Picking the Correct SCSI Hardware

Three different types of SCSI cable are commonly used: 25-pin, Narrow (50-pin) and Wide (68-pin). Wide SCSI host adapters typically include at least one Narrow and one Wide connector. Three types of termination are in common use:

• Passive, which is used on the oldest and low-end devices

• Active, which is used on mid-range devices

• Low-voltage differential (LVD), which is used on the latest high-end devices. Active and passive terminations are known collectively as single-ended; they're somewhat interchangeable, although active almost always works better. High-voltage differential, or HVD, termination is also an option on some more obscure SCSI variants. As a general rule, you shouldn't mix termination types, although you can usually get away with mixing active and passive termination. Different SCSI variants support different maximum cable lengths, which you should never exceed. Table 2.2 summarizes the features of different SCSI varieties.

Table 2.2: SCSI Hardware Types

SCSI Type

Speed

Termination

Cable Type

Maximum Cable Length

SCSI-1

5MB/S

single-ended

25- or 50-pin

6m

SCSI-2

5MB/S

single-ended

50-pin

6m

Fast SCSI-2

10MB/S

single-ended

50-pin

3m

Fast/Wide SCSI-2

20MB/S

single-ended

68-pin

3m

UltraSCSI

20MB/S

single-ended

50-pin

3m or 1,5mf^

UltraWide SCSI

40MB/S

single-ended

68-pin

3m or 1,5mf^

Ultra2 Wide SCSI

80MB/S

LVD

68-pin

12m

Ultra3 SCSI orUltra160 SCSI

160MB/S

LVD

68-pin

12m

Ultra320 SCSI

320MB/S

LVD

68-pin

12m

Ultra640 SCSI

640MB/S

LVD

68-pin

12m

^Maximum cable length is 3m for four or fewer devices and 1,5m for five or more devices.

Picking an appropriate SCSI host adapter can be tricky if you need to mix device types, especially those that use different types of termination or cable widths. Some high-end SCSI host adapters support two SCSI chains, which can make such mixtures easier to implement; otherwise, you may do well to buy a low-end SCSI adapter for low-end devices and a high-end adapter for faster devices.

Unlike ATA controllers, SCSI host adapters have no lowest-common-denominator mode; Linux must support your model adapter to provide even minimal functionality. If you want to boot from a SCSI disk, the driver must be compiled directly into the kernel. In this case, the SCSI adapter must also include a SCSI Basic Input/Output System (BIOS), which enables the computer's BIOS to see the hard disk to begin the boot process. Most high-end and mid-range SCSI adapters include a BIOS, but many low-end adapters don't.

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