Re: Desperate situation
- From: Gene Heskett <gene.heskett@xxxxxxxxxxx>
- Date: Tue, 14 Feb 2006 23:42:39 -0500
On Tuesday 14 February 2006 21:59, Jeff Vian wrote:
[...]
Yeah, my post wasn't very clear. Two channels per controller,
master/slave device per channel, master device should be on the
connector closest to the controller because of noise issues and
that's why some half-bright got the idea that it should just be left
up to cable select anyway, ...
NOTE: If you have a CS cable putting the master in the middle
probably will not work. The CS cable has the master connector at the
end and the slave in the middle.
I have always been told (even in hardware schooling) that you should
always have a device at the cable end to prevent 'ringing' and noise
caused by signal reflection from an unterminated cable end. This is
mandatory on SCSI and highly recommended on IDE cabling. (it is
sometimes unnecessary, not because the problem does not exist, but
because the data transfer is so [relatively] slow that some
interference does not impact performance seem by the user).
I never install a single device on a cable unless it is connected to
the end of the cable. This by default makes the single device the
master, and thus implies a reason for putting the master connector at
the end of the cable.
And it's no surprise that it sometimes "just works" for some people,
and sometimes just can't be made to do what you really wanted it to
do.
ATA confuses me. It irks me no end that ATA has become mainstream,
while SCSI gets no respect.
SCSI gets a lot of respect. It also costs a lot more $ than IDE/ATA
so it is harder to get the home user to justify the extra expense.
That $ factor, and the reputation for needing a nearly endless supply of
virgins to be sacrificed over it to make it work in the presense of
hidiously designed, then further emasculated by the bean counters
changing parts without telling engineering because this parts 5% of the
cost of the part the engineers specified, terminations, or should I say
miss-terminations. I scsi bus, built with the usual 220/330 ohm
passive resistor packs for termination works great on paper, and still
works pretty error free if the isolation diode the engineers wanted,
either a power GE, better yet a schotkey was used, both of which will
have less than say .2 volts of voltage drop across them.
But some bean counter sees the cost and decides an SI diode, with .75
to .8 volts of drop is good enough and the board gets populated and
shipped before the engineers that have a clue discover it.
Whats the problem? Well, with no diode, you get a logic one voltage of
about 3 volts, which is comfortably in excess of the standard ttl logic
one voltage of 2.4 volts, a 600 millivolt noise and echo margin.
Stick an SI diode with a voltage drop of .7 volts in series with the
terminations power supply, and you've lost about .45 volts at the logic
line itself, leaving only a 150 millivolt echo and noise margin. You
need a chicken or goat occasionally to make that work. Its there to
keep the power supply in the outboard box with the scsi stuff in it,
from backfeeding its 5 volt line into the hosts 5 volt line,
effectively preventing you from turning the host off!
Then toss in that most computer psu's fade downward with age, and the
5.03 volts you had when it was fresh a year ago, is now about 4.88
volts. Now the logic one noise/echo margin is reduced to 30
millivolts, and you can sacrifice all the virgins on the planet without
making it reliable enough to even boot up.
The bottom line is that when active terminations are used, the
terminator chip regulates this voltage internally and can tolerate a
fadeing psu well enough that when the box goes down, it will probably
be from some other low voltage induced failure, not a scsi bus error.
Active terms also have the advantage of much less power dissipation, so
the moral of this tome is to, when dealing with building scsi stuff,
demand and get from the suppliers, devices that are actively terminated
for both ends of the cable. If the one you are dealing with cannot
supply such devices, then I'd suggest you look elsewhere for a similar
device.
To say what I meant to say, I removed the _second_ controller card
(not slave controller or whatever it was I said) and have both
drives on the mobo's controller's primary channel, with the CD drive
all by itself on the secondary channel (pretending to the BIOS that
it's SCSI no less). Grub is on the drive on the "master" connector
(closest to the controller on the cable, strapped CS), along with
FC3, and two BSDs. The drive connected to the slave controller
(which is actually there to be a spare because I don't trust ATA
drives) has another BSD and sometimes other Linux on it.
Thats probably wrong, unless its actually being seen as /dev/hdb due to
the CS working and its not the last drive.
If the CS master, which does the terminating of the cable, then it must
be on the END of the cable to do this correctly. Is this drive on the
end of the cable then seen as /dev/hda?
[...]
--
Cheers, Gene
People having trouble with vz bouncing email to me should add the word
'online' between the 'verizon', and the dot which bypasses vz's
stupid bounce rules. I do use spamassassin too. :-)
Yahoo.com and AOL/TW attorneys please note, additions to the above
message by Gene Heskett are:
Copyright 2006 by Maurice Eugene Heskett, all rights reserved.
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