The term "SCSI cable" usually refers to a complete cable, including the wire,
connectors and possibly a terminator as well. On this page I want to start by looking at
the cable itself, the actual wires that make up the "overall cable". There are a
number of different types of cables available; these are combined with various connector
types to create specific cable implementations.
SCSI cables come in two distinct varieties: external and internal.
External cables are used to connect SCSI devices that do not reside inside the PC, but
rather have their own enclosures and power supplies; internal cables connect SCSI devices
installed within the PC system box. These cables are totally different in construction,
primarily because the external environment represents much more of a risk to data
corruption. This means external cables must be designed to protect the data traveling on
the cable. Internal cables don't have this problem because the metal case of the PC shields the components inside from
most of the electromagnetic and radio frequency noise and interference from the
"outside world". Thus, internal cables can be made more simply and cheaply than
external ones.
Let's start by looking at external cables. These are commonly called shielded
cables because they are made specifically to protect the data they carry from outside
interference. They have a very specific design in order to ensure that data traveling on
the cable is secured, including the following properties:
- Twisted Pair Wiring: All the wires in the cable are formed into pairs,
consisting of a data signal paired with its complement. For single-ended signaling, each
signal is paired with a "signal return" wire--a fancy name for a ground wire.
For differential signaling, each "positive" signal is paired with its
corresponding "negative" signal (see the description of
differential signaling for an explanation of this). The two wires in each pair are
then twisted together. This twisting improves signal integrity compared to running all the
wires in parallel to each other. So an external narrow cable with 50 wires actually
contains 25 pairs; a 68-wire cable 34 pairs. (This sort of wiring is also commonly used in
other applications, such as network cabling, for the same reason.)
- Shielding: The entire cable is wrapped with a metallic shield, such as
aluminum or copper foil or braid, to block out noise and interference.
- Layered Structure: The pairs of wires aren't all just tossed into the
cable at random; instead, a structure of layers is used. The "core layer" of the
cable contains the pairs carrying the most important control signals: REQ and ACK (request
and acknowledge). Around that core, pairs of other control signals are arranged in a
"middle layer". The outer layer of the cable contains the data and other
signals. The purpose of this three-layer structure is to further insulate the most
important signals to improve data integrity.
External cables have a round cross-section, reflecting the circular layers mentioned
just above. Needless to say, these cables aren't simple to manufacture! All this precise
engineering doesn't come without a cost: external SCSI cables are generally quite
expensive. For internal cables all these special steps are not required to protect the
data in the wires from external interference. Therefore, instead of special shielded,
multiple-layer construction, internal devices use unshielded cables, which are
flat ribbon cables similar to those used for floppy drives and IDE/ATA devices. These are
much cheaper than external cables to make.
|
Close-up view of an external SCSI cable.
Note the round shape of the cable's cross-section,
and the labeling, which indicates that this is LVD-compliant,
shielded cable, using AWG 28 conductors. |
Original image © Computer Cable Makers, Inc.
Image used with permission. |
Even with internal cables, there are differences in construction (beyond the width
issue, 50 wires for narrow SCSI or 68 wires for wide SCSI). One issue is the thickness of
the wires used; another is the insulation that goes over the wires. Better cables
generally use Teflon as a wire insulation material, while cheaper ones may use PVC
(polyvinyl chloride; vinyl). Regular flat cables are typically used for single-ended SCSI
applications up to Ultra speeds (20 MHz).
|
An assortment of different internal ribbon cables used
for
connecting SCSI hardware. Note that some are strictly flat
cables, but the one on the far left and the one third from the right
are partially flat and partially twisted pair cable. |
Original image © CS Electronics
Image used with permission. |
For Ultra2 or faster internal cables using LVD signaling, the poor electrical
characteristics of cheap flat ribbon cables begin to become an issue in terms of signal
integrity even within the PC. Therefore, a new type of internal ribbon cable was created
for these cables, which actually combines some of the characteristics of regular internal
and external cables. With these ribbon cables, pairs are twisted between the connectors on
the cable--just like in external cables--but the ribbon remains flat near where the
connectors go, for easier attachment. The return to pair twisting improves performance for
high-speed SCSI applications, while increasing cost somewhat, though not as much as if
external cables are used. This technology is sometimes called "Twist-N-Flat"
cable, since it is partially flat and partially twisted-pair.
Next: SCSI Connector Types
