[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces
and Configuration | Small Computer Systems Interface (SCSI) | SCSI Cables and Connectors ]
General Cable and Connector Issues
The main reason why there are so many types of SCSI cables is simply that there are so
many types of SCSI--and so many different ways of implementing them. This great
flexibility is actually one of the key strengths of the interface. The design of any SCSI
cable is based on a combination of different attributes chosen to implement a particular
kind of SCSI bus.
Each SCSI cable must meet the specific electrical requirements associated with the SCSI
signaling speeds and methods it supports. This refers not just to obvious matters--such as
how many pins are on a particular connector type, or which signals are carried on which
wires--but the more complex factors that are the domain of electrical engineering
professionals. For example, the thickness of each wire in a cable, the characteristic
impedance of the cable, materials used for the wires, connectors and covers, and so on.
The following are other factors that have an impact on the design of SCSI cables, as
well as on the selection of cables to meet a particular application:
- Cable Type: SCSI is different from most PC interfaces in that it
supports both internal and external devices. These use drastically different types of
cabling, because the environment inside the PC is very different from that outside it.
Both internal and external cables come in a variety of styles themselves. This is all discussed in more detail on this page.
- Connector Type: Different types of connectors are used for different
kinds of SCSI. These are only partially dependent on the type of physical cable used; to
some extent, connector types are "mixed and matched" with cable technologies to
make particular cables. See here for more.
- Cable Length: The maximum length of a SCSI cable is dictated by the
signaling type and signaling speed of the interface; this summary
table shows the length limits for all the different types of SCSI. However, not all
cables are built to the maximum length. Cables of all different lengths are made to suit
different needs and budgets (most people don't need 12-meter-long cables for LVD devices,
for example, even though they are legal.)
- Number of Connectors: Cables vary in terms of the number of connectors
they include. Generally speaking, longer cables have more connectors, allowing more
devices to be attached to the same SCSI bus segment. Specialized cables may have fewer
connectors; for example, LVD cables can be 25m in length instead of the usual 12m if they
are used "point to point"--just two devices on the cable.
- Connector Spacing: Some types of SCSI have limits regarding how closely
two connectors can appear on the cable. If the cable has many connectors you may have to
leave some of the connectors unused for maximum performance. In all cases, it is
recommended that devices be evenly spaced across the cable.
- Termination: Some cables have a built-in terminator at the end of the
cable while others require the addition of a separate terminator. See the page on termination for more.
- General Quality: The overall quality of a SCSI cable is very important,
but is not something tangible that can be easily measured or quantified. Remember that not
all cables are created equal. SCSI cables are often the culprits in problematic SCSI
buses, so don't skimp on the quality of your cables.
Warning: Some companies
sell cables with labels such as "SCSI-1 cable", "SCSI-2 cable" or
"SCSI-3 cable". With the possible exception of "SCSI-1", these are
extremely vague terms that do not tell you nearly enough about the cable to decide if it
is the one you want. These terms refer to SCSI standards,
which define SCSI families, not specific types.
Next: SCSI Cable Types
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