Low End Mac Reader Specials

Memory To Go Special: MacPro 8 Core 8GB kit $232 / 4GB kit $116 / 2GB kit $72. New Macbook 2GB DDR3-$65. HARD DRIVES available -- Free shipping / LIfetime warranty.

Download Typestyler, still the Ultimate Styling Tool for Internet, Print and Video Graphics. Works great in Classic with a Native OS X Version on the way. Free Tryout: www.typestyler.com

LA Computer Company: Specials on AppleCare, iMac's, MacBook Pros and more. Optical Drives for Apple iBooks, Powerbooks, MacBooks, MacBook Pros in Stock. Call 1-800-941-7654 Click Here.

OWC: Big Drives, High Performance - Not High Prices! SATA 3.5" up to 1.5TB. Notebook up to 500GB. FW up to 6.0TB. 1.0TB Drive Models from as low as $97.99 www.MacSales.com

Poker Mac Don't install Parallels to play poker online! Poker Mac will show you how to download and install a native Mac poker application such as Full Tilt Poker Mac.

Laptop Hardware Provided by TechRestore - Overnight Mac & iPod Repairs.

Compare products like desktop computers, apple laptops, apple macs, and LCD Monitors side by side! All the information and reviews to make the best purchasing decision for new mobile phones, sat nav systems, or MP3 players. The Ciao online shopping community makes searching products easy for you.

Low End Mac's Online Tech Journal

SCSI History

R. R. "Dallas" Cook-Robinson originally posted this to the SuperMacsemail list. It is reprinted with his permission.

After lurking the SuperMacs list for a few months, I finally decided it was time to post a contribution of my own. I left my asbestos underwear at the cleaners, so be kind, would you?

I've seen a lot of issues discussed and a lot of opinions expressed (sometimes stated as fact, unfortunately), but nothing seems to have generated as much confusion as SCSI. SCSI isn't the black art it sometimes seems to be, but the various terms different people use for the same thing makes it tough to sort out. I hope the following lessens the confusion.

SCSI is a set of standards for parallel (multi-wire) data transfer among physically separate devices. The original standard (SCSI) has been rewritten twice to give us what is now called SCSI-1, SCSI-2, and SCSI-3. Each standard defines electrical and physical properties mostly of interest to electrical engineers, but people like simple names so various implementations acquired catchy titles which don't always make that much sense.

SCSI-1

The original standard, now called SCSI-1, had only one flavor, so there is little confusion about it. It is the 8-bit wide, 5MBps (maximum) stuff most of us know as the standard Mac SCSI interface. It supports 8 SCSI IDs, 0-7, one of which must be a host computer. It's great advantage is the 20' (6m) maximum SCSI chain length it supports and the large number of devices available, particularly scanners and small removable media drives.

SCSI-2, Fast and Wide

SCSI-2 started the trend toward multiple implementations of the same standard - and multiple (and confusing!) names. SCSI-2 implementations are known as Fast because the maximum transfer rate was doubled to 10 MegaTransfers per second. A MegaTransfer is one million bus transfers without regard to bus width.

This was important, because SCSI-2 introduced bus widths wider than one byte to the SCSI standard. (While 32-bit wide transfers are a part of the standard, only 8- and 16-bit implementations have been marketed.) Thus, we get Fast SCSI, an 8-bit data interface with a maximum transfer rate of 10 MBps, and Wide SCSI, a 16-bit one with a maximum of 20 MBps - the same electrical characteristics and limitations, but twice the number of data lines for twice the throughput. Wide SCSI sometimes goes by the name Fast and Wide SCSI, but that usage is dying out.

The addition of the wider data path made it simple to add an address line to the standard, so all Wide SCSI implementations support 16 IDs (0-15). The SCSI-2 standard supports all the IDs, so Wide SCSI can support up to 15 devices plus a host.

The improved transfer rate came with the penalty of reduced SCSI chain length, limited to 3 meters (9.8'). It is important to understand that ribbon and simple twisted pair cables, while cheap, are not optimum transmission devices for high speed digital signals. These signal pulses bounce back along wires whenever they encounter any change in the construction of the wiring and generate crosstalk, ghost signals in adjacent wires. The longer the cable and the faster the pulse rate, the more these effects degrade the signal, placing limits on cable length and/or transfer speed.

SCSI-2 opted to reduce length to boost transfer speed.

SCSI-3, Ultra SCSI

By the time SCSI-3 came along, inexpensive electronics that could resolve faster pulses were practical in consumer gear. This let engineers double the internal clock rate of SCSI processors, and Ultra SCSI was born with 20 MegaTransfers per second performance. In theory, both narrow (8-bit) and wide (16-bit) Ultra SCSI interfaces are defined, but it is rare to see a device with narrow Ultra SCSI fitted to it. Thus, Ultra SCSI devices operating on a wide bus can do 40 MBps. If they are included on a narrow chain, they max out at 20 MBps, just like Wide SCSI.

Unfortunately, increasing the clock rate doesn't change the behavior of the cables, so Ultra SCSI is sensitive to cable length. 3 meter (9.8') Ultra SCSI chains will work reliably, but only when no more than 5 total devices (including the host) are attached. Limiting the chain length to 1.5 meters (4.9') allows 6-8 total devices on the chain. For reasons I won't go into here, 8 total devices is the practical limit for Ultra SCSI.

Ultra2, LVD SCSI

But there is more to the SCSI standards than discussed so far. One of the specs is the electrical transmission method that defines voltages and other characteristics on each wire in the interface. Without getting into a long discussion, suffice it to say other implementations that sacrificed cost and compatibility for longer cable lengths have been around since SCSI-2, but they don't apply to desktop systems most users will encounter.

However, under SCSI-3, a new, relatively inexpensive and completely compatible electrical standard that allowed long cable lengths was defined. This is Ultra2, also called LVD SCSI. By devoting two wires to each signal and only looking at the difference between them, the limitations of cable effects on signal integrity is greatly reduced. This doesn't double the numbers of wires in the cable, because conventional designs pair each signal wire with a common ground wire, but it does double the number of line drivers required, so it is more expensive.

Ultra2 takes advantage of this connection standard to produce a bus with a 40 MegaTransfer per second maximum transfer rate, a maximum cable length of 12 meters (29.4'), and support for 16 device IDs on the SCSI chain. Since it is invariably offered only in a wide version, the maximum transfer rate is 80 MBps.

To sum it all up, here's a list of the major characteristics of each SCSI flavor:

SCSI-1

5MBps
<6 meter (19.6') cables
8 IDs, 8 supported

SCSI-2

Fast SCSI
10 MBps
<3 m (9.8') cables
8 IDs, 8 supported

Wide SCSI
20 MBps
<3 m (9.8') cables
16 IDs, 16 supported

SCSI-3, Ultra SCSI(specs vary by cable length and bus width, but no separate naming conventions)

Narrow Ultra
20 MBps
8 IDs

Wide Ultra
40 MBps
16 IDs

5 devices supported on <3 meter (9.8') cables, 6-8 devices supported on <1.5 Meter cables

Ultra2 SCSI

80 MBps
<12 Meter (29.4Ç) cables
16 devices supported in wide implementations

Other Resources

Recent Online Tech Journal Columns

Links for the Day

Recent Content on Low End Mac

Recent Deals

About LEM | Support | Usage | Privacy | Contacts

Entire Low End Mac website copyright ©1997-2009 by Cobweb Publishing, Inc., unless otherwise noted. All rights reserved. Advice presented in good faith, but what works for one may not work for all. Please report errors to .
  LINKS: We allow and encourage links to any public page as long as the linked page does not appear within a frame that prevents bookmarking it.
  Access our RSS news feed at http://lowendmac.com/feed.xml.
  Email may be published at our discretion; email addresses will not be published without permission, and we will encrypt them in hopes of avoiding spammers. If you prefer your message not be published, mark it "not for publication." Letters may be edited for length, context, and to match house style.
  PRIVACY: We don't collect personal information unless you explicitly provide it. For more details, see our Terms of Use.
  Low End Mac is an independent publication and has not been authorized, sponsored, or otherwise approved by Apple Inc. Apple, the Apple logo, Macintosh, iBook, iMac, eMac, iPod, iPhone, PowerBook, MacBook, MagSafe, Mac Pro, Apple TV, and AirPort are registered trademarks of Apple Inc. Additional company and product names may be trademarks or registered trademarks and are hereby acknowledged.


Navigation

Used Mac Dealers
Apple History
Video Cards
Email Lists

Favorite Sites

MacSurfer
MacMinute
MacInTouch
MyAppleMenu
InfoMac
Macs Only!
The Mac Observer
Accelerate Your Mac
RetroMacCast
PB Central
MacWindows
The Vintage Mac
   Museum
DealMac
DealsOnTheWeb
Mac2Sell
ramseeker
Mac Driver Museum
JAG's House
System 6 Heaven
System 7 Today
the pickle's Low-End
   Mac FAQ
Abandonware
   Petition
Mac vs. PC Info

Affiliates

The Apple Store
Mac Connection
MacMall
TechRestore
ExperCom
Crucial Memory
batteries.com

Have a question?
Ask an expert!

Advertise

All of our advertising is handled by BackBeat Media. For price quotes and advertising information, please contact at BackBeat Media (646-546-5194). This number is for advertising only.

Open Link

Problems viewing this page with Internet Explorer 5.5 or 6? It works fine in other browsers, including IE 7. We recommend Firefox for those using Windows, as it is standards based and more secure than IE 6 (and earlier). More LEM visitors use Firefox than any other browser.