Yesterday
Historically, serial connections
have often been regarded as the poor cousins when speed was the criterion.
Their tasks have included printers (almost universally replaced eons ago
by the faster parallel port), a mouse port and a modem. Speed was - until
recently - the fiat of parallel bus connections: for example, printer parallel
ports, IDE/EIDE hard drive connectors and SCSI
Today
Serial connections are reborn:
they can boast speeds that shame their parallel bus competition and they
are poised to make inroads into other traditional parallel connection territory
Most desktop users today
have Ultra DMA/66 as the hard drive interface "flavour of the year." This
interface is capable of data transfers up to 66 MB (bytes, not bits) per
second (burst mode) although the sustained rate is proportionately lower.
SCSI is an option; however, for most users it suffers a significant cost
disadvantage. Not only are the hard drives more expensive but also users
must purchase the host adapter. Most home and small business users opt
to use the built-in IDE/EIDE connectors
As processor power and speed
accelerate, data must arrive faster to prevent a bottleneck. To compensate,
several design approaches, singly or in combination, are available: for
example, faster platter rotation speeds, greater areal density, RAID drive
arrays, larger on-drive caches and faster algorithms
Indeed, a faster protocol
- Ultra DMA/100 - has appeared and chipsets to support it are becoming
available. Already, the next increment, Ultra DMA/133 awaits official blessing
Tomorrow
High speed serial connections
were resurrected with the arrival of Universal Serial Bus (USB). Motherboard
support is integrated with connectors on ATX form boards; a wide
range of peripherals (for example, keyboards, mouse, printers, scanners
and digital cameras) utilize the convenience of hot plugging. USB is only
the pioneer; indeed, it is scheduled for an imminent speed boost. Also
waiting in the wings is IEEE 1394 (aka Firewire - trademark of Apple Computers,
or iLink - trademark of Sony, or high speed serial bus or high speed serial
interface) and Serial ATA. IEEE 1394 was originally relegated to digital
video interconnections; however, it has gained prominence as other peripherals
(chiefly hard drives) arrive in retail channels
Serial ATA
The incremental progression
of "Ultra" may soon be replaced by an entirely different protocol: Serial
ATA. Gone would be ribbon cables: the hallmark of the present parallel
signal interface. The major problem with the present scheme is signal noise
and crosstalk between adjacent wires. To minimize this interference Ultra
DMA/66 (and beyond) specifies an 80 conductor ribbon instead of the more
common 40 wire; for backward compatibility the familiar 40 pin connector
remains. These connectors are significantly more expensive locally. Switching
to a serial interface allows fewer wires and lower voltages; both are conducive
to less signal noise and crosstalk (with less heat and better cable organization
as freebies)
Features/Benefits of Serial ATA
- Increased transfer rates - initially
up to 150 MB (again, bytes not bits) per second (MBps) with projected incremental
increases to 600 MBps)
-
a round, 8 pin (PS/2 mouse connector
style), seven conductor, cable instead of a wide ribbon - promoting better
airflow and requiring less motherboard space
-
no need to daisy-chain (each
drive will have an individual cable)
-
point-to-point protocol - the
end of master/slave jumpers (Each drive would have a single-purpose connector
that would connect to a motherboard hub. For those versed in LAN topology
this would be a star configuration.)
-
parallel operation of devices
(currently, only one drive on a bus can be working)
-
first party DMA (allows device
access to host memory - uncertain of details but supposedly improved from
the current algorithm)
-
lower logic signal voltages
and differential signalling (500 millivolts proposed versus the current
5 volts) - less noise, crosstalk
-
reduced motor voltage (3.3 versus
5.0) for less heat production: a boon for portables
-
transparent support for CDs,
CD-R, CD-RW, DVDs, tape devices, high capacity removable media devices,
zip drives
-
intended only for internal use
- no external connectors anticipated for peripherals (marketing strategy
to keep costs down; avoids bogging down hard drive performance with slower
peripherals)
-
one meter maximum cable length
(ideal for Ultra is less than 0.5 meter) permitting greater flexibility
of internal case layout
-
forward and backward compatibility
with existing hardware provided by adapters (both new drives to the old
interfaces and old drives to Serial ATA)
-
OS and BIOS transparent (Microsoft
Windows 9x, Me, and 2000; Linux and presumably Mac)
-
compatible with all current
software (register level transparent)
-
device driver changes not necessary
-
hot pluggable at manufacturer's
option (less expensive hard drive arrays?)
-
provisional timetable: 1.0 release
fall 2001, first commercially available products late 2001, broad adoption
in 2002/li>
-
identical connectors for desktop
(3.5 inch form factor) and mobile (2.5 inch form factor) drives
-
cost competitive with present
ATA/ATAPI
USB
The specification for the
current Version (1.1) supports transfer rates of 1.2-1.5 MBps; the proposed
2.0 specification would raise it to 60 MBps (in 2002) while providing backwards
and forwards compatibility. That is, a version 2 device would work when
connected to a version 1.1 interface (only at the slower rate) and a version
1.1 device would work when connected to a verson 2.0 port (but would not
speed up). For external peripherals like scanners this offers the expectation
of SCSI-like performance without the additional cost of the host adapter
USB was originally intended
for low-impact, narrow bandwidth peripherals (like keyboards and mice).
Indeed, the USB chipset and cabling designs are not ideal for wide bandwidth
devices like hard drives. Intel, the parent of USB, in conjunction with
Microsoft originally indicated that IEEE 1394 was the high-end counterpart
and co-existence was the operating strategy. Are the gloves off? This new
variant offers comparable performance to IEEE 1394 (SCSI beware) and enjoys
de facto motherboard integration that reduces costs (IEEE 1394 is integrated
on few systems; most implementations utilize a PCI expansion card)
USB Specifications
-
intended only for external devices
-
integrated on motherboard
-
cable up to 6 meters per link
-
foolproof, one-way cables (not
IEEE 1394 interchangeable)
IEEE 1394
This serial protocol was
faster than the original implementation of USB and entered the arena in
digital video applications. Given this niche application, higher implementation
costs and "no other game in town" it has languished in the PC marketplace.
Some manufacturers are offering "Firewire" hard drives that use a conventional
hard drive in a suitable external package with this interface but with
a considerable price premium. By increasing USB's data transfer rates into
IEEE 1394 territory, the latter would be hard-pressed to break out of its
mold given the higher cost of implementation. Still, what it does, it does
well and for technical reasons USB is not ideally engineered to provide
large amounts of data at high speed (mass storage data transfers)
IEEE 1394 Specifications
-
aimed at high bandwidth peripherals
(digital video) and mass storage
-
higher implementation costs
(also possibly royalties)
-
originally much faster than
USB 1.1; roughly comparable to USB 2.0 (IEEE 1394 is scheduled to increase
in 2005 to 200 MBps)
-
intended for external devices
only
-
motherboard integration seems
unlikely (Intel holds the reins here and a more complicated protocol requires
supporting chipsets that are at a cost disadvantage when compared to USB)
-
foolproof, one-way cables (not
USB interchangeable)
-
up to 4.5 meters per link
Conclusions
Given the momentum behind
Serial ATA (most major manufacturers are committed) it would appear to
be a "no brainer." Faster processors require greater data bandwidth and
the current protocol has reached the end of its lifespan. The new approach
will offer greater data throughput, headroom for future speed increases,
internal design improvements (better airflow, less heat generation) and
ultimately lower costs. During transition, inexpensive (hopefully) adapters
will allow gradual phase-in with minimal disruptions
USB appears to have the upper
hand. It is positioned to support a diverse range of peripherals and motherboard
integration offers the benefits of low cost and end-user simplicity. Otherwise,
both buses are surprisingly alike. Casual end-users, I suspect, will be
quite satisfied with the already present USB. For them, USB fulfills their
peripheral connection requirements. IEEE 1394 has made no attempt (to date)
to move into the USB arena of connecting a mouse or keyboard
The exotics of digital video
aside, consider connecting a scanner. Given the choice, would you purchase:
a USB version that you can literally plug in and run the software; a SCSI
version that requires obtaining and configuring a host adapter, setting
the device ID and arranging termination in addition to the software;
or, an IEEE 1394 version that requires purchasing a card, installing software
drivers and then the scanner software. Yes, IEEE 1394 is arguably easier
than SCSI but why would you purchase, at additional expense, any add-in
board (regardless of protocol), with the complications of setup (tongue
in cheek: Plug and Pray) and drivers when your motherboard has all you
need?
Surprisingly, IEEE 1394 and
USB (Version 2.0) are more alike than different with specifications that
are very similar. Marketing considerations originally positioned them to
address, respectively, the high end and the low end of the peripheral arena.
If IEEE 1394 were integrated on the motherboard, instead of an expansion
card, could/would end- users notice much difference? Would they care? In
both cases, I doubt it.
Originally published: April, 2001