SSD Interfaces

Solid State Drives are available with a variety of system interfaces based primarily on the performance requirements for the SDD in the system. Also, since SDDs are generally used in conjunction or interchangeable with magnetic disk drives, a common mass storage bus interface is used in most cases. This also allows the system software to manage both drive types in a similar way, making system integration nearly plug-and-play. These common interface types include SATA, Fibre Channel, SAS, and ATA/IDE. 

There are also interfaces initially designed for other purposes but have been adopted by SSDs in some cases. On the low end, the Universal Serial Bus (USB), initially designed for managing peripherals like printers, keyboards, and mice, soon was adopted as a convenient and economical means to provide removable storage using FLASH Thumb Drive as a replacement for floppy drives in many systems. This interface is also commonly used as a secondary bus option on many form factor SSD drives. Form factor drives include 1.8-inch. 2.5-inch, and 3.5-inch form factors.

The IEEE 1394 Firewire interface is another special application case. Invented by Apple Computer, this interface was designed initially to allow streaming digital video (DV) communications between professional cameras and computers as a replacement for analog A/V interfaces and as an alternative to parallel busses such as the SCSI (Small Computer Systems Interface). As disk storage speeds advanced, system designers soon discovered great benefits in using a 1394 Firewire interface on disk drives to allow real-time DV recording and playback in professional DV editing systems, and even sometimes in the camera itself.    
 
There are also bridge boards that can transition from one interface to another. This is beneficial if the native interface on the SDD, such as IDE, does not match the common interface used in the system, such as IEEE 1394 or USB. The use of bridge boards does introduce performance limitations in most cases, and compatibility across a wide range of configurations can be a problem. Most system designers try to avoid the use of bridge boards if possible.
The interfaces used by most of the SDDs on the market today are summarized in the table below.
 
Interface
 
Description
Details
Standards
SATA
Serial ATA
 
 
A serial implementation of the Parallel ATA interface (also called the IDE) used on floppy drives and early magnetic disk drives.
A point-to-point system that utilizes a 7-conductor cable with two differential [Tx/Rx] pairs to each drive. A separate 15-pin power connector is used.
 
Data throughput potential is 150, 300, or 600 Mbytes/s, based on the version used. The max unshielded cable length is 1 meter, 2 meters if shielded, or up to 8 meters using the xSATA version. 
Serial ATA Revision 3.0, 5/2009
T13 committee
FC
Fibre Channel
A multiple rate serial  Gbit,
Multiprotocol interface.
 
Fibre Channel was developed as a modern serial interface for Storage Area Networks (SAN) where an array of drives in the same cabinet, room, or facility where shared by a set of servers.
A bidirectional interface that supports SCSI, IP, ATM, HIPPI, and/or IEEE802.2 over copper or fiber optic cables. Rates from 1 to 10 Gbits/s based on version used.
 
Twister pair: 33 meters
Coax: 75 meters
Fiber Optic: 10 kilometers
 
In addition, FC can be configured in an arbitrated loop containing up to 127 devices, Point to Point,  or in a switched fabric.
 
SSD drives using Fibre channel support the twisted pair electrical interface and most use the 40-pin SCA-2 connector. This connector contains 4 twisted pairs for the base FC signals as well as various configuration, power, and ground pins. Maximum length 1 meter.
 
SSD drives utilize the Fibre Channel Protocol FCP-SCSI protocol command set. (SCSI - Small Computer System Interface).
T10 committee
SAS
Serial Attached SCSI
A 3 or 6-Gbit rate serial implementation of the 8/16/32-bit wide parallel Small Computer Systems Interface (SCSI)
A point-to-point full duplex system. It utilizes a 4 two differential [Tx/Rx] pairs to each drive. A separate 15-pin power connector is used when combined with SATA, or can be integrated into a single connector. There are various connectors utilized.
 
Data throughput potential is 3 or 6 Gbits/s with 12 Gbits/s planned for 2012 release.
 
 The max cable length is 10 meters. 
Serial Attached SCSI - 2.0 {SAS-2.0), 11/2007
T10 committee
ATA/IDE
Advanced Technology Attachment/Integrated Drive Electronics
Various generations of the parallel drive interface used in PCs since 1986.
 
Speeds from 16 to 133 Mbytes/s possible.
A master-slave 16-bit parallel bus allowing up to 2 devices controlled by one master. 
 
A single 40-pin connector is used with a maximum 18-inch cable length standard. Enhanced versions have 80 pins.
 
Adopted for PCMCIA and Compact Flash SSD devices.
 
ANSI X3.221-1994
T13 committee
PCIe
Peripheral Component Interconnect Express
A serial version of the PCI bus. A hub is used on the backplane to allow data rates up to 4 Gbits/s per lane.
This is an internal interface, so a SSD would be on a circuit board and plugged into a PCIe slot in the motherboard.
 
 
PCI Express 2.0
PCISIG group
 
USB
Universal Serial Bus
A simple serial bus with integrated power.
 
Base data rate is 12 Mbits/s. Later versions support up to 480 Mbits/sec.
A 4-pin bidirectional serial bus using a hub and spoke topology in order to connect up to 128 devices under the control of one master controller.
 
The current USD 2.0 standard allows operation 5 meters per cable but supports a string of cables with intermediate hubs to achieve a 30 meter maximum cable length.
 
SSD thumb drives and many form factor drives provide a USD interface as a secondary access channel. This is possible since the controller chip used in many form factor SSD drives only needs to use a few pins to integrate this interface into the drive design.
USB 2.0 Specification USB.org
IEEE 1394
Firewire
A serial bus system originally designed for digital video  (DV) applications.
 
Allows rates from 400-3200 Mbits/s
 
Usually on a bridge card.
A point-to point half duplex system allowing cable lengths up to 4.5 meters. Also utilizes a hub/tree system like USB. Unlike USB, 1394 does allow peer-to-peer communications without intervention of the system processor.
 
It offers similar functionality as USB 2.0 but with more data saturation capability, higher power distribution capability, and higher data reliability – critical for many applications.
IEEE 1394

Related Guides:

Return to the SSD Guide