT was the first
with disk, and it used the ST-506 interface.


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Old 02-03-2008, 01:59 PM
Timothy Murphy
 
Default Difference between IDE and SCSI ??

Bob Kinney wrote:

> I can understand your confusion, because Fedora has begun labeling *all*
> internal storage devices as "sd", as in "SCSI device", whether or not it
> actually is on a SCSI or IDE chain.

Surely it is more than just labelling.
Fedora (and Linux generally, to a lesser extent)
now uses SCSI emulation to talk to IDE disks.

Incidentally, your posting was difficult to follow.
You seemed to be replying to yourself.

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