PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

13.2 Serial ATA

The Serial ATA Working
Group (http://www.serialata.org) is a group of
companies led by APT, Dell, Intel, Maxtor, and Seagate. In August
2001, this group released the Serial ATA Specification
1.0, which defines a replacement for the parallel ATA
physical storage interface.

Serial ATA (SATA) drives
and interfaces were originally expected to ship in volume in late
2001, but various technical and marketing reasons delayed deployment
by a year or more. By late 2002, SATA motherboards and drives were in
limited distribution. Maxtor and Western Digital had planned to ship
SATA drives in 2002, but failed to do so, leaving Seagate as the only
hard drive maker shipping SATA drives in significant quantities
through early 2003.

Motherboards also lacked native SATA support until spring 2003.
Beginning in fall 2002, a few premium motherboards such as the Intel
D845PEBT2 and the ASUS A7N8X Deluxe incorporated SATA support, but
those transition motherboards merely grafted on SATA support using
third-party support chips. Native chipset-level SATA support had to
wait for the arrival in spring 2003 of motherboards based on Intel
Springdale- and Canterwood-series chipsets.

Despite the slow start, SATA is on track to replace PATA,
particularly for hard drives. By late 2003, most mainstream hard
drives and motherboards will be native SATA products. Although SATA
will rapidly become the standard for hard drives, most new
motherboards will also have PATA interfaces well into 2005. This is
true because manufacturers of ATAPI devices, such as optical drives,
will be slower to convert to SATA. PATA is good enough for ATAPI
devices, and we suspect ATAPI device makers are concerned about
complicating manufacturing, inventory, and distribution by producing
both PATA and SATA models.

You needn't worry about current PATA motherboards and drives being orphaned. During the transition, we expect most motherboards to have both PATA and SATA interfaces embedded. Also, inexpensive PCI SATA interface cards are available from Adaptec, Promise, SIIG, and others. We expect PATA drives to be available through 2006, and possibly into 2007 or later.

SATA can be used
to connect internal storage devices such as hard drives, optical
drives, and tape drives to the PC motherboard. Serial ATA
1.0, called Ultra SATA/1500 or
SATA/150, operates at 1.5 Gb/s, and provides 150
MB/s read/write performance for storage peripherals. Serial
ATA II (Ultra SATA/3000 or
SATA/300) is already in the works, and will
operate at 3.0 Gb/s. Serial ATA III
(Ultra SATA/4500 or
SATA/450) is planned, and will operate at 4.5
Gb/s. Table 13-6 compares features of PATA, SATA,
and other current high-speed bus standards.

SATA II is more than just a faster version of SATA 1.0. SATA II is to
be introduced in two phases.

Phase 1 interfaces and drives, which we expect to ship in late 2003
or early 4003, target the small server and network storage markets.
Phase 1 maintains the 150 MB/s data rate of SATA 1.0, but adds
SCSI-like features such as command queuing, data scatter/gathering,
and out-of-order execution and delivery. Phase I enhances
manageability with features such as fan control, activity indicators,
temperature control, and new device notification, and extends cabling
lengths to allow rack-mounted hot-swappable arrays. Using these new
features requires updated drivers and OS support (versus SATA 1.0,
which is fully compatible with standard ATA drivers), but SATA II
interfaces will be compatible with SATA 1.0 drives.

Phase 2 interfaces and drives, which we expect to ship during 2004,
target the midrange server and network storage markets. Phase 2
increases the data rate to the second-generation 300 MB/s and
includes various physical and topological improvements intended to
support larger arrays of drives. As with Phase 1, Phase 2 requires
updated drivers and OS support, but maintains backward compatibility
with SATA 1.0 drives.

We don't expect to see SATA III products until at
least 2005, so we'll reserve comment on them until
we know a bit more about them.

13.2.1 SATA Features

SATA has the
following important features:

Current ATA standards use 5.0V or 3.3V (ATA-100/133). These
relatively high voltages in conjunction with high pin densities make
100 MB/s the highest data rate that is realistically achievable. SATA
uses 500 millivolt (0.5V) peak-to-peak signaling, which results in
much lower interference and crosstalk between conductors.

SATA replaces the 40-pin/80-wire parallel ATA ribbon cable with a
seven-wire cable. In addition to reducing costs and increasing
reliability, the smaller SATA cable eases cable routing and improves
airflow and cooling. An SATA cable may be as long as 1 meter (39+
inches), versus the 0.45-meter (18 inch) limitation of standard ATA.
This increased length contributes to improved ease of use and
flexibility when installing drives, particularly in tower systems.
The smaller and less-expensive SATA connector replaces the large,
cumbersome 40-pin connectors used by standard ATA.

In addition to three ground wires, the seven-wire SATA cable uses a
differential transmit pair (TX+ and TX-) and a differential receive
pair (RX+ and RX-).

In addition to using differential signaling, SATA incorporates
superior error detection and correction, which ensures the end-to-end
integrity of command and data transfers at speeds greatly exceeding
those available with standard ATA.

SATA appears identical to PATA from the viewpoint of the operating
system. This means that current operating systems can recognize and
use SATA interfaces and devices using existing drivers.

Unlike PATA, which permits connecting two devices to a single
interface, SATA dedicates an interface to each device. This helps
performance in three ways. First, each SATA device has a full 150
MB/s of bandwidth available to it. Although current drives are not
bandwidth-constrained by PATA interfaces, as faster drives become
available this will become an issue. Second, PATA allows only one
device to use the channel at a time, which means a device may have to
wait its turn before writing or reading data on a PATA channel. SATA
devices can write or read at any time, without consideration for
other devices. Third, if two devices are installed on a PATA channel,
that channel always operates at the speed of the slower device. For
example, installing a UDMA-6 hard drive and a UDMA-2 optical drive on
the same channel means the hard drive must operate at UDMA-2. SATA
devices always communicate at the highest data rate supported by the
device and interface.

SATA backers appreciate that there will be a transition period during
which PATA and SATA must coexist. Inexpensive dongles will adapt PATA
devices to SATA interfaces, and SATA devices to PATA
interfaces.

SATA 1.0 drives are available in two types, and it's worth knowing the difference. Native SATA drives, such as the Seagate 7200 series, use SATA 1.0 protocols end to end, and support the full 150 MB/s data rate. Bridged SATA drives are actually PATA drives with interface circuitry that links interface-side SATA protocols to drive-side UDMA-6 or UDMA-7 protocols, using buffering to accommodate the differing data rates. The throughput of bridged SATA drives is limited to that of the underlying PATA UDMA protocol. In theory, that should make no difference because even the fastest ATA drives cannot saturate UDMA-6, but in practice a bridged drive may be slower than a native SATA drive.

13.2.2 SATA Connectors and Cables

SATA uses
simplified connectors and cables. Connectors are keyed unambiguously.
The 15-pin SATA Power Connector, shown in Table 13-7, and the seven-pin SATA Signal
Connector, shown in Table 13-8, each use
a single row of pins with 0.050 inch (1.27mm) spacing. SATA makes
provision for hot-plugging using blind backplane connectors. The
mating sequence for such connectors is shown in Table 13-7 and Table 13-8 in the Mating
column. Connections for ground pins P4 and P12 are made first,
connections for the precharge power pins and the remaining ground
pins are made second, and connections for the signal pins and
remaining power pins are made third.

The two ends of an SATA signal cable assembly use identical
receptacles. Either cable receptacle can mate to the signal segment
of the device plug connector or to the host plug connector. The SATA
specification defines the allowable length of an SATA signal cable as
to 1 meter. As intriguing as it is to ponder the implications of a
0-meter cable, the real meaning of that part of the specification is
that an SATA device can legally be connected directly to the host
signal connector, without using a cable. Figure 13-3
shows two SATA signal connectors, with a motherboard mounting hole
shown at the upper right for comparison. Note the seven contacts on
the connector and the unambiguous keying. The 15-pin SATA power
connector uses a similar physical connector, also with unambiguous
keying.

Figure 13-3. SATA interface ports

In addition to superior electrical characteristics and greater
allowable length, one major advantage of SATA cabling is its smaller
physical size, which contributes to neater cable runs and much
improved airflow and cooling. Figure 13-4 shows an
SATA signal cable on the left and a UDMA ATA cable on the right. Even
allowing for the fact that an ATA cable supports two devices,
it's clear that using SATA conserves motherboard
real estate and greatly reduces cable clutter inside the case.

Figure 13-4. SATA cable (left) and UDMA 80-wire ATA cable

13.2.3 Configuring SATA devices

There's simply
not much to say about configuring SATA devices. Unlike with PATA,
with SATA you needn't set jumpers for Master or
Slave (although SATA does support Master/Slave emulation). Each SATA
device connects to a dedicated signal connector, and the signal and
power cables are completely standard. Nor do you have to worry about
configuring DMA, deciding which devices should share a channel, and
so on. There are no concerns about capacity limits because all SATA
devices and interfaces support 48-bit LBA. The BIOS, operating
system, and drivers all recognize an SATA drive as just another ATA
drive, so there's no configuration needed. You
simply connect the signal cable to the drive and interface, connect
the power cable to the drive, and start using the drive. Everything
should be that simple.