Definition of SAS in English

Serial Attached SCSI is short for SAS according to abbreviationfinder. It is a serial interconnection system between SCSI devices, successor to parallel SCSI (Small Computer System Interface), although it still uses SCSI commands to interact with SAS devices. Increases speed and allows quick connection and disconnection. It is compatible with Serial ATA (SATA) and the devices can communicate through protocols.


The organization behind the development of the SAS specification is the SCSI Trade Association. It is a California- based non-profit organization that was formed in 1996 to promote the use and awareness of Parallel SCSI.

The first version appeared at the end of 2003: SAS 300, which achieved a bandwidth of 3Gb / s, which considerably increased the speed of its predecessor (SCSI Ultra 320Mb / s). The next evolution, SAS 600, achieves a speed of up to 6Gb / s, while it is expected to reach a speed of around 12Gb / s.

One of the main characteristics is that it increases the transfer speed by increasing the number of connected devices, that is, it can manage a constant transfer rate for each connected device, in addition to ending the limitation of 16 devices existing in SCSI, it is for This means that SAS technology is expected to replace its predecessor SCSI.

The connector is the same as the SATA interface and allows to use these hard drives, for applications with less need for speed, saving costs. Therefore, SATA disks can be used by SAS controllers but not vice versa, a SATA controller does not recognize SAS disks.

The communication

Communication protocols

  • SSP (Serial SCSI Protocol), which allows the transport of SCSI commands between SCSI devices,
  • STP (Serial ATA Tunneled Protocol), which allows the transport of ATA commands to multiple SATA devices,
  • SMP (Serial Management Protocol), which allows the transport of domain management functions.

SAS devices can be Initiators (controllers), Targets (peripherals) or Target / Initiators (they can sometimes function as a controller and sometimes as a peripheral), and can support one or more transport protocols (SSP, STP and / or SMP).

SAS domain

A SAS domain is a system made up of SAS devices that communicate through an interconnection subsystem, which can be:

  • A set of physical links that interconnect a SAS Initiator with a SAS Target, or
  • A set of physical links and expanders that interconnect more than two SAS devices.

SATA Targets can be part of a SAS domain as long as they are connected to expanders that support the STP protocol.

Initiators that intend to communicate with SATA Targets in a SAS domain can only do so using the STP protocol (that is, controllers that only support the SATA protocol cannot be used in SAS domains, but only in SATA domains). The expanders that support STP act as a bridge between both protocols (STP and SATA), thus acting as a bridge between a SAS domain and one or more SATA domains.

Allowing SATA devices to connect to a SAS domain allows concurrent sharing of access to ATA peripherals by multiple STP Initiators, which cannot be done in normal, parallel, or serial (SATA) ATA domains.

The physical level

The phys and the physical links Each of the point-to-point connections in the interconnection subsystem is known as a physical link, and is made up of 2 pairs of differential signals, a pair to transmit in one direction and another pair to transmit in the direction. opposite, allowing information to be transmitted in both directions simultaneously.

The devices are electrically connected to a physical link through a transceiver called a phy. SAS devices can have one or more phys.

The normalized transmission speeds for the moment are 1.5 Gbps and 3 Gbps (1,500 and 3,000 Mbaud respectively), equivalent to a speed in the physical link of 150 and 300 MB / s respectively (since each character consists of 10 bits, due to the encoding used 8b10b). The information is transmitted in NRZ code, and the maximum differential voltage value allowed in the receiver is +/- 800 mV (the minimum value allowed depends on the speed of the link, being +/- 160 mV approximately for 1.5 Gbps and +/- 135 mV approximately for 3 Gbps). Physs can support different roles (Initiator or Target) and / or different protocols (SSP, SMP and / or STP), but they can only use one role and one protocol during each connection.

SAS cables and connectors

SAS supports 4 types of connections: 1- External connections via cable
External SAS devices (Initiators, Targets and Expanders) use a 25-pin connector, which includes 4 physical links (4 x 4 pins) and ground signals (9 pins), and connect to cables

External SAS, which can include conductors for one, two, three, or all four physical links. External SAS cables and connectors do not have power contacts or LEDs.

2- Internal connections direct
the targets SAS internal always use a connector 29 includes two pins that physical links (4 signal pins and 3 mass per link), power (14 pins in total to 12 V, 5 V, 3.3 V and ground) and Ready LED (1 pin). SAS connector panels always use a 29-pin connector to allow direct connection of the Target. Internal Initiators and Expanders can also use the 29-pin connector to allow direct connection of the Target.

SATA devices can be connected directly to a SAS connector panel or to a SAS expander with a 29-pin connector, as the 22-pin connector on the SATA device is physically compatible with the SAS 29-pin connector, using the power pins and of the primary physical link and leaving the 7 pins of the secondary physical link free (SATA devices only have one phy). The LED signal of the SAS connector matches the reserved pin of the power segment of the SATA connector.

3- Internal Single Port Cable Connections Internal single port
SAS cables use a 29-pin connector on the side of the Target, but only include conductors for power, the LED, and the primary physical link. These cables clear the secondary physical link of the Target.

On the other hand, the internal SAS Initiators and expanders that allow cable connection have 7-pin connectors identical to the signal connector of a SATA controller (1 physical link). The physical link conductors of the internal SAS cable terminate opposite the Target in a 7-pin connector identical to that used on the SATA signal cable.

This cable allows the connection of the primary physical link from an internal SAS Target to an internal Initiator or expander, and also allows the connection of a SATA device to an internal expander.

4- Internal Connections via Dual Port
Cable Internal dual port SAS cables use a 29-pin connector on the side of the Target, and include conductors for power, the LED, and both physical links. The conductors for each physical link of the internal SAS cable terminate opposite the Target in a 7-pin connector identical to that used on the SATA signal cable.

This cable allows the connection of the two physical links of an internal SAS Target to two ports of Initiators or internal expanders. It can also be used to connect a SATA device to an expander, although the secondary physical link connector would not connect anywhere. Cable lengths, when using 24 gauge solid conductor screened twinax cable and quality passive components, can be up to 8 meters.

Serial Attached SCSI