15 May
2014
15 May
'14
1:06 p.m.
On May 15, 2014, at 14:47, Mouse <mouse at rodents-montreal.org> wrote:
Both are correct. It's an important distinction to make;
if you're co-opting a 40-pin 100-mil connector for some
other purpose because IDE cables are common as dirt,
you need to be aware of the multitude of pins that
are tied together at the connector as ground. If you're
clever and have enough pins to spare, you can make
use of the improved signal quality, but you can do some
real damage if you just try to reuse them willy-nilly.
In any case, you can use either cable with most IDE drives,
but you won't get above ATA-33 without using an
80-conductor cable. 80-conductor cable, as long as it's
following the standard, also has the "cable select" conductor
disconnected for the far end plug. Also, in my personal
experience, 80-conductor cables have the "key" pin
plugged a lot more often than their 40-conductor brethren,
making them a right pain when you have a board that
doesn't have the key pin snipped.
- Dave
I don't
recall the specifics as to why they used 80 conductor wired
for a 40 pin connection; I believe it had something to do with
reducing crosstalk & interference at the higher speeds of the (at
that time) newer hard drives.
The version I saw is that the other half of the wires are grounded at
the CPU end and exist as shielding for the other lines - basically, as
you say, to reduce crosstalk and other intra-line interference.
While I'm a good deal less sure of this, I also have the impression
that there is some other difference - an otherwise unused pin grounded
in the connector? - which (a) serves to notify the host electronics
that it's got an 80-wire cable and (b) means that the host connector is
operationally different from the drive connectors (which isn't the case
for 40-wire cabling).