SFF-8639 is an extension of the 8482 SAS Connector, which has higher signal quality requirements (to support 12Gb/s SAS and Gen3 PCIe) and adds support for SATA Express and 4 lanes of MultiLink SAS or PCI Express.
The connector has a total of 6 High-Speed signal paths; strangely, each specification only ever uses up to 4 of them at any time.
The standard defines a backplane connector that is designed to support the following interfaces:
- Single Port SATA — all existing SATA Drives as defined by the SATA Specification Revision 3.1
- Two Port SATA Express — new SATA Express drives as defined in the soon-to-be-ratified SATA Express Specification
- Dual Port SAS — all existing dual port SAS Drives up to 6Gb/S (SFF-8482) and new 12Gb/S Drives (SFF-8680)
- MultiLink SAS — new 4 lane SAS Drives (SFF-8630)
- PCI Express — PCI Express disk drives with up to 4 lanes of PCI Express data (SFF-8639, SSD Form Factor V1.0) now known as U.2 or SFF.
Whilst the connector is physically compatible with all of the drive types listed above, systems using this connector will probably only support a subset of these interfaces. The initial use of the connector will be for the new U.2 NVMe Solid State Drives (SSDs), with several vendors already working on products.
A major point of interest here is that the connector will be hot-plug compatible. This will be the first major design for PCIe storage that supports hot-plug, and there is likely to be a wide range of testing issues around this.