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Appendix A: Introduction to SAS ● 56
Direct-attach Connections
In a direct-attach connection, SAS or SATA disk drives are connected directly to a SAS card
with SAS cables. One disk drive is attached to one SAS connector with one SAS cable (or
multiple disk drives are attached to one SAS connector with one fan-out cable). The figure on
page 25 shows an example of direct-attach connections.
The number of direct-attached disk drives is limited to the number of phys supported by the
SAS card. (Note that there may be multiple phys within a single connector. See page 55.)
Backplane Connections
In a backplane connection, disk drives and SAS cards are attached to and communicate with
each other through a system backplane. The figure on page 26 shows an example of backplane
connections.
The number of end devices is limited to the number of slots available on the backplane. For
example, the Adaptec S50 enclosure, which contains an expander, is a backplane connection
that supports up to 12 SAS or SATA disk drives.
Some backplanes support daisy-chain expansion to other backplanes. For example, you can
daisy-chain (connect one to the next) up to nine Adaptec S50 enclosures to a single SAS card in
a host system.
SAS Expander Connections
A SAS expander device literally expands the number of end devices that you can connect
together. Expander devices, typically embedded into a system backplane (see page 26), support
large configurations of SAS end devices, including SAS cards and SAS and SATA disk drives.
With expander devices, you can build large and complex storage topologies.
There are two types of SAS expanders: fanout expanders and edge expanders. Each performs a
different role in a storage system. (For more information about how SAS expanders work, refer
to the STA Web site at www.scsita.org.)
You can connect up to 128 SAS ports to an edge expander. (A single edge expander can
therefore support up to 128 SAS addresses.)
You can connect up to 128 edge expanders to a fanout expander.
You can use only one fanout expander in any single SAS domain (a topology of SAS—and
possibly SATA—end devices and expander devices). A single SAS domain can therefore
comprise up to 16,384 SAS ports (and therefore up to 16,384 SAS addresses).