12.004 What is "SCSI-2" and how is it different from SCSI-1?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
12.004 What is "SCSI-2" and how is it different from SCSI-1?
For hard drives, "SCSI-2" basically means that the drive supports a stricter
command set. The physical interface is usually identical.
For other device types, "SCSI-2" means a lot more, because the original SCSI
standard didn't define much in the way of device types and command sets, so
most devices use proprietary command sets. SCSI-2 standardises the command sets
for most types of devices.
There are three special types of interface that you might see mentioned:
"Fast SCSI" supports data transfer at twice the speed of the original SCSI
standard (10 MB per second vs 5 MB per second). This will not be a
compatibility issue, as it is just the maximum transfer speed supported by the
drive. The Apple II cannot transfer more than one megabyte per second.
"Wide SCSI" uses a different cable arrangement to double the width of the
data path (16 bits instead of 8 bits). A wide SCSI drive cannot be used with
an Apple II, unless it can also operated in "narrow" mode with the original
50-pin connector. (There is also "Fast Wide SCSI", which doubles the data
rate and the width of the bus.)
"Differential SCSI" involves a different type of interface to the computer,
where every data signal has a balanced positive and negative pair of wires,
rather than a single wire and a ground line. I believe it has a different type
of connector. Differential SCSI drives cannot be used with an Apple II."
Some drives use a proprietary connector, but the standard (narrow, non-
differential) SCSI bus uses the same 50-pin connector for SCSI-1 and SCSI-2.
The only significant problem you might run into is termination, and supply of
termination power. SCSI-2 devices tend to be fussier about termination than
older devices.
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