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Products > Interface > Serial ATA >

What is Serial ATA ?


The widespread popularity of broadband and digital media has fuelled a dramatic expansion in hard-disk applications. Moreover, the rising density and speed of hard-disk drives is creating an increased demand for host-drive interfaces with faster transfer rates.

Until recently, the Parallel ATA interface was the most common type of storage device interface. However, as hard-disk data transfer rates started approaching the Parallel ATA interface's maximum transfer rate of 100 MBps, the performance improvement has reached to a technical limit, and thus the need for a breakthrough in storage device interface technology. It was at this point, in 2001, that with an eye to the future, a scalable, performance-enhancing interface standard—Serial ATA—was established.

While maintaining software compatibility with Parallel ATA, the Serial ATA standard applies high-speed serial data transfer technology to the I/O interface to archive a data transfer rate of 300 MBps – three times as fast as Parallel ATA (Ultra ATA/100). Serial ATA is also superior in terms of reliability and space saving. 2005 will undoubtedly see Serial ATA become the PC storage interface standard, and will herald the start of its widespread employment in storage devices for digital home appliances.


Features of Serial ATA

Serial ATA Parallel ATA (Ultra ATA/100)
Transfer rate 150 or 300 MBps 100 MBps
Signal lines 2 pairs (4 lines) per channel 40 lines per channel
Cable type Shielded 4-core parallel coaxial cable 80-core flat cable
Maximum cable length 1- to several-meter-cable, backplane (up to 40 inches) 46 cm (18 inches)
Connection type 1-to-1 (1 device per cable, point-to-point connection)
1-to-1 (1 device per cable, point-to-point connection)

1-to-15 (PM*1 used)
1-to-15

2-to-1 (PS*2 used)
2-to-1 		Shared bus connection
Master-slave (2 devices per cable)
Connection type
Number of connected HDDs*3 120 devices (= 15 devices per PM x 8 channels per host)
Number of connected HDDs 		24 devices (= 2 devices per channel x 12 channels per host)
Number of connected HDDs
Major applications PC internal, PC external, servers, storage PC internal
Hot plug Possible Not possible
Command queuing*4 Available (NCQ) Partially available (TCQ)
Reliability (CRC support) Supported Partially supported (ATA register data not supported)
Cost performance Good Good



Notes

  1. PM: Port multiplier
    Enables an increase in the number of connectable devices. Similar to Ethernet or USB hub.

  2. PS: Port selector
    A switch used to share a device between two hosts. Usually used to raise the reliability of the system by switching to the remaining host when one of two hosts is faulty, thus creating a "fault-tolerant" system.

  3. Number of connected HDDs
    Although not restricted in either specifications of Serial ATA or Parallel ATA, this number is estimated as the practical limit per host.
    It is assumed that in the case of Serial ATA, a 1-to-15 port multiplier (PM) is connected to each channel of an 8-channel host.

  4. Command queuing
    A method whereby several commands are issued by the host without waiting for completion of disk reading/writing by the HDD device. The HDD device reads/writes the disk in the optimal order, thus improving the performance. TCQ (tagged command queuing) used by Parallel ATA did not gain popularity because hard-disk manufacturers disliked the effect-reducing large overhead. However, with NCQ (native command queuing) used by Serial ATA, manufacturers had an input from the initial specification creation stage, resulting in expressed support from a large number of vendors.