Subject: Re: AHA-2940 UW SCSI adapter problems?
To: None <netbsd-users@NetBSD.org, port-i386@NetBSD.org>
From: der Mouse <mouse@Rodents.Montreal.QC.CA>
List: port-i386
Date: 07/19/2005 12:27:41
>>> isn't that an LVD disk?
>> LVD disks will happily fall back to SE SCSI if needed.
> Oh joy, it will work then?  I've just googled quickly for that, and
> don't understand a word of it (LVD vs. SE) but I'll spend some more
> time on it today.

LVD stands for Low-Voltage Differential.  SE stands for Single-Ended.
(There is also HVD, High-Voltage Differential.)

Back when SCSI was first designed, it was what is now called SE: each
signal wire was referenced to a common signal ground.  It was fairly
well done; about half the pins are ground pins, so there isn't too much
of the ground bounce problem.

But even with properly-made cables, where each signal wire is a twisted
pair with a ground wire, there's a limit on the speed you can push that
kind of wiring to.  So someone decided to try differential.

Differential, versus signal-and-ground, refers to the way the
electrical signals are referred.  A differential signal takes two
wires, and what matters is the voltage between them, rather than either
one's voltage compared to something else.  To indicate one logic state,
wire A is driven positive and wire B negative; for the other, the other
way around.  Neither one is referenced to a common ground potential.

Differential requires more complicated electronics, but in general you
can get significantly higher speed out of it (both versus
single-ended).

A differential receiver can handle a single-ended (signal-and-ground)
wire pair, but connecting a na´ve differential transmitter to a
signal-and-ground pair is likely to burn out the driver that's trying
to drive the ground line away from ground.  And indeed, you do not want
to connect a HVD disk to a SE SCSI bus.

With LVD, though, they were smart.  A LVD disk has electronics to
notice when half the signal pins are tied to ground, and it switches
over to SE-style communication.

> Are SCSI disks really all they are cracked up to be?  Is SCSI vs. ATA
> just a religious war, or are there technical merits to SCSI?

There are real differences.  I understand they are less now than they
used to be, but since I use old computers I tend not to know much about
that.  For me personally, there are two reasons I prefer SCSI: (1)
reliability (in my experience, SCSI disks break significantly less
often than IDE disks) and (2) portability between machines (I have
about four machines capable of speaking IDE; I have dozens that speak
SCSI).

Both of these are quite likely artifacts of my collection, but that
doesn't make them any less advantages for me....

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