eSATA

Standardized in 2004, eSATA (e standing for external) provides a variant of SATA meant for external connectivity. While it has revised electrical requirements and the connectors and cables are not identical with SATA, the protocol and logical signaling are compatible on the (internal) SATA level:

• Minimum transmit potential increased: Range is 500–600 mV instead of 400–600 mV.
• Minimum receive potential decreased: Range is 240–600 mV instead of 325–600 mV.
• Identical protocol and logical signaling (link/transport-layer and above), allowing native SATA devices to be deployed in external enclosures with minimal modification
• Maximum cable length of 2 metres (6.6 ft) (USB and FireWire allow longer distances.)
• The external cable connector equates to a shielded version of the connector specified in SATA 1.0a with these basic differences:
  • The external connector has no "L"-shaped key, and the guide features are vertically offset and reduced in size. This prevents the use of unshielded internal cables in external applications and vice-versa.
  • To prevent ESD damage, the design increased insertion depth from 5 mm to 6.6 mm and the contacts are mounted farther back in both the receptacle and plug.
  • To provide EMI protection and meet FCC and CE emission requirements, the cable has an extra layer of shielding, and the connectors have metal contact-points.
  • The connector shield has retention springs in on both the top and bottom surfaces.
  • The external connector and cable have a design-life of over five thousand insertions and removals, whereas the internal connector is specified to withstand only fifty.

Aimed at the consumer market, eSATA enters an external storage market served also by the USB and FireWire interfaces. Most external hard-disk-drive cases with FireWire or USB interfaces use either PATA or SATA drives and "bridges" to translate between the drives' interfaces and the enclosures' external ports; this bridging incurs some inefficiency. Some single disks can transfer 157 MB/s during real use,^[8] about four times the maximum transfer rate of USB 2.0 or FireWire 400 (IEEE 1394a) and almost twice as fast as the maximum transfer rate of FireWire 800. The S3200 FireWire 1394b spec reaches ~400 MB/s (3.2 Gbit/s), and USB 3.0 has a nominal speed of 5 Gbit/s. Some low-level drive features, such as S.M.A.R.T., may not operate through some USB [2] or FireWire or USB+FireWire bridges; eSATA does not suffer from these issues provided that the controller manufacturer (and its drivers) presents eSATA drives as ATA devices, rather than as "SCSI" devices, as has been common with Silicon Image, JMicron, and NVIDIA nForce drivers for Windows Vista. In those cases SATA drives will not have low-level features accessible. Firewire's future 6.4 Gb/s (768 MB/s) will be faster than eSATA I. The eSATA version of SATA 6G will operate at 6.0 Gb/s (the term SATA III is being eschewed by the SATA-IO to avoid confusion with SATA II 3.0 Gbit/s, which was colloquially referred to as "SATA 3G" [bps] or "SATA 300" [MB/s] since 1.5 Gbit/s SATA I and 1.5 Gbit/s SATA II were referred to as both "SATA 1.5G" [b/s] or "SATA 150" [MB/s]). Therefore, they will operate with negligible differences between them.^[21] Once an interface can transfer data as fast as a drive can handle them, increasing the interface speed does not improve data transfer.
Most computers have USB ports, and many computers and consumer electronic appliances have FireWire ports, but few devices have external SATA connectors. For small form-factor devices (such as external 2.5-inch (64 mm) disks), a PC-hosted USB or FireWire link can usually supply sufficient power to operate the device. However, eSATA connectors cannot supply power, and require a power supply for the external device. The related eSATAp (but mechanically incompatible) connector adds power to an external SATA connection, so that an additional power supply is not needed.^[22] Some e-sata ports double as eSATA/USB.
Desktop computers without a built-in eSATA interface can install an eSATA host bus adapter (HBA); if the motherboard supports SATA, an externally-available eSATA connector can be added. Notebook computers can be upgraded with Cardbus^[23] or ExpressCard^[24] versions of an eSATA HBA. With passive adapters, the maximum cable length is reduced to 1 metre (3.3 ft) due to the absence of compliant eSATA signal-levels.