More on master and slave...and cable select

Subject: More on master and slave...and cable select
From: "David Neeley" <dbneeley -at- gmail -dot- com>
To: techwr-l -at- lists -dot- techwr-l -dot- com
Date: Mon, 19 Mar 2007 11:54:33 -0500

First, I should say that learning about the ATA interface (including IDE
hard drives) is of limited utility. Intel's latest motherboard chipset,
shown last week at CEBIT, has *NO* IDE host adapter. It supports only SATA
(and perhaps serial SCSI, although I am not sure of that...that may be left
to the province of a separate card when needed). We are already seeing SATA
CD and DVD drives beginning to appear.

(Note that although commonly referred to as an "IDE controller"--the plug
and associated hardware on the motherboard is a "host adapter" and *not* a
controller at all. It is merely an entry port onto the peripheral bus for
the drives.)

It turns out that serial transfer can be much quicker than parallel,
although that may seem counter-intuitive.

In the ever-present quest for more performance, hard drives are also
beginning to have much larger solid state memory incorporated--far larger
than traditional cache memory. These hybrid hard drives will use flash (or
other non-volatile memory coming out soon) to store frequently used
data--such as the major portion of the operating system--so that retrieval
is much faster than with the physical access part of the disk. This will
enable very fast booting, for instance, while reducing power requirements
further than is possible at the moment.

You should understand that the IDE interface is a parallel interface.
Originally, it was available with either a 40-pin cable or a 44-pin cable
(the latter used the extra four leads to supply power for those drives
without separate power connectors). By the time the ATA-4 spec came along, a
new cable was introduced with 80 leads. Every other one was a ground cable,
to preserve data integrity as the interface became faster and faster. That
was the version of the spec that also incorporated ATAPI support, for full
inclusion of more standardized CD and other peripheral support. Prior to
that, you may or may not have had ATAPI support, and not long before *that*
there were many non-standard interfaces for CDs, tape drives, etc.

Cable select requires a special cable--not just any ol' IDE cable you happen
to have sitting around. It was primarily implemented so the large OEM
manufacturers had one less setting to get right when they installed hard
drives on the assembly line. There is no other advantage to that setting.
Why Al's new drive did not work in the chain that had been working with the
cable select setting I do not know--unless he also substituted the new cable
for the old one when he replaced the drive.

By the way, the preferable way to set up an IDE drive cable is to put the
master on the end, with the slave drive in the middle. Theoretically, that
will give the best performance, although it obviously works both ways.

Most folks that use two hard disks and one or two CD drives have the hard
disks on one cable and the CDs on the other. That, too, is seldom the best
method--although the physical setup of doing it better can be tricky.
Remember that the two disks cannot transfer data on the same physical cable
at one time--so in some cases it is much quicker to have them on separate
channels. Let us say you have one drive for the operating system and the
other for data--a relatively common setup. When they can both operate
simultaneously, it should stand to reason that they can work more quickly
being on separate cables entirely.

Similarly, if you have the two CD devices on the same cable, there are times
when they cannot operate optimally, either. On average, then, it is best to
have one hard drive and one CD on one channel, the others on the other
channel. Which CD (if you have two) you have on which channel depends upon
which one is the most commonly used to transfer data to the hard
drive...those should be on separate cables.

Again, this is often difficult to attain using garden variety cables in the
typical case--but it *is* optimal for performance, assuming you can route
the cables properly in a manner that still allows for proper cooling of the

Now, back to the job search...


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