Archived posting to the Leica Users Group, 2001/04/03
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Ah, yes. The SCSI morass.
This is a Leica topic because of high-speed film scanners.
The best place to read about SCSI is the SCSI Trade Association web
site; see the excellent table at
http://www.scsita.org/aboutscsi/index01.html
(If you're a computer weenie, like me, and you want the raw
full-throttle truth, you want to read the T10 Technical
Committee web site at http://www.t10.org/ )
Basically, SCSI has three things:
1. How fast data is sent.
2. How much data is sent in one thunk.
3. How clever are the electrical and logical protocols between
sender and receiver.
SCSI-1 was
1. 5 megathunks per second\
2. 8 bits per thunk.
3. Very stupid.
"Fast" SCSI refers to sending more thunks per second.
"Wide" SCSI (an obsolete term, since it's now all wide) referred to
sending more information per thunk.
"Ultra" referred to a certain combination of speed, width, and voltage
signalling.
"Ultra2" is like Ultra but it allows fancier electrical signalling to
be negotiated, for faster speeds.
"Ultra3" is also like Ultra, but has more fancier electrical
possibilities and more ways that the sender and receiver can make peace
with each other.
Etc.
In terms of our Leica-users computers, if you are using Ultra2 or
beyond, then (as Dennis said) you can mix and match devices of
different speeds, provided that none of those devices wants to use LVD
signalling. You can't use LVD signalling unless every device is LVD,
because LVD stands for Low Voltage Differential, and if the bus is
using low-voltage signalling and some device wants high-voltage
signalling, the high-voltage device won't even see the low-voltage
devices, and will step on them. So if there's a high-voltage device on
a SCSI chain, the controller will tell the low-voltage devices that
they have to slow down and use high voltage.
Low voltage is faster than high voltage because it has less
electrons to move around. It takes longer to send a pulse that will
make the other end see 4 volts than to send a pulse that will make the
other end see 1.3 volts.
I am not aware of any scanners that use LVD signalling. Every scanner
I've ever seen, if it has a SCSI interface, has an SE interface, which
is high voltage.
Any computer designer who designs a computer with an external connector
on an internal LVD SCSI chain should be executed. They know this, so
they don't do it.
Putting this all together, the summary result for photographers is:
it's always a fine plan to plug a SCSI device
into an external connector that is marked
"Ultra2" or beyond. If the external connector is
not Ultra2, then you will have to do some
thinking about whether you are slowing down your
computer by doing this, but if the external
connector is Ultra2 or beyond, no thinking is
required.