Authors:
Jay Neer, Advanced Technical Marketing Manager, Industry Standards, Molex
With Contributions by Harry Mason, Director, Industry Marketing, LSI Corp. and
Susan Bobholz, Product Marketing Manager, Intel
Introduction
The first article in this series of three articles discusses SAS connectivity and introduced the basic concepts of how SAS devices connect together as well as how SATA devices connect into a SAS infrastructure. The second article discussed how SAS connectivity has evolved over time.
This third and final article in the series provides a review of the connectors and cable assemblies released for each revision level of the SAS standard.
SAS has experienced very fast growth based on the enhancements described in the previous articles. These expanded opportunities have created a need for the standardization of new connectors on several occasions. This article will show the chronology of these changes, by the SAS standard revision level, to provide an understanding of why convergence is important for revision SAS 3.0. Graphics are included for each of the revision levels to provide a visual connection to the product requirements as defined on the Advanced Connectivity Roadmap. The need for connector convergence in revision 3.0 becomes apparent and appropriate as the article unfolds.
Please read all three articles to understand the complete SAS connectivity story.
The release of Serial Attached SCSI (SAS) in 2004 included a blended family of interconnects.
Connectors were purposely chosen so that there could be inter-mateability between the SAS and SATA protocols when desired. Keying was available but optional, except for Hard Disk Drives (HDDs).
SAS 1.0
HDD connectors included a combined power and a signal connector for docking to a backplane, a cabling solution with a combined power and signal plug cable assembly as well as independent power and signal cables and their connectors. The new SAS HDD interface was identical to the existing SATA HDD connector except that it was hard keyed to prevent SAS drives from plugging into SATA systems – see Figure 1. SATA HDDs could plug into SAS ports as the blocking key was added to the HDD side of the interface rather than the backplane/cable side.
Figure 1 SAS HDD Connector (SFF-8482) Figure 2
SAS HDDs have a second I/O port. The contacts for this port were added to the back side of the blocking key of the SAS HDD connector. Figure 2 shows the mating/backplane side.
SFF 8482 provides the mechanical detail for the SAS HDD connector interface.
External I/O Connector
An existing I/O connector design was chosen for the SAS external I/O connector. The connector was already approved and in use for Ethernet CX4, 4x Fibre Channel and 4x InfiniBand SDR applications. These standards all implemented a pull-to-release latching version of the connector. The SAS/SATA choice however was to implement the jack screw attachment option instead, which provided a natural key between the two groups of applications. Optional keys were also defined for both SAS and SATA so that the two similar jackscrew applications could be keyed if desired. The connector interface provides four channels over eight differential pairs.
Figure 3 – SAS 4x External I/O Connector (SFF-8074)
Internal SAS Connector
Internal SAS connectors included the existing x1 SATA standard connector. This connector provides a single channel of data – no sidebands are provided. A new 4x internal multilane connector (SAS 4i) was developed to provide a common SAS/SATA cabling solution for a 4x channel connection. The solution included provision for eight side band signals in addition to the eight differential pairs (4 channels) for the data. SAS 4i can be paired with another SAS 4i, with (1-4) SATA x1 connectors or with an HDD cable receptacle.
Figure 4 – SAS 4i Internal Connector (SFF-8484)
SAS 1.1
New applications quickly drove the need to provide higher data rate and higher density connectors for SAS 1.1. The Mini-SAS external and internal solution doubled the data rate and the density of the connector interface for the external implementation and quadrupled the density for the internal implementation. Hard keys were defined for the external I/O “in ports, out ports and end-point ports.” The cable plugs defined “in port plus end point” and “out port plus end point” key slots. This combination enabled an “in port” on one end and an “out port” on the other end with the “end point” enabled on both ends so that one cable would service all applications defined in the standard.
Figure 5 – Mini-SAS 4x External I/O Connector (SFF-8088)
Figure 6 – Mini-SAS 4i Internal I/O Connector – (SFF-8087)
SAS 2.0
The connectors themselves did not change for SAS 2.0. New applications, however, needed longer cable lengths. The SAS 1.1 external connector design did not physically change from an interoperability perspective; cable assembly specifications were revised to enable extended length cable assemblies.
A new mechanical key was required to block the newer extended length cable assemblies from plugging into older systems due to the higher voltage required to drive the longer length.
Figure 7 – Mini-SAS 4x External I/O Connector – (SFF-8088)
SAS 2.1
Applications, packaging/form factor size reductions along with increasing data rate requirements have again driven a change to the SAS connector landscape. From an application perspective, cable management brings the requirement for more contacts within the external connector interface. As a result, the number of contacts has increased from 26 to 36. Server enclosure packaging has driven the requirements for four 4x ports down to a low profile PCIe card from a full size card. The need for a bigger pipe has brought the introduction of an 8x external connector solution. Longer distance requirements led to incorporation of an optical cable, at least for vertical solutions as soon as possible. As a result the menu for connectors has dramatically expanded for SAS 2.1.
Figure 8 – Mini-SAS 4x Active (SFF-8088)
– Mini-SAS HD 4x & 8x (SFF-8644)
– QSFP+ 4x (SFF-8436)
Figure 9 – Mini-SAS HD 4i & 8i Internal I/O Connectors
A downsized HDD connector was introduced in SAS 2.1 to accommodate 1.8″ HDDs and SSDs.
The second SAS port remains on the top side of the drive plug and two additional contacts have been added for additional system functions. The interface is also appearing on 2.5″ devices.
Figure 10 – Micro-SAS Connector (SFF-8486) (Courtesy Molex)
SAS 3.0
The Advanced Connectivity Roadmap defined by the SCSI Trade Association in conjunction with the INCITS T10 Technical Committee brings a converged connector interface to SAS. The Mini-SAS HD I/O defined in SAS 2.1 was designed to accommodate the 12 Gb/s data rate projected for SAS 3.0 as well as the form factor requirements driven by the user community.
The Mini-SAS HD connector can be designed into SAS 2.1 applications today to take advantage
of numerous advantages as well as in preparation for 3.0, such as:
– Higher port density for adapter cards, servers, and switches
– The ability to plug two 4x or one 8x cable into the same 8x host receptacle
– The ability to reduce the number of SKUs – with cable management, the need for unique host
designs based on the Mini-SAS mechanical keying definitions is eliminated. Referring back to
Figure 8, there are nine physical Mini-SAS guides shown for a single guide – that is reduced to
one with Mini-SAS HD
– Special adapter cables that physically mate back to other SAS 2.1 connectors will not be
needed for 3.0 implementations
– Systems will be able to accept an optical cable assembly
Figure 11 – Mini-SAS HD 4x and 8x (SFF-8644)
The unshielded internal connectors shown for the SAS 2.1 revision also apply for SAS 3.0.
No additional connectors are shown in the SAS roadmap.
Please refer to Figure 9 for SAS 3.0 Internal Connectors
Conclusion
As the industry continues to change and evolve, it can depend on SAS keeping pace. As 1.8″ SSDs have emerged, connectors have been defined to enable those SAS and SATA devices to connect into a SAS system.
Connectivity has always been of primary importance to SAS. New connectors and cables have been added to the standard as required by new applications. SAS 3.0 will continue to advance connectivity while bringing many of the current connectivity options into a more cost-effective, converged solution for most providers. Reducing the number of host receptacles to a single version that accepts both passive and active copper as well as optical cables provides a simple, cost effective solution for end users.
