eSATA on motherboard (was: boot from "USB Device")
- From: "Timothy Daniels" <SpamBucket@xxxxxxxxxxxxxxxx>
- Date: Fri, 1 Feb 2008 12:11:03 -0800
While the software may not know the difference between
SATA and eSATA, there are signaling level requirements
on eSATA due to the longer cable that eSATA permits
(up 2 meters). And on motherboards that have the eSATA
feature (e.g. Dell's Optiplex 755), an eSATA connector
resides on the motherboard:
http://support.dell.com/support/topics/global.aspx/support/dsn/en/document?c=us&cs=19&dl=false&l=en&s=dhs&docid=322F45CF605E5D50E040A68F5B284B8C&doclang=en
The following paragraphs, taken from SIIG's white paper
on eSATA, discusses the eSATA signalling levels:
http://www.sata-io.org/documents/External%20SATA%20WP%2011-09.pdf
"ELECTRICAL SIGNALING REQUIREMENTS
"One of the features identified above is the ability to have
a longer cable, up to 2 meters, for an external application.
Since the original Serial ATA specification was designed
for the internal 1-meter cable, there was not sufficient design
margin in the spec to drive the longer cable. The Serial ATA
specification sets minimum and maximum transmit voltages
that must be sent from a Serial ATA host or device, and also
a minimum voltage that a receiver must be able to decode
properly. For the internal cable at a speed of 1.5 Gbps, the
transmission voltage that must be sent from the host to the drive,
or vice versa, ranges from 400 to 600 mV. The receiver must
be able to decode voltages between 325 and 600 mV to
account for some loss in signal though the cables and connectors.
"With the 2-meter cable, in order to account for any additional
losses over the cable, the minimum voltage transmitted is raised
from 400 to 500 mV, and the minimum receiver sensitivity is
further decreased to 240 mV. These changes accommodate
any additional degradation within the longer cable or additional
connectors in the signal path. It should be noted that the signal
levels vary slightly for the 3 Gbps signaling rate, and that the
compliance points where these signals need to be verified are
at the point where the external cable is plugged in, not at the
silicon device.
"Since many of the existing disk drives and Serial ATA chipsets
were originally designed with only the internal signal levels in mind,
they may not be able to meet the more rigorous requirements of
the external interconnect levels. For this reason, the spec allows
a buffer chip to be used in either the device or host system to
provide the increased signaling levels or receiver sensitivity where
necessary."
*TimDaniels*
"Ben Myers" wrote:
This is because true and honest-to-gosh eSATA cannot be
distinguished from internal SATA by the software. After all,
eSATA is nothing more than a SATA cable that ultimately
connects to a motherboard SATA connector + external 5v/12v
power.
SATA connected to a USB adapter like the one in my computer
toolbox is yet another USB device to contend with... Ben Myers
"Tom Scales" wrote:
It may be an OS limitation. For example, Vista will not boot
from a USB or Firewire drive. I think it will from an eSATA
-----Original Message-----http://support.dell.com/support/edocs/systems/xps420/en/OM/HTML/appendi
From: Timothy Daniels
Posted At: Thursday, January 31, 2008 2:24 PM
Posted To: alt.sys.pc-clone.dell
Conversation: boot from "USB Device"
Subject: boot from "USB Device"
Virtually all of Dell's current PCs include "USB Device"
as a bootable device type in their BIOSs' Boot Sequence
of device types. Here is the setup specs for Dell's XPS 420
desktop:
x.htm#wp1054035
Check out the Option Settings under the "Boot Sequence"
section. It refers to "USB Device" as a "memory device",
not a "storage device". And when I explicitly asked Dell's
Tech Support whether that would include USB hard drives,
the answer was "No". Would someone here tell why the
Dell PCs can boot from USB flash drives and not from
USB hard drives?
*TimDaniels*
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