This question has been in my head for a while so I'm going to ask the accumulated wisdom of the forum for help! :~
I have two oldie IDE hard drive ribbon cables, one apparently has 40 pins and 40 conductors on the ribbon cable. On the other hand, the other newer one has 40 pins but 80 conductors, much thinner wires.
So why 40 pins and 80 conductors? How are the 80 conductors used?
Thank you!
Every other wire is connected to GND. This improves shielding and reduces crosstalk enabling the cable to carry signals at higher data rates. In a 40 conductor cable the data signals were all next to each other and would interfere with one another at high frequency. With an 80 pin cable they are each separated by a GND so the interference is less.
This issue bit me when trying to use a 80 conductor cable with 40pin connectors for a led cube I built.
The older IDE type with 40 conductor cable are a straight pin to pin connection at the connector ends so you can use the cable anyway you wish, but the cable with 80 conductors shares many pins at the connector ends for the common ground wires, so cannot be used as a 40 pin straight through connector cable.
Lefty
stimmer and lefty,
Thank you both, very much! I'll test my 80-wire cable to see which pins are all connected. So it seems I should go with the good-old 40-pin 40-conductor cables. Any source for new 40-pin cables or should I consider a crimper and a big roll of cable?
It seems that using a 80-wire cable is a bad idea for passing connections:
Here is from wiki:
Pin 19, 22, 24, 26, 30, and 40 are not separate, but connected to the ground.
Pin 20
In the ATA standard pin 20 is defined as (mechanical) key and is not used; i.e., this socket on the female connector is often obstructed, and a cable or drive connector with a pin in this position cannot be connected, making it impossible to plug in a connector the wrong way round. However, some flash memory drives can use pin 20 as VCC_in to power the drive without requiring a special power cable.
Pin 28
Pin 28 of the gray (slave/middle) connector of an 80 conductor cable is not attached to any conductor of the cable. It is attached normally on the black (master drive end) and blue (motherboard end) connectors.
Pin 34
Pin 34 is connected to ground inside the blue connector of an 80 conductor cable but not attached to any conductor of the cable. It is attached normally on the gray and black connectors.
If you are going to build a lot of custom length cable I would get
a roll and the IDC connectors. These crimp nicely using a bench
vise.
Just happened to end up with 40pin connectors on my
Sanguino compatible boards 
http://wiblocks.luciani.org/NB2/index.html
(* jcl *)