By Jay Neer, Molex
The initial release for Serial Attached SCSI (SAS) connectors and cables was a bit different from the typical release of a new I/O standard. The SCSI Trade Association (STA) was committed to enabling Serial ATA (SATA) connectivity with SAS, so close coordination was required between the two standards organizations to achieve this goal. The idea was to create one-way interconnectivity to permit SATA hard-disk drives (HDDs) to plug into SAS backplanes, while preventing the dual-port SAS HDDs from plugging into SATA backplanes.
Early HDD interconnectivity
To achieve the one-way interconnectivity goal, blocking key combinations were implemented for both HDD plug connectors and their mating backplane receptacle connectors. The HDD cable-attach receptacle connectors were also included. In addition,t he single-port SATA HDD cables and connectors and their companion SATA power cables were incorporated into the SAS standard. The internal multilane connectors, designated as SAS 4i, were not keyed as there were eight side-by-side signal contacts available for electronic keying should an OEM need to identify various functionality aspects for a particular application. Several recommended versions were included in the standard.
Keys were required for the external I/O connectors and were created to prevent interconnectivity between the SAS 4x external and the SATA 4x external connectors. External keys were optional in the original release but were made mandatory when the first revision of the standard was released. The new mini-SAS connector keys 2 and 4 were defined for “data out,” whereas keys 4 and 6 were defined for “data in” and key 4 for “endpoint” solutions. The interconnect scenario provides a seemingly large number of connector and cabling variations until one looks at the broad range of applications that continue to be developed for SAS.
Connections inside the box
A summary of the variations that have been standardized for connectors within the box is shown in Figure 1. The figures represent the maximum density that can be accommodated on a low-profile PCIe add-in card per the PCI SIG-controlled PCIe CEM (card electromechanical) specification. Server manufacturers continue to migrate to system designs specifying only low-profile cards rather than the older high-profile cards, driving the size reductions for the connectors. The advent of SAS switches has also contributed to the pursuit of smaller, denser connectors. In addition, switches drive the need for longer cable lengths, which created the need for active cables to be incorporated in the latest rev of the standard.
The names assigned to the connectors used are per the SAS standard. The initial release of SAS 4i appears on the left of Figure 1, with later generations following to the right. Each new SAS connector generation has at least doubled the density of the previous generation, as well as enabled a doubling of the data rate through the interface. The figures simplistically show each new generation connecting to itself. In practice however, adapter cables are available that enable any connector on any end of any cable assembly, including direct attach to HDDs, as shown. Recommended pinouts for all of these combinations are defined in the standard. Board-mount connectors are available that accept cables either parallel or perpendicular to the board. Second-generation cable plugs (mini-SAS 4i) are available that provide for the cable itself to exit the receptacle pointing to the left, the right, pointing up, pointing straight out, pointing down, and also back over itself. (Not all are shown.)
The latest rev of the SAS standard also introduces an 8x internal connector to the matrix. The multi-plugging feature of the new Mini-SAS High Density (HD) connector enables either an 8x cable or two 4x cables to plug into the same 8x board-mount receptacle. This feature provides functional flexibility without the need for additional stock keeping units (SKUs) for adapter and system board makers.
New, higher-capacity storage devices are seen moving from the initial 3.5-inch form factor down to the 2.5-inch form factor as well as to the emerging 1.8-inch form factor. Reductions in packaging density in the HDD designs have also driven the size of t their I/O connectors down. Solid state devices (SSDs) are now available in both the 2.5- and 1.8-inch form factors, opening up additional market opportunities for SAS. The 1.8-inch devices introduce the reduced size Micro-SAS connectors. The same one-way blocking per key concept introduced with the original 3.5-inch SAS HDD connector has been employed for the Micro-SAS connector solution. The storage devices may be docked to either mid/backplane or cabled receptacles, as shown in Figure 1.
Connections outside the box
External I/Os have followed the same form-factor reductions as the internal I/Os. The major difference between the two is the need for more effective electromagnetic interference (EMI) shielding between the board-mount connectors and their system bezel or faceplate as the frequencies have increased. The overall shield required on the external cable assemblies adds to the size of the cable diameter. This makes it more difficult to terminate the wire to the smaller connectors, to mechanically support the weight of the cable hanging from the connector, and also to route and manage the cables once they are installed at the end customer. As a result, both internal and external board-mount connectors from the latest generation are attached with a screw to provide an appropriate strain relief.
Both second- and third-generation connectors have used the same physical connector, or connector sub-assembly, for both internal and external connector implementations. This serves to reduce manufacturing costs for connector suppliers, which in turn, lowers the total cost of the SAS solution. It also reduces customer connector qualification expense for systems suppliers. The external I/O can be segregated into a number of groups, sich as passive/active, managed/unmanaged, and copper/optical. Passive/unmanaged are highlighted in Figure 2 and external active cables are highlighted in Figure 3.
Figure 2 provides the same left-to-right progression for external connectors as Figure 1 provides for the internal connectors. The same packaging reduction issues and data-rate increases track for both internal and external solutions. One of the primary differences between first generation and second generation is that external connectors were the definition of a more rigid keying specification for the second generation. The keying scheme differentiated ‘out-port’ from ‘in-port’ connectors and from ‘end-point’ connectors. More information was needed to optimize the performance at a given length, and it was becoming more difficult to keep track of what each combination of mechanical keys represented. So the decision was made to switch to electronic keying for third-generation connectors. Other reasons are described later in reference to active cables.
Several bulk cable suppliers offer a premium-grade, smaller-diameter cable that can provide a cable-management benefit to some systems.
The methodology chosen for the electronic keying of the third generation of the SAS external I/O is a subset of the Quad Small-factor Pluggable+ (QSFP+) specification defined in SFF-8436. This enables a smooth transition from QSFP+ to the Mini-SAS HD connector for the converged next generation of the SAS standard. The Mini-SAS connector does not have enough contacts to enable cable management, however longer active-copper cables have been released in the latest rev of the SAS standard. The QSFP+ interface was primarily adopted to enable active optical links in vertical solutions.
The next revision of the external standard will bring a converged connector interface back to the SAS user community. This interface will enable 8x and 4x passive and active copper assemblies, active 4x optical cable assemblies, and pluggable 4x optical modules that will all mate to the Mini-SAS HD board-mount connector receptacle. All versions will be capable of operating as 3, 6, and 12 Gbits/sec. See Figure 4 for an illustration.
The converged HD pluggable solution also provides a highly competitive connectivity solution for the advent of SAS switches. The port density is 2x the port density of QSFP+. As a result, QSFP+ applications requiring a belly-to-belly solution can now be implemented with a single-sided Mini-SAS HD solution with no upside-down cables. In addition, the length of the copper traces on the host board can be reduced approximately to half, which improves the signal quality.
The interconnect progression from the single external and internal I/O interface to four I/Os is a testimony to the growing popularity of SAS. Performance gains throughout the SAS ecosystem all point to additional application development for SAS.
Jay Neer is strategic development manager, industry standards with Molex Inc. He represents Molex at T10/SCSI, T11/Fibre Channel, and SFF standards committees as well as the SCSI Trade Association, the Fibre Channel Industry Association, and the InfiniBand Trade Association. Neer has been named on more than 30 patents related to connectors and packaging. He holds degrees from the State University of New York in Tool Design and a Bachelor of Science degree in Technology from Excelsior College.
