CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [166]
SCSI is an old technology dating from the late 1970s, but it has been updated continually. SCSI is faster than ATA (though the gap is closing fast), and until SATA arrived, SCSI was the only good choice for anyone using RAID (see the “RAID” section a little later). SCSI is arguably fading away, but it deserves some mention.
SCSI Chains
SCSI manifests itself through a SCSI chain, a series of SCSI devices working together through a host adapter. The host adapter provides the interface between the SCSI chain and the PC. Figure 11-23 shows a typical SCSI PCI host adapter. Many techs refer to the host adapter as the SCSI controller, so you should be comfortable with both terms.
Figure 11-23 SCSI host adapter
All SCSI devices can be divided into two groups: internal and external. Internal SCSI devices are attached inside the PC and connect to the host adapter through the latter’s internal connector. Figure 11-24 shows an internal SCSI device, in this case a CD-ROM drive. External devices hook to the external connector of the host adapter. Figure 11-25 is an example of an external SCSI device.
Internal SCSI devices connect to the host adapter with a 68-pin ribbon cable (Figure 11-26). This flat, flexible cable functions precisely like a PATA cable. Many external devices connect to the host adapter with a 50-pin high-density (HD) connector. Figure 11-27 shows a host adapter external port. Higher-end SCSI devices use a 68-pin HD connector.
Figure 11-24 Internal SCSI CD-ROM
Figure 11-25 Back of external SCSI device
Figure 11-26 Typical 68-pin ribbon cable
Figure 11-27 50-pin HD port on SCSI host adapter
Multiple internal devices can be connected simply by using a cable with enough connectors. Figure 11-28, for example, shows a cable that can take up to four SCSI devices, including the host adapter.
Figure 11-28 Internal SCSI chain with two devices
Assuming the SCSI host adapter has a standard external port (some controllers don’t have external connections at all), plugging in an external SCSI device is as simple as running a cable from device to controller. The external SCSI connectors are D-shaped so you can’t plug them in backward. As an added bonus, some external SCSI devices have two ports, one to connect to the host adapter and a second to connect to another SCSI device. The process of connecting a device directly to another device is called daisy-chaining. You can daisy-chain as many as 15 devices to one host adapter. SCSI chains can be internal, external, or both (see Figure 11-29).
SCSI IDs
If you’re going to connect a number of devices on the same SCSI chain, you must provide some way for the host adapter to tell one device from another. To differentiate devices, SCSI uses a unique identifier called the SCSI ID. The SCSI ID number can range from 0 to 15. SCSI IDs are similar to many other PC hardware settings in that a SCSI device can theoretically have any SCSI ID as long as that ID is not already taken by another device connected to the same host adapter.
Figure 11-29 Internal and external devices on one SCSI chain
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NOTE Old SCSI equipment allowed SCSI IDs from 0 to 7 only.
Some conventions should be followed when setting SCSI IDs. Typically, most people set the host adapter to 7 or 15, but you can change this setting. Note that there is no order for the use of SCSI IDs. It does not matter which device gets which number, and you can skip numbers. Restrictions on IDs apply only within a single chain. Two devices can have the same ID, in other words, as long as they are on different chains (Figure 11-30).
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