Differential Signaling - Compaq BL10e - HP ProLiant - 512 MB RAM Manual

Differential signaling

All SAS devices have connection points called ports. One or more transceiver mechanisms, called
phys, are located in the port of each SAS device. A physical link, consisting of two wire pairs,
connects the transmitter of each phy in one device's port to the receiver of a phy in another device's
port. The SAS interface allows the combination of multiple physical links to create two (2x), 3x, 4x, or
8x connections per port for scalable bandwidth. A port that has one phy is described as ―narrow‖
while a port with two to four phys is described as ―wide.‖
SAS uses differential signaling to transfer data over a physical link (Figure 2), which reduces the
effects of capacitance, inductance, and noise experienced by parallel SCSI at higher speeds. SAS
communication is full duplex, which means that each phy can send and receive information
simultaneously over the two wire pairs.
Figure 2. In differential signaling, positive minus negative equals 1500–900 = 600mV or 900–1500 = -600mV.
The physical link rates for SAS and SATA technologies are listed in Table 2.
Table 2. Physical link rates per direction
Physical link rate Generation
1.5 Gbps SAS-1, SAS-1.1, SATA Revision
1.0
3 Gbps SAS-1, SAS-1.1, SATA Revision
2.0
6 Gbps SAS-2, SAS-2.1, SATA Revision
3.0
Bandwidth
150 MB/s
300 MB/s
600 MB/s
4x bandwidth
600 MB/s
1200 MB/s
2400 MB/s
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