Hardware Reference
In-Depth Information
conductor cable.
Note the keying on the 80-conductor cable that is designed to prevent backward installation. Note
also that the poorly constructed 40-conductor cable shown in
Figure 7.6
lacks keying. Most good 40-
conductor cables include the keying; however, because it is optional, many cheaply constructed
versions do not include it. Keying was made mandatory for all 80-conductor cables as part of the
standard.
Longer or Rounded Cables
The official PATA standard limits cable length to 18 inches (0.46 meters); however, many of the
cables sold are longer, up to 36 inches (0.91 meters) or more in length. I've had many readers write
me questioning the length, asking, “Why would people sell cables longer than 18 inches if the
standard doesn't allow it?” Well, just because something is for sale doesn't mean it conforms to the
standards and will work properly! I see improperly designed, poorly manufactured, and
nonconforming items for sale all the time. Many people have used the longer cables and their systems
seem to work fine, but I've also documented numerous cases where using longer cables has caused
problems, so I decided to investigate this issue more thoroughly.
What I discovered is that you can use longer 80-conductor cables reliably up to 27 inches (0.69
meters) in length, but 40-conductor cables should remain limited to 18 inches, just as the standard
indicates.
In fact, an attempt was made to change the PATA standard to allow 27-inch cables. If you read
www.t13.org/Documents/UploadedDocuments/technical/e00151r0.pdf
, you'll see data from a
proposal that shows “negligible differences in Ultra DMA Mode 5 signal integrity between a 27-inch,
80-conductor cable and an 18-inch, 80-conductor cable.” This extended cable design was actually
proposed back in October 2000, but it was never incorporated into the standard. Even though it was
never officially approved, I take the information presented in this proposal as empirical evidence for
allowing the use of 80-conductor cables up to 27 inches in length without problems.
To that, I would add another recommendation, which is that in general I do not recommend “rounded”
ATA cables. A rounded design has not been approved in the ATA standard, and there is some
evidence that it can cause problems with crosstalk and noise. The design of 80-conductor cables is
such that a ground wire is interspersed between each data wire in the ribbon, and rounding the cable
causes some of the data lines to run parallel or adjacent to each other at random, thereby causing
crosstalk and noise and resulting in signal errors.
Of course, many people use rounded cables with success, but my knowledge of electrical engineering
as well as the ATA standard has always made me somewhat uncomfortable with their use.
PATA Signals
This section describes in more detail some of the most important PATA signals having to do with
drive configuration and installation. This information can help you understand how the cable select
feature works, for example.
Pin 20 is used as a key pin for cable orientation and is not connected to the interface. This pin should
be missing from any ATA connectors, and the cable should have the pin-20 hole in the connector
plugged off to prevent the cable from being plugged in backward.
Pin 39 carries the drive active/slave present (DASP) signal, which is a dual-purpose, time-
multiplexed signal. During power-on initialization, this signal indicates whether a slave drive is
