Hardware Reference
In-Depth Information
I once had a student ask me how to tell an 80-conductor cable from a 40-conductor cable.
The simple answer is to count the ridges (conductors) in the cable. If you count only 40,
it must be a 40-conductor cable, and if you count to 80...well, you get the idea! If you ob-
serve them side by side, the difference is clear: The 80-conductor cable has an obviously
smoother, less ridged appearance than the 40-conductor cable.
Note the keying on the 80-conductor cable that is designed to prevent backward installa-
tion. Note also that the poorly constructed 40-conductor cable shown in
Figure 7.6
lacks
keying. Most good40-conductor cables include the keying; however,because it is option-
al, 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 improp-
erly designed, poorly manufactured, and nonconforming items for sale all the time. Many
peoplehaveusedthelongercablesandtheirsystemsseemtoworkfine,butI'vealsodoc-
umented 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
integritybetweena27-inch,80-conductorcableandan18-inch,80-conductorcable.”This
extendedcabledesignwasactuallyproposedbackinOctober2000,butitwasneverincor-
porated into the standard. Even though it was never officially approved, Itake the inform-
ationpresentedinthisproposalasempiricalevidenceforallowingtheuseof80-conductor
cables up to 27 inches in length without problems.
Tothat,Iwouldaddanotherrecommendation,whichisthatingeneralIdonotrecommend
“rounded”ATAcables.AroundeddesignhasnotbeenapprovedintheATAstandard,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
