Cables and Connectors

Unlike most computer interfaces, SCSI supports both internal and external devices. That is, you can attach a SCSI device either inside the computer's main case or outside that case. (When mounted outside, the device normally resides in its own case.) To support this operation, most SCSI host adapters have at least two connectors, one of which is accessible from outside the computer (see Figure 9.1). Some host adapters have three or even four connectors in order to support both inside and outside connectors and two bus widths, as described later in "Varieties of SCSI."

Chapter 9

Internal External

Connector Connector

Figure 9.1

The internal and external SCSI connectors enable you to attach both internal and external devices to a single SCSI bus.

Internal External

Connector Connector

Figure 9.1

The internal and external SCSI connectors enable you to attach both internal and external devices to a single SCSI bus.

Caution

Although some SCSI host adapters have three connectors, you should never attach 9 devices to all three connectors of most adapters. SCSI chains should be one-

dimensional, each device falling along a single line. It's OK if the SCSI host adapter a C

falls midway along this line, but the line must not split into a Y-shape. Adapters with a I

four connectors usually support two separate SCSI busses, so you can attach devices to EEH

all four connectors. S S

SCSI cabling comes in a confusing array of options. Figure 9.2 shows just two types of SCSI cable: An internal 50-pin ribbon cable and an external 50-pin high-density-to-25-pin cable. Internal SCSI cables usually have enough connectors to link several SCSI devices using just one cable. External cables, by contrast, are two-device items. Most external SCSI devices include two connectors, so it's possible to link devices one after another.

Part II

Figure 9.2

SCSI cables come in a wide variety of lengths and connector types. This figure shows two types of SCSI cable: an internal 50-pin ribbon cable and an external 50-pin high-density—to —25-pin cable.

Figure 9.2

SCSI cables come in a wide variety of lengths and connector types. This figure shows two types of SCSI cable: an internal 50-pin ribbon cable and an external 50-pin high-density—to —25-pin cable.

If you have a SCSI cable that has the wrong connector type on one end, it's often possible to purchase a converter, such as the one shown in Figure 9.3. These converters are usually less expensive than a new SCSI cable, but they can also be awkward. In some cases, they add enough weight to the connection that they can come loose or even fall off completely. I therefore recommend avoiding such converters whenever possible.

Some of the most common SCSI cable and connector types include

• 50-pin ribbon cable Shown in Figure 9.2, 50-pin ribbon cable is the workhorse of older SCSI implementations. It's used for internal connections to older SCSI hard disks, and is still used for non-disk devices such as tape backups and CD-ROM drives.

• 68-pin ribbon cable 68-pin ribbon cable is physically narrower than 50-pin ribbon cable, and uses connectors with pins spaced closer together. It serves the same basic function as 50-pin ribbon cable, linking internal devices. Most recent SCSI hard disks use 68-pin connectors.

Chapter 9

Figure 9.3

SCSI converters let you plug one type of cable into a mismatched socket.

Figure 9.3

SCSI converters let you plug one type of cable into a mismatched socket.

• 25-pin external cables 25-pin cables should never have happened, but they did. 25-pin connectors are common on some inexpensive external SCSI devices, and some low-end SCSI host adapters have 25-pin external connectors. The connector on the left side of the external cable shown in Figure 9.2 is a 25-pin external SCSI connector.

• 50-pin Centronics cables 50-pin Centronics connectors look like oversized parallelport connectors on printers (see Figure 9.4). They're used on many external SCSI enclosures, and some older SCSI host adapter cards used them as well. They're a good choice for connecting 50-pin external SCSI devices.

• 50-pin high-density cables 50-pin high-density connectors are commonly used as external connectors for SCSI host adapters. (The external connector in Figure 9.1, the right end of the external cable in Figure 9.2, and the visible end of the converter in Figure 9.3 all use these connectors.) Note that high-density connectors tend to be smaller than conventional connectors with fewer pins. These connectors are easy to use, but they sometimes work themselves loose, particularly when paired with an adapter like the one shown in Figure 9.3.

• 68-pin external cables These cables use connectors that look much like the 50-pin high-density connectors, except that they include 18 more pins. They're used to link external versions of modern hard disks to SCSI host adapters.

EE h

Part II

Figure 9.4

50-pin external devices often use Centronics-style SCSI connectors.

Figure 9.4

50-pin external devices often use Centronics-style SCSI connectors.

Which cable type you need depends on the devices you intend to link. Also, be aware that some SCSI variants require special characteristics in their cables. For instance, Ultra2 SCSI devices require special cables, although the connector types are the same as I've just described.

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