At 01:39 12/31/1999 , Brad Loomis wrote:
>I am looking at 18.2 GB 7200RPM, IBM drives. I have an Adaptec
>2940UW controller [with] a 4.5GB [68 pin] drive on it. I know
>that U2W is backward compatible, so do I need to get a 68 pin
>drive, or an 80 pin and adapt it? If I got the 68 pin, would
>that put the hex on a controller upgrade it the future? As far
>as performance? And what about LVD, yeah or nay? I realize that
>some of the newer drives require a separate terminator

Hi Brad

As you noted, you can use any U2W drive with your present Adaptec
2940UW controller. Some U2W drives will automatically sense the lower
voltage and switch to SE (regular UW) mode but some (IBM comes to
mind) will work as SE only if you first set a jumper on the drive.

An 80 pin drive (called SCA) will work fine on normal 68 pin cables
as long as you have an 80pin-to-68pin converter. (These converters
might cost $20 to $40.) I don't see any reason to buy an 80 pin drive
unless you are going to use a "hot swap" tray to hold the drive or
unless you can get it for a really good price.

One thing to remember is that in general, neither U2W (aka LVD) nor
SCA drives have termination circuitry on board. So you need to
either have a regular SE drive like your 4.5GB 68 pin drive at the
end of the cable with its termination enabled, or have a separate
termination module at the end of the cable, or get an 80pin-to-68pin
converter that adds termination. An 80 pin drive with one of these
80-to-68 converters attached (one capable of terminating the bus)
is completely equivalent to a normal wide SCSI drive.

If you do get an Ultra2 SCSI controller in the future, you might
consider getting one like Adaptec's 2940U2W which lets you put SE
devices (like your 4.5GB drive) on one cable and U2W LVD devices
on a separate cable. (In other words, there are two electrically
independent branches of the SCSI bus on a 2940U2W.) There are two
sockets on the 2940U2W...one for a U2W cable and one for a UW cable.
There's even a third socket for a 50 pin SE cable.

Reviews at http://www.storagereview.com/ state that no performance
loss was perceived when using a U2W drive in plain Ultra Wide mode.
However, they also noted (in another article) that using a U3 drive
in U2 mode did hurt performance a lot. (U3 is a newer standard
which offers double the bandwidth of U2.)

U2W allows you to use much longer cable lengths (as much as 4 times
longer than UW) and runs at twice the frequency...which doubles band
width. For example, if you have two 10,000 RPM U2W drives that can
each transfer 25MB/sec, and they are transferring data simultaneously,
and they're running in SE mode, you will only get 40 MB/sec total.
(40MB/sec is the limit for UW SCSI. Admittedly the drives may not be
transferring data simultaneously too often.) But If you had *three*
of these drives, they could all be pumping out 25MB/sec and U2W
would allow you the full 75MB/sec...since U2W allows for 80MB/sec
total. If the combined maximum *external* transfer rate of all your
drives is not greater than 40MB/sec, there is no gain in Ultra2...
apart from being able to use longer cables.

Regards,
Bill

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