Quote:> I'm going to buy an Adaptec AHA-2940UW SCSI controller card. It has a
> 68-pin high-density external connector. I am looking at a Seagate
> Barracuda 9LP 4.5GB Ultra Wide SCSI hard drive, but I find that it comes
> in 3 different 68-pin models. They are:
> W = 68-pin Wide SCSI connector
> WD = 68-pin Wide Differential SCSI connector
> LW = 68-pin Wide SCSI connector, low-voltage Differential
> I would guess I just want the model that ends in W, but if anyone could
> help me out with what I should do, that would be great. I think the WD
> and LW models may have something to do with reducing the effects of
> noise. If it matters, I will not be chaining very many SCSI devices...
You need the "W" model. This uses a single line to transmit data. The
"WD" uses two lines (+D and -D), which makes for safer transmission.
"LW" looks like the Ultra2 variant of "WD".
Anyway, all these (at least W and WD) are _not_ compatible at all. The
2940UW needs "W" drives; connecting a "WD" can kill the hostadapter
and/or the drive.
I'm not entirely sure about the "LW" --- if this is Ultra2, you could
use it, but it doesn't help you since it will operate in "W" mode
anyway (Ultra2 devices operate as "Ultra" if there is at least one
Ultra device on the bus --- the hostadapter in your case), it will
just be more expensive, but without _any_ advantage (on the 2940UW).
If you are chaining _many_ SCSI devices, don't forget that Ultra _and_
UltraWide can handle at most 7 devices (plus the hostadapter). This
is different from non-Ultra SCSI, where Wide busses can handle 15
devices.
Also, if you have more than 3 devices (plus hostadapter), max. cable
length is restricted to 1.5m. So you can just about forget chaining
many external devices.
But then, people overclock their systems, use bad cabling, so I guess
many will tell you that things work fine with >7 devices or 10m cables.
You are operating outside the specs then, which may or may not work,
can fail sometimes and work most of the time etc.
If you overclock your system you can just as well operate outside the
SCSI specs; if you like a stable and reliable system, I recommend
to do things right.
Christian
--
Christian Stieber http://www.informatik.tu-muenchen.de/~stieber
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