3.3. Objective 4: Set Up Non-IDE Devices
SCSI is an interface for streaming devices and block storage devices such as tape drives, hard disks, CD-ROMs, and other peripheral instruments. SCSI is the standard interface on server-style PCs, Unix workstations, and many older Apple models (mostly 604 and earlier systems). Desktop PCs and newer Apple systems (G3 and above) usually opt for the IDE (ATA)-style disk interfaces because they are less expensive. The advantage that SCSI has over IDE is that it offers much more flexibility and expandability, as well as faster throughput.
SCSI defines a bus to which multiple devices are connected. The medium is a high-quality cable or a series of cables connected to daisy-chained devices in series. One of the devices in the chain is the SCSI controller, which is the host interface to the other connected SCSI devices. The controller and each of the other devices on the bus is assigned a permanent SCSI address, also known as the SCSI ID, which defines each SCSI device uniquely on the bus. The controller can access devices individually by using the unique SCSI address to access a specific device.
22.214.171.124. SCSI types
The world of SCSI can be a little confusing, despite the standards set by ANSI. The original SCSI-1 interface is a 5 MBps 8-bit interface. It uses a 50-pin Centronics connector, similar to but larger than those found on most printers. This interface is still in popular use today, although the connector is usually replaced by a 50-pin Micro-D connector. (This connector is similar to the DB-25 used for serial ports but has a much higher pin density.) As performance demands have escalated, manufacturers have begun offering enhanced products with faster data transfer rates. Current interfaces include:
Recent developments have yielded additional SCSI interface types with up to 160 MBps throughput, and efforts continue to keep SCSI competitive with other technologies. As performance increases, however, constraints on cabling and connectors become more significant. Such constraints are a major factor in deploying large SCSI-based systems. Also, with the variety of connectors, cables, and transfer rates available in the SCSI standards, it's important to plan carefully. The other inhibiting factor, at least on the consumer level, is that SCSI hard drives tend to cost two to three times the amount of similar-sized IDE drives.
3.3.2. SCSI IDs
Each device on a SCSI bus, including the controller, has an address based on a binary reading of the address lines. The 8-bit SCSI buses have three address lines and thus will accommodate 23=8 devices, including the controller. For the 16-bit buses, there are four address lines resulting in a possible 24=16 devices. This results in a maximum of 7 and 15 devices, respectively. These addresses can be configured using jumpers (typical for disk drives) or switches. SCSI addresses run from 0 to 7 for 8-bit buses and from 0 to 15 for 16-bit buses. It is customary for the controller to occupy address 7 for both bus widths. Disks and other devices must be assigned a unique address on the bus, and they must be provided with proper termination, which is discussed later in this section.
126.96.36.199. SCSI logical unit numbers
Some SCSI devices, such as RAID controllers, appear to the SCSI controller as a disk drive with a single SCSI address. For the controller to access multiple logical devices using a single SCSI address, an accompanying logical unit number (LUN), is reported to the controller. Single disks and tape drives usually only report LUN zero, but sometimes they report the same on all LUNs, and this must be coped with.
188.8.131.52. Linux SCSI disk device files
On Linux systems, IDE disk devices are known as /dev/hda, /dev/hdb, /dev/hdc, and /dev/hdd. For SCSI, a similar pattern emerges, with /dev/sda, /dev/sdb, and so on. The first partition on disk /dev/sda will be /dev/sda1but remember that the partition number has nothing to do with the SCSI ID. Instead, the letter names of the Linux SCSI devices start with sda and proceed across all SCSI IDs and LUNs. The numbers are sequentially assigned to partitions on a single ID/LUN combination.
For example, a SCSI-2 bus with two disks, a tape drive, a RAID controller with two LUNs, and the SCSI controller might be assigned addresses as shown in Table 3-2.
Another facet of SCSI that can be confusing is termination . A SCSI bus can be considered a cable with devices connected along its length, but not at the ends. Instead of devices, the ends of the SCSI bus have terminators, which are simple electrical devices that condition the signal and reduce electrical noise on the bus. Termination can be particularly problematic if you attempt to mix 8- and 16-bit devices on a single bus and use an 8-bit terminator, leaving half of the 16-bit SCSI bus unterminated.
Termination devices aren't always separate pieces of equipment. In fact, most device manufacturers include termination circuitry on their devices, so the application of an external terminator device is not necessary. SCSI controllers can terminate one end of the SCSI bus while an external terminator or a disk's internal terminator is used on the other end. Whichever type of terminator is being used, you must be sure that exactly one terminator is placed at each end of the SCSI bus (for a total of exactly two terminators), otherwise the bus may fail.
184.108.40.206. SCSI controllers on PCs
Most PCs don't come with integrated SCSI controllers, but a number of add-on cards are available. SCSI controllers have their own firmware installed along with an accompanying BIOS, which has its own configuration menus. If you're using SCSI on a PC, it's important to be able to manipulate these settings appropriately.
Like the BIOS, a SCSI controller BIOS usually has a keyboard combination, announced at boot time, to enter the setup utility. Once the utility is launched, you can control a number of aspects of the controller, including: