[vox-tech] Useless IDE hard drive info... pin 21.

[vox-tech@lists.lugod.org]

Okay I figured I'd mail this off for interest sake... 

  Many of you may have noticed that IDE hard drives are missing
one of the 40 pins in the connector and opposite the missing pin 
there is a notch in the frame around the pins... both of those 
features are supposed to act as wards against someone plugging 
the cable in backwards.

  To work with the wards mentioned above some IDE cables have a red strip,
a extra bit of plastic plugging one of the pin holes, AND a chunk of
plastic attached to the outside of the frame (which goes in the notch
mentioned above).  I say some because some cables don't have any of
those, most have a pick-and-match sort of set of features.

  Some of you may have found that if you plug a cable with no wards in 
backwards the system tends to not POST... (power it on and nothing 
appears to happen, often the drives don't even spin up).

  Well maybe one or two of you have wondered what would happen
if you take a cable with the little bit of plastic plugging the
pin hole (but no chunk of plastic on the outside) flip it around,
so it is backwards, then shove it hard enough into a hard 
drive that it seats.

  If you are luckiest you will just bend the hard drive pin that 
is in the way.  If you are un-luckiest the pin will break off.
  In some cases the pin will push into the frame of pins and break contact
with the drive controller card.  Well the surprise is that if you go into
the BIOS and disable DMA mode the drive is mostly functional...


  You will see that pin 21 is the DMA Request pin... that tells the drive a 
DMA mode request is coming, without it no DMA transfers work but you can 
use one of the slower PIO modes.

  http://www.cablingdirectory.com/pinouts/internal/ATA40PinCablePinout.htm

  The pin-out list from this page is correct, the diagram is not... 
from what I understand IDE pins are numbered one above the next, not
length wise.  So it is like:
  1  3  5
  2  4  6
not like:
  1  2  3
 21 22 23


  If you decide that you want to repair this, you'll notice that the hard
drive controller card attached to the pin frame, the card tends to be
attached to the hard drive body via some torc screws (something smaller
than size T-10, but if you push real hard a T-10 (which can be found at 
most hardware stores) will spin the screw).  After taking the card off the
drive, pushing the pin back into place, bending the cross metal piece
back into place, and use of a soldering iron with a fine tip... the
drive can resume DMA mode.

    Wondering why hardware gets on my nerves sometimes,
      Mike

ps:
  If you attempt the repair, just be sure to cover the card body with a 
piece of newspaper so if you touch the iron to the board you don't melt it.

pps:
  This is why I like cables without the little bit of plastic blocking a
pin, but with a big chunk of plastic on the outside.