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SCSI Cable Types

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.

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).

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

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