Call it scuzzy, but have some respect
- By Michael Cheek
- Jun 16, 1997
SCSI missed the boat when it came to nifty naming. Users refer to the Small Computer
System Interface as "scuzzy," though in reality it's anything but.
Upcoming interfaces such as the Universal Serial Bus could displace SCSI on the desktop
before long, but SCSI has a strong foothold with users of high-end workstations and
servers. It's the preferred way to connect scanners and removable-media drives.
Why? Because SCSI's 10-megabyte/sec-or-faster access rates make the speediest parallel
port seem pokey at only 2 megabytes/sec.
But trying to order SCSI products can stump a wizard. Call a reseller to ask about SCSI
adapter cards and you'll get: "Is that SCSI-1, -2 or -3? Fast? Wide? Or Fast and
Wide? Or Ultra? UltraWide? What about connectors-50-pin Centronics, 50-pin mini, 25-pin
standard or 68-pin?"
To avoid SCSI insanity, remember that the American National Standards Institute has
adopted only two SCSI standards at this time: SCSI-1 and SCSI-2. A third standard is in
the works, and several products already follow SCSI-3 specifications. SCSI-2 is the
connection you'll see most often today.
SCSI-1 became an industry standard in 1986. Only ancient PC products still have its
5-megabyte/sec, 25-pin connector, which looks like a parallel cable.
SCSI-2 became official in 1994, with 50 pins but still the 5-megabyte/sec speed.
Subsets of that standard also appeared: Fast, Wide and Fast-Wide. This is where SCSI names
start getting a little muddy.
Fast SCSI doubles SCSI-2's transfer speed to 10 megabytes/sec through a 50-pin
connection, known as mini 50-pin or high-density 50-pin. Wide SCSI doubles the data path
of basic SCSI-2 to transfer 10 megabytes/sec with a 68-pin adapter. Combine Fast and Wide
and you get a Fast-Wide SCSI-2 adapter that can achieve speeds of 20 megabytes/sec with
Ultra is a term that applies to the emerging SCSI-3 standard, which may double the
speed yet again. An Ultra SCSI-3 connection bursts up to 20 megabytes/sec over its 50
wires. UltraWide SCSI-3 sends 40 megabytes/sec over its 68-connection cable. Fast Ultra
and Fast UltraWide do not exist-at least not yet.
These speeds show why SCSI is popular for internal hard drives and other storage
devices. The Enhanced Integrated Device Electronics (EIDE) interface on many systems
delivers only 13 megabytes/sec-much slower than most SCSI-2 and all SCSI-3 connections.
SCSI, unlike EIDE, lets you daisychain several devices together. Most SCSI devices have
an auto-termination feature to tell the adapter where the chain ends.
SCSI-1, Fast SCSI-2 and Ultra SCSI-3 each allow eight device addresses, numbered 0
through 7. Because the SCSI adapter card usually uses address 7, and address 0 or 6 is for
bootable SCSI hard drives, avoid those three addresses when installing your new SCSI
You can attach devices safely at addresses 1 through 5, linking from one to another
without adding another SCSI adapter card. Wide SCSI-2, Fast-Wide SCSI-2 and UltraWide
SCSI-3 allow 16 addresses.
USB looks like a strong contender to succeed SCSI in some areas. At 1.5 megabytes/sec,
it transfers more slowly, but it can daisychain up to 128 devices-far more than SCSI.
USB won't gain much popularity for storage devices except perhaps tape backups. But
don't discount it-its data speeds are comparable to a T1 Internet connection. You'll find
USB popping up on scanners, modems, cameras, printers and monitors.
A likelier successor to SCSI is the 50-megabyte/sec FireWire (also known by its
Institute of Electrical and Electronic Engineers number, 1364). Faster and thinner than
SCSI, FireWire needs only six wires-two twisted pairs, a power and a ground-to connect up
to 63 devices.
A possibly faster SCSI successor for high-end storage devices is Serial Systems
Architecture (SSA), which may be backward-compatible with SCSI. It uses a thinner cable,
plugs in up to 127 devices and gives throughput of 80 megabytes/sec or faster.
USB is just now emerging in devices. FireWire isn't expected to take hold until 1999,
and SSA is only starting to be talked about. So for a while, SCSI rules. If there's a
SCSI-4, it might slim down enough to compete with the thinner cables and faster speeds of
the other emerging interfaces.