Re: tlug: Some hardware questions...

[Howard Abbey <habbey@example.com>]

Well, since Jonathan asked for more info...
Just call this the confusing world of SCSI.

Karl-Max Wagner wrote:
> OK. But some of the questions will require a somewhat complex
> answer. 
And complex answers often need correcting.  (Sorry; overall it was
a good explanation.)

> SCSI is a peripheral bus system. It allows the connection of at
> most 7 peripheral devices. 
For SCSI-1.  Newer standards (below) alow 16 total (-1 for PC
host interface.)

> However, many SCSI controllers have a lot of advanced
> functionality built in which the IDE manufacturers then scramble
> up to implement, too. Actually there are real IDE adapter
> controllers now that implement a lot of the functionality good
> SCSI adapters have. However, this is in its infantility still.
SCSI contollers often take most of the load of controlling the
drive away from the CPU.  I haven't seen IDE do this yet, but if
they're starting to, great!

Another advantage of SCSI is that it is what is used in high-end
systems.  Much of the stuff built for them are built VERY tough,
and very fast.  IDE is catching up on speed, but reliability is 
still SCSI.

> > What's the difference between SCSI II, Wide and UltraWide?
> 
> One beautiful day they found out that 16 data lines aren't
> enough and beefed that up to 32 lines and called the result Wide
> SCSI. AFAIK UltraWide is the next step and has 64 lines.
> Beautiful side effect: they are - obviously - mutually
> incompatible. They forgot to implement a variable bus width.
> Megabummer.
The line numbers are off.  First 8, then 16, data lines.  The
next steps are only improving quality, and speed, not # of lines.
And some are (sort of) compatible.

I have a feeling the formatting will get munged, but...
Table of SCSI standards:  
Spec. #	Name	     Sales name  Speed MB/s  *Compatible
First	asynchronous SCSI   SCSI  	  7	with this?
	a.k.a. Single Ended
^^^\- still commonly used in scanners, other slow stuff
SCSI-2	synchronous SCSI    SCSI-II	  10	yes
	a.k.a. Fast SCSI
	or Fast-10
	or Single Ended Fast
	    " Wide	     unpopular	  20	yes, sort of
		needed 2 cables I think...

	Differential Fast   Differential 20?	NO! 
	-will fry equipment if plugged in to wrong type.
	    " Wide???

SCSI-3  
	Single Ended Fast-20 Ultra	  20	yes
	    " Wide	     	  40	with adapter
	Low Voltage Differential(LVD)
	                     Ultra-2	  80,160??	yes

* By compatible, I mean you can use all the devices together.  
With Fast-20, the fast devices will go fast, the slow ones slow.
LVD reverts back to Fast-20.

Now you all know why SCSI can be so confusing, especially when 
sales people themselves mix things up and create new names.

> > If you can point me to somewhere where this is covered in detail,
> > please do.
> 
> The official ANSI SCSI specs cover that in more detail than you
> probably like......
DEFINITELY.  I had to study them at IBM.  Good sleeping material.
They're available at:
ftp://ftp.symbios.com/pub/standards/io/x3t10/drafts/

A summary is at:
I couldn't find one, so I can't check to see if my memory is 
correct.  :(  I found something, but, it's not a summary, it's a 
listing of SCSI adapters for sale, but gives examples of each 
standard and data rate:
http://www.symbios.com/semi/scsichra.htm


> > Zip Drive
> > ---------
> > I have an Ultrawide-SCSI Adaptec card. I almost bought an Iomega Zip
> > Drive today, before noticing it said it was SCSI II.

> > Do I need some kind of adpater to slot it in?
> 
Ultrawide = SCSI-3 => only need an adapter & cable.
This is the setup I use for ALL my SCSI devices, including my
SCSI zip drive.  

However, I've yet to see an Ultrawide-SCSI adapter card that didn't
also include a narrow adapter plug internally.  My guess is that you 
don't even need an adapter.  (Mine are in an external SCSI box, so an
adapter for the external wide plug on my SCSI card is needed.)

One other confusing thing to note.  Internal SCSI cables:
The 'wide' cables with more lines are physically narrower than the
'narrow' cables.

> > Does a zip drive use an internal or external power source?
Since it was SCSI-II, I'll guess it was internal, and therefore
will run off the internal power, just like hard drives do.

And the kicker:
Jonathan Byrne asked:
> What about things like Ultra-ATA that claim a 33 MB/sec tranfer 
> rate, the same as UltraWide SCSI?  Do they really do this, or 
> is this a theoretical capability of the drive that cannot be met 
> by the adapter card in the computer?
Ultra-ATA's 33 MB/sec is the theoretical capability of the adapter
card in the computer and drive, that rarely (read almost never) can
be met by the physical drive.  The fastest drives (SCSI of course) 
out there can sustain a speed of 10-15 Meg a second.  All drives have
buffers in them (< 1 meg), and that's what might actually use the
33MB/s  bandwidth, assuming you're OS doesn't do any caching (yeah,
right.) Typical drives max out at sustained rates of 8MB/s or so, I
think. So why all this fuss over fast bus speeds (33 or 80 MB/s), when
the drive can't keep up?!?  For SCSI, it makes sense.  7 devices, all
wanting to be used, at ~10MB/s each = 70MB/s.  Excellent for a RAID
system.

BTW, I'll take this moment to HIGHLY recommend Linux's software RAID
setup for anyone with SCSI.  If you have 2 drives, you can get about
1.5 times the speed.  (If it was ideal, it would be 2 times the speed.)
And don't think you have to lose space.  That's only if you want 
increased reliability (which the driver doesn't easily provide anyway,
last time I checked).    I use it for some ancient small (~100 meg ea.)
drives to bring them to a reasonable partition size & speed.

Later,
Howard
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