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[ 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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