[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces
and Configuration | Small Computer Systems Interface (SCSI) | SCSI Data Transfer Modes and Feature Sets ]
SCSI Transfer Mode and Feature Set Compatibility
I sometimes call the various types of SCSI "flavors"; well, if that's so, the
way the interface is heading we'll soon be in Baskin Robbins territory. ;^) The sheer
number of different kinds of SCSI can certainly make the interface seem overwhelming! How
does a SCSI user make it all of this hardware work together? Fortunately, while the
standards and feature sets can be quite confusing, the hardware is actually
well-engineered, and the standards are designed to allow different hardware types to work
together fairly readily.
It's important to remember that a key design goal of all SCSI standards is backwards
compatibility. Few people want to buy new hardware that won't work with their older
hardware. Therefore, in most cases, at least in theory, you can mix older, slower hardware
with newer, faster hardware. You can, again in theory, put a brand-new Ultra160
SCSI hard disk on the same SCSI bus with a decade-old SCSI-1 host adapter (albeit with
added hardware and suboptimal results.) This is generally true, but note the important
qualifier: in theory. Since changes are always being made to the signaling and
other aspects of the interface, there is no guarantee that any two very different pieces
of SCSI hardware will work together.
There are no hard and fast rules regarding the compatibility of different SCSI transfer
modes and feature sets, especially if they are very different in terms of key attributes.
Here are some issues that you should keep in mind as you consider device compatibility:
- Age: The greater the difference in age between two devices, the
greater the difficulties associated with getting them to work together. The extreme
example I gave above of trying to get an Ultra160 drive to work with a SCSI-1 host adapter
(or vice-versa) would probably not be much fun. :^) However, mixing Ultra160 and Ultra2
devices is fairly straightforward.
- Drive and Host Speed Negotiation: You can use faster
drives on slower host adapters or vice versa, but communication will only occur as fast as
the slowest device can handle. For example, you can connect a Wide Ultra SCSI drive to an
Ultra160 host adapter, but the drive will only run at a maximum of 40 MB/s throughput, not
160 MB/s.
- Signaling: Mixing different types of signaling on the same bus can lead
to problems ranging from slowdowns to disaster. :^) The older (high
voltage) differential signaling is not electrically compatible with either
single-ended or LVD devices, and should never be mixed with
those types, or you risk disaster such as smoked hardware. Multimode LVD devices
can be mixed with SE devices, but they won't function at Ultra2 or higher speeds if you do
so.
- Bus Width: You can mix wide and narrow devices
on the same SCSI bus, but there are specific requirements in
doing this, to ensure that the bus functions properly.
- Packages: If you want to be sure that a particular SCSI implementation
will work, buy a complete system or SCSI "package" (including a host adapter,
drives, cables and terminators) from a reputable dealer.
- Ask For Help: If you are considering a particular hardware combination,
ask for advice on it before trying the setup. Unless the setup is extremely strange,
someone may have already done what you are contemplating. A good place to try is the comp.periphs.scsi newsgroup.
 Next: SCSI
Protocols and Interface Features
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