HDD Cabling 101

Are you confused by the jargon that surrounds hard drives and its cabling? Then let me try and set your mind at ease – I don’t intend for this to be an exhaustive account of all the different cabling / interface types* but will concentrate on those associated with current and fairly recent desktops. So here is my HDD Cabling 101. Or as we Brits would call it, an introduction.

In short there are two main types, IDE and SATA. However these are often given alternative names and in some cases have been renamed to try and avoid confusion but in reality have only added to it. Physically the two cables are radically different in size and make up with the older IDE cable having 40 pins (some modern iterations have 80 wires but retain the 40 pins) on the end of a flat, typically grey, ribbon cable. A SATA cable on the other hand, is much more compact having only 7 (or 8 if you include the coding notch) pins.

As with any 101 we can’t totally avoid history. The IDE was originally a Western Digital design and stood for Integrated Drive Electronics. That said, I was never a huge fan of history and there are lots of other pages on the web that can give you much greater detail. Suffice to say WD invented the IDE and that has subsequently been known down
the years as:

  • IDE – Integrated Drive Electronics
  • EIDE – Enhanced IDE
  • ATA – AT** Attachment
  • ATAPI – AT Attachment Packet Interface
  • ATA-1
  • ATA-2 and
  • PATA – Parallel ATA

It is this last that has really lead to the confusion many of my friends have today. PATA, or Parallel ATA is a retroactive name given to the IDE/AT interface back in 2003 when SATA was introduced to the market.

40 & 80 wire ATA cables

40 & 80 wire ATA cables

In reality, what you need to know for all of the above is that they all use the 40 pin flat ribbon cable to interface to your computer. The only change to this standard was the introduction of the 80 pin cable. This was introduced with the Ultra DMA/33 (UDMA) standard. As mentioned above, the cable doubles its number of wires to 80 but retained the same 40 pin connectivity. This allowed for backwards compatibility and meant if I purchased a UDMA drive but only used a 40 pin cable then it would gracefully degrade to the slower speed. So what did / do the extra 40 wires do? They provide what is known as capacitive coupling which in essence reduces the crosstalk across the
original 40 wires and allows for the higher data transfer rates.

The only other item to note is that all of these older style drives use a 4 pin Molex connector for power.

Serial ATA (SATA) is the relatively new boy on the block, having been released to the world in 2003. You will see it referred to as:

  • SATA
  • SATA 1
  • SATA 1.5
  • SATA II
  • SATA 2
  • SATA 3
  • SATA 6

(colour groupings indicate these are functionally identical, just a different naming convention)

The basic requirement was to give desktop hard drives a boost in data throughput without huge increases in cost – prior to SATA the debates used to centre around why a home user would want the power, expense and confusion that was SCSI. With SATA it started to eliminate those arguments and allow home users access to the power, onfigurability
and scalability of technologies like SCSI. Choices like hot swapping or drive redundancy (also know as RAID) were suddenly within the reach of mere mortals and by extension smaller Church / Business / Non-Profit setups.

SATA cable

SATA cable

The cable for a SATA drive is radically different though and is only 7 pins wide with an equally compact connector. The other differences between SATA and PATA cables is that SATA is a one to one affair and the length of cable you can use is  twice that of an IDE ***. This eliminates cabling and jumper confusion and also eases cabling routes through the PC. The main issue I am faced with by friends, etc is that not all SATA drives are made equal and the problems arise when you try to connect power to them.

The early generation SATA drives still use the trusty 4 pin Molex – however more and more they are adopting the newer 15 pin wafer like connector of the data cables. Like the Molex they are keyed so as to make incorrect alignment and mis-identification harder. For those that don’t yet have the latest PSU’s cable of handling the newer power sockets directly then fear not as there are adapters that are easily and cheaply available to convert from the 4 pin Molex to the 15 pin SATA power strip.

And that, ladies and gents, is that. I trust you’ve found this useful as well as a little educational and will hopefully serve to clarify what cable you require for which hard drive.

* Should you really wish to know, then ask away and I’ll write a “HDD Cabling 201″ post – though I’m not sure even I fancy that task.
** AT was always thought to refer to the fact that it was introduced for the IBM PC/AT (Advanced Technology) family of computers but to avoid any trademark issues it was simply known as the AT Attachment or ATA.
*** Though in reality I have used far longer than the IDE specified maximum length of 45cm without problems.

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0 Responses to HDD Cabling 101

  1. Very good article — I haven’t kept up on all the changes/developments so this is a good refresher. I guess I’ll work on building my next NAS with SATA. :-)

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