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2torial #0431:
Learn2 Connect External SCSI Devices (Macintosh OS)
SCSI, not $%@*!
The term SCSI (pronounced "skuzzy") sounds like a dirty word, but
it actually stands for Small Computer System Interface. Essentially,
it's how all Macintoshes (except for a few obsolete ones) prior
to the iMac and the new blue-and-white G3s "talk" with external
devices such as hard drives, CD-ROM drives, and removable media.
Some PCs also rely on SCSI.
Making things a little more complicated is the development of Fast/Wide
SCSI, a newer standard that increases the amount of information
streaming through the cable connecting your computer with those
external devices. Although it's not exactly as simple as plugging
in a toaster, SCSI does make it pretty easy to expand your computing
capability in unit-by-unit increments.
Note: If you have an iMac, this 2torial does not apply to
you. You have what's called USB (universal serial bus) ports. To
add a USB peripheral, simply plug it in and load the appropriate
drivers. USB devices are hot-swappable, meaning after you plug in
your new device, you don't need to reboot your computer before using
it.
Also worth mentioning is Firewire, an up-and-coming replacement
for the waning SCSI interface. Firewire is much like USB in that
you simply plug in the new device and go, but it's much faster and
more expandable. Drivers are automatically loaded by the device
itself. A Firewire chain can hold up to 127 devices and operates
at speeds much faster than SCSI and USB. Featured in the blue and
white G3s, Firewire devices are just beginning to hit the market.
Keep in mind that there are a few rules that govern all modern
SCSI "chains" (the collection of SCSI devices attached one to another,
and then to your computer):
- A maximum of seven external devices may be connected to your
computer in each SCSI chain.
- The combined length of all cables used cannot be longer than
18 feet, 6 feet is the recommended total length for older (pre-1992)
computers.
- The chain must be "terminated" at the final device. Some devices
have built-in "active" termination, and should be placed at the
end of your chain. If none of your devices have this feature,
you may use the external "passive" terminator that came with the
device or purchase an "active" terminator from a source such as
APS Technologies.
Each device must have an individual "ID number" assigned to it.
No device on the same chain may duplicate another's ID number. Typically,
no device can be set to the address 7 (seven), since that is reserved
for the computer's CPU (central processing unit).
 Set the ID numbers
Somewhere, on almost all external devices, there is a means of
mechanically setting the SCSI ID number. Generally, there's a small
numeral on the back of the device set between two push tabs; push
one tab and the numeral is reduced by one, push the other and the
numeral is increased by one. Sometimes the numeral is hidden by
a sliding door, and on some cheaper devices the numeral is permanently
set, and cannot be changed. What you want to do is:
- First determine the order in which you would like to connect
your devices, from your computer to the first one and then from
one to another.
- Next, for best results, set the device first in line from the
computer to a low ID number, then set each additional device to
a higher number than the one before it. Don't worry too much if
sequential numbering is not possible, everything will probably
work just fine as long as no numbers are duplicated. Keep in mind
that some devices, notably scanners and older CD-ROM drives, can
be temperamental. They should be placed at the end of the chain
(farthest away from the computer, hopefully with the highest number).
 Make the connections (for standard SCSI)
This step describes the connection process for the standard SCSI
device. If you're working with a Fast/Wide SCSI device, skip to
the first bulleted item below.
First: Take a look at your desk area and clear a space for your
external devices. Make sure there are enough power outlets available,
and that all power cords will reach the devices. Turn off your computer
(save your files!), and power down all the external devices.
- Take the DB 25-pin to 50-pin cable and fit the smaller end of
it over the SCSI port on the back of your computer, then screw
down the fasteners on each side. The port will have this symbol
over it:
- Now take the larger end of the cable and fit it over either
of the two connectors on the external device. Screw down the fasteners...or
if the fasteners are clips, clip them firmly into place
- If you only have the one external device, you need to terminate
it now. If it has built-in termination, make sure that the indicator
light on the device is lit. Otherwise fit either your passive
or active terminator over the second connector on the device.
- On the other hand, if you have additional devices to install,
take a 50-pin to 50-pin cable and fit one end over the second
connector on the first device. Now fit the other end over either
connector on the next device. Repeat this step until you run out
of devices (or money to buy them).
- Remember that the last device in the chain needs to be terminated,
either by built-in termination or by placing a terminator over
the second, exposed connector. The more devices in the chain,
or the more temperamental those devices are, the more likely you'll
need "active" termination to make everything work correctly all
the time.
 Make the connections (Fast/Wide SCSI)
- Take the 68-pin to 68-pin cable and fit one end of it over the
port on the adapter card. Make sure it's fastened securely.
- Now take the other end of the cable and fit it over either of
the two connectors on the external Fast/Wide device. Again, make
sure it's fastened securely.
- If you only have the one external device, you need to terminate
it now. It appears that all Fast/Wide SCSI devices currently being
manufactured incorporate built-in "active" termination. There
will be an indicator light, and a small switch to turn the termination
off and on, make sure the indicator is lit.
- On the other hand, if you have additional devices to install,
switch the termination off the first device, take another 68-pin
to 68-pin cable, and fit one end over the second connector on
that device. Now fit the other end over either connector on the
next device. Repeat this step until you run out of devices.
Remember that the last device in the chain needs to be terminated,
so make sure that the indicator is lit on that device.
 Turn it on!
O.K. everything's connected now, right? Power up those external
devices first, and insert any removable cartridges or tapes, then
power up your computer. The devices will show up on your desktop,
and you'll be able to double-click the icons to access them. Congratulations!
You're using your computer to its full potential!
Keep in mind, though, that if you'd like the option of powering
up some devices at different times than others (or changing removable
cartridges while working), you'll need additional software such
as SCSI Probe, HDT Toolkit, or shareware that you can download off
the Web.
 Troubleshoot
The SCSI standard (or rather, standards) make peripheral connection
a pretty straightforward process; most of the time, the steps in
this 2torial should have you up and running. Nevertheless, problems
can arise. Here are a few situations that you might run into:
- Non-standard SCSI connectors on devices. Some devices, such
as Zip drives, don't have 50-pin Centronics-style connectors on
them. All you have to do to integrate them into your multiple-device
chain is buy a cable that has the standard 50-pin connector on
one end, and whatever connector the device uses on the other.
These devices generally ship with a cable that will allow you
to connect them to your computer by themselves, and if they are
the only device in your chain...well, no worries.
- Trying to mix Fast/Wide devices with standard devices. If you
really need to do this, make sure the Fast/Wide device is first
in the chain, and get a 68-pin to 50-pin Centronics-style cable
to attach the standard devices to the Fast/Wide one. Be aware,
however that performance of that Fast/Wide device will greatly
suffer.
- Multiple SCSI chains. Some of the new high-end computers support
two or more SCSI paths. This is your best solution to mixing Fast/Wide
and standard devices on one computer, having dedicated chains
for each.
- Bad cables. Cables will go "bad" if they're flexed too far or
have a manufacturing defect, or were just of poor quality to begin
with. If you're having trouble, try switching cables around. The
fix could be as simple as buying a new one.
-end-
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