SCSI Small Computer Systems Interface

SCSI is one of the oldest and most evolved standard interfaces still in use. Originally developed by Shugart Associates in 1979, the standard spawned multiple revisions of ANSI standards: SCSI-1, SCSI-2, and SCSI-3, each with significant performance enhancements. (ANSI is the standards organization that rules these things. They move slowly, but like a glacier, they also have tremendous force.)

A useful but confusing aspect of the standards is the fact that although each revision defines new features and performance levels, many of these features are optional. Thus, two drive or controller manufacturers may elect to support different subsets of the features of a standard, and both can still call their products compliant to the same standard. To differentiate their products from each other, the manufacturers' marketing folks invented a bunch of "marketing names" for certain common subsets of the standard features. And of course, these names rapidly supplanted the standard names in common usage.

The end result was a proliferation of similar-but-different names which can be damn near impossible to sort out. So, we will place all the SCSI cards on the table for you, and help make sense of it.

First, we'll briefly define the three real ANSI standards, a bunch of the terminology and acronyms, and the dozen most common marketing names. Then we'll tell you the important stuff — the drives and controllers that Glyph sells, and why we think these are the right ones for you.

Standard: SCSI-1

The SCSI-1 standard is obsolete, and was withdrawn some time ago by ANSI. Old SCSI-1 devices can still be used, of course, and they can be run on faster/wider busses, with some restrictions; but their presence will degrade the operational characteristics of the bus (i.e. slow it down).

Standard: SCSI-2

SCSI-2 doubles the maximum bus speed from 5 MHz to 10 MHz ("Fast"), and doubles the data width from 8 bits to 16 bits (2 bytes = "Wide"). Running at 10 MHz and 16-bit width ("Fast/Wide") allows data transfer rates up to 20 MB/sec. SCSI-2 also permits more devices on the bus (16), defines active termination for higher data reliability, and defines "differential" voltage signals at 5V ("HVD"). Software-wise, it supports command queuing and expanded command sets to support optical and removable media.

Back-compatibility with SCSI-1 devices was maintained fairly well -- in general, older SCSI-1 devices will operate on a SCSI-2 bus, of course at the SCSI-1 performance level. Their presence on the bus makes the entire bus run at the reduced performance, and any faster or wider SCSI-2 devices on it also are forced back into SCSI-1 operation.

Standard: SCSI-3

But naturally, device manufacturers don't wait around for ANSI to release the final standard — they design, build, and ship lots of products in the meantime. Since the working draft standards documents are available for public reference, they use these drafts as a starting point and the manufacturers define what is actually getting built and sold.

The SCSI-3 standard introduces higher data speeds (and thus higher bandwidths), 3-Volt differential signalling ("LVD"), double-clocking, and other performance enhancements in the command set. It also restricts the number of devices on the bus, and the length of the bus cabling, based on the type of signalling in use.

Summary of the ANSI SCSI Standards

        * data speed x2 means double-edge clocking

Signalling — Single-ended is Like a Guitar What?

Voltage is easy. The older high voltage is 5 Volts (not really that high, eh?) that used to be common with TTL logic chips. The lower voltage is 3.3 Volts, which is common these days with faster CMOS chips. The important thing here is that you not place a low-voltage device on a high-voltage bus — you'll blow up the low-voltage chips.

Now hang in here with us, we're going to draw an analogy between SCSI and audio, becauses single-ended vs. differential SCSI is just like "unbalanced" and "balanced" in audio signals.

A guitar cord has only one signal conductor (other than ground), and is known as "unbalanced". And as you know, guitar cords are prone to noise pickup, high-frequency loss if it's a long cable, etc. Same thing with SE "single-ended" SCSI — each signal is carried on one wire of the cable. So it's not very robust against noise and losses, and that's why the maximum allowable cable lengths are so short for SE at the higher data speeds.

On the other hand, an XLR microphone cable has two signal conductors (in addition to ground), hot(+) and cold (-), and is known as "balanced". And as you know, balanced signals can travel farther without high-frequency loss and they pickup less noise. Same thing with DIFF "differential" SCSI (LVD and HVD) — each signal is carried on two wires (+) and (-) of the cable. So it's a lot more noise-immune, doesn't suffer as much high-frequency loss, and thus you can run these screaming data rates like 160 and 320 MB/sec.

The reason you can use the same cable for both SE and HVD/LVD SCSI is that if you start with a differential scenario, and ground all the (-) pins of the differential signals, you end up with a single-ended scenario. Just as if you tied the cold pin of your microphone cable to pin 1 (ground).

Icons, too?

Enter the Ultra Name Game

(These names make you wonder, though... who would name something "Ultra", knowing it was going to be left in the dust in a matter of months? Well, yeah, there's always "X-treme"... maybe that's next... X-treme-SCSI, and then X-treme-SCSI-2, and ... Just so you know, we're not really picking on marketing folks, we like 'em... especially Glyph's marketing folks who, let's face it, are extremely cool.)

Ahem. The list of names, please (drum-roll)...

Name	Std	Width @ Speed
SCSI	SCSI-1	1 Byte data @ 5 MHz
Wide	SCSI-2	2 Byte data @ 5 MHz
Fast	SCSI-2	1 Byte data @ 10 MHz
Fast-Wide	SCSI-2	2 Byte data @ 10 MHz
Ultra	SCSI-3	1 Byte data @ 20 MHz
Ultra-Wide	SCSI-3	2 Byte data @ 20 MHz
Ultra2*	SCSI-3	1 Byte data @ 40 MHz
Ultra2-Wide*	SCSI-3	2 Byte data @ 40 MHz
Ultra3	SCSI-3	2 Byte data @ 40 MHz x2
Ultra160	SCSI-3	2 Byte data @ 40 MHz x2
Ultra160+	SCSI-3	2 Byte data @ 40 MHz x2
Ultra320	SCSI-3	2 Byte data @ 80 MHz x2

* Unfortunately, because the narrow Ultra2 never got very popular, a lot of people refer to Ultra2-Wide as Ultra2, with the "Wide" left implied. We've been known to do this ourselves, in keeping with common industry usage. When you hear Ultra2, it actually almost always refers to Ultra2-Wide.

Glyph Sez: Here's What You Want

	Data Width (max)	Data Speed (max)*	Data Bandwidth (max)	Signals	Devices, Cable Length (max)
Ultra	1 Byte	20 MHz	20 MB/sec	HVD SE	HVD: 8 dev, 25 m SE: 8 dev, <=1.5 m SE: 4 dev, >1.5 m
Ultra2-Wide ("Ultra2")	2 Byte	40 MHz	80 MB/sec	LVD SE	LVD: 16 dev, <=12 m LVD: 2 dev, >12 m SE: 8 dev, <=1.5 m SE: 4 dev, >1.5 m
Ultra160	2 Byte	40 MHz x2	160 MB/sec	LVD SE	LVD: 16 dev, <=12 m LVD: 2 dev, >12 m SE: 8 dev, <=1.5 m SE: 4 dev, >1.5 m
Ultra320	2 Byte	80 MHz x2	320 MB/sec	LVD SE	LVD: 16 dev, <=12 m LVD: 2 dev, >12 m SE: 8 dev, <=1.5 m SE: 4 dev, >1.5 m

The above is primarily for the hard drive mechanisms. SCSI controller cards (the PCI adapter boards that go in your Mac or PC) often have many selectable capabilities and you can tune the performance of the system based on how you configure the card. We can help you work that stuff out, of course.

Selecting the proper mating set of drives and SCSI controller for your application is something where you want to get in touch with our Application Specialists and interact a little, give some background, get some advice. We here at Glyph pride ourselves on being able to sift through all this and distill it down, but when it comes to making the best decision about your own situation, you can count on Glyph to support you directly too.

Wow!

Resources

The T10 Technical Committee
T10 Working Draft Standards
SCSI Trade Association
The PC Guide, SCSI Data Transfer Modes and Feature Sets