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.