Are You Up-to-speed on SAS Backplane Design?

Author: Dennis Alexander

Although the design of a Serial Attached SCSI (SAS) backplane is not a trivial matter, the design task is much simpler than the design and layout of a parallel SCSI backplane. A parallel SCSI backplane has 27 differential signaling pairs that must be routed in a daisy-chain fashion from the I/O connector to each of the disk drive connectors that reside on a common SCSI bus. SAS is a point-to-point interconnect comprised of only two differential signaling pairs per drive (transmit and receive) that are routed between the I/O connector and each disk drive connector. Four signaling pairs are required for each drive if dual-port operation is supported.

Parallel SCSI drive backplane connectors also have four SCSI ID pins per drive that must be connected and six single-ended signal pins per drive that can optionally be used. SAS has one single-ended signal that may be used to indicate drive ready status, but no SCSI ID pins.

Power and ground connections are basically equivalent in design complexity for the two protocols, however SAS has an optional +3.3 volt supply input to support future SAS drives that may require 3.3 volts.

The SAS protocol allows SAS backplanes to support Serial ATA (STA) disk drives. Parallel SCSI backplanes can only support parallel SCSI devices.

SAS Backplane Architecture
Disk drive backplanes may be categorized into two families; internal server and external storage unit backplanes. The architecture of a SAS backplane may vary considerably depending upon the requirements of a specific server or external storage unit, however a SAS backplane will most likely be comprised of one or two I/O connectors, a drive connector for each drive, an LED and drive PRESENT management chip and two or three LEDs for each disk drive.

Most external storage unit backplanes will likely need to support the dual-port feature of SAS drives while most mainstream servers may not require this feature. Server backplanes with more than eight drives will probably need a SAS expander to expand the host controller’s interconnect port. External storage units typically have their expanders on I/O cards, not on the drive backplane.

Another design difference may be the type of I/O connector used on the backplane. An internal server backplane may use one or two of the recently defined multilane internal serial-attachment connectors (defined in SFF 8484 at:, depending on the number of drives the backplane supports. This connector supports up to four SAS lanes. A multilane internal serial-attachment cable would be used to cable the server backplane to the host SAS controller. An external storage unit will likely use two redundant high-speed blade-and-socket PCB-to-PCB connectors to connect the backplane printed circuit assembly (PCA) to the I/O controller PCA or to a mid-plane PCA.

Components of a SAS Backplane</b
I/O Connector: The preferred I/O connector for server backplanes is the multilane internal serial-attachment connector. This connector has eight signal pins to support four SAS drives and six pins for the SGPIO (Serial General Purpose Input/Output) side-band signals. The SGPIO bus is used for efficient LED management and for sensing drive PRESENT status. See SFF 8485 for the specification of the SGPIO bus.

To support the dual-ported SAS drive feature, a second multilane connector will be required to support the second drive port on each SAS drive connector. If support for more than four drives is required, additional multilane connectors could be added or a SAS expander could be added to the backplane.

SAS Drive Connector: The SAS drive connector has 29 pins and supports both SAS and SATA disk drives. The connector has two sets of SAS transmit and receive pairs to support dual-port SAS drives. SATA drives do not support dual porting. The connector also supports + 12 volts, + 5 volts, and + 3.3 volts and has pre-charging pins for each of these voltages. A small value resistor placed between each supply voltage and the pre-charge pins limits the inrush current to the drives during a hot-plug insertion. Today most SAS and SATA drives use + 12 volts and + 5 volts. In the future some drives may use + 3.3 volts instead of + 5.5 volts. The SAS drive connector is defined in the Serial Attached SCSI – 1.1 (SAS-1.1) specification at:

LED Management: The backplane may contain LEDs to indicate drive status. Light from the LEDs could be transmitted to the outside of the server by using light pipes mounted on the SAS drive tray. A small microcontroller on the backplane, connected via the SGPIO bus to a host bus adapter (HBA) or a SAS RAID controller, could control the LEDs.

Drive PRESENT Circuitry: The presence or absence of a drive may be detected by sensing the voltage level of one of the pre-charge pins before and after a drive is installed. See the Serial ATA II: Extensions to Serial ATA 1.0a, revision 1.1 specification at: for a recommended sensing circuit.

Other Considerations: There are many other factors to consider when designing a SAS drive backplane. Some of these factors are; air flow for cooling the drives through or around the backplane; PCB signal impedance for high-speed serial data lines; crosstalk between the high-speed data lines; rigidity of the backplane to support drive insertion and extraction; serviceability and proper decoupling or filtering of the supply voltages. Attention to these details is crucial to the success of a well-designed SAS backplane.

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