peripherals may not work, or may conflict with critical devices, such as disk controllers and network cards. Here are some points to consider when installing certain devices under Linux:

Video Cards: Linux works with all video cards in simple text mode. Unless you need X-windows capability, you have no need to install an X server on your Linux system — especially if you are running it as a server. X-windows uses a lot of RAM, and you must also carefully check that you have drivers available for your video card to work in X-windows.

Sound Cards: Generally, you have no need for a sound card on a server system, but if you need it on a desktop multimedia system, keep a few things in mind. Be careful when choosing which I/O port, IRQ, and DMA resources that the sound card will use. Network and SCSI cards are often configured with the same resources. This will cause a resource conflict, and it may disable your SCSI hard drive bus, resulting in a system that won’t boot. In the case of a network card conflict, the server won’t be able to communicate with the network.

Installing and Configuring SCSI Devices

7.3 Assure that system hardware is configured correctly prior to installation (e.g., IRQs, BIOS, DMA, SCSI settings, cabling) by identifying proper procedures for installing and configuring SCSI and IEEE 1394 devices

7.6Remove and replace hardware and accessories (e.g., cables and components) based on symptoms of a problem by identifying basic procedures for adding and removing field replaceable components

7.7Remove and replace hardware and accessories (e.g., cables and components) based on symptoms of a problem by identifying common symptoms and problems associated with each component and how to troubleshoot and isolate problems

7.10 Identify proper procedures for diagnosing and troubleshooting SCSI devices

SCSI (Small Computer Systems Interface) is an advanced system bus built for high performance disks and peripheral communications. Although much more expensive than IDE/ATA, SCSI is preferred in server installations for its robust performance and flexible expansion options. Many devices can be chained together on a SCSI bus, which is required for large disk space installations and fault-tolerant RAID systems.

SCSI definitions

Before describing some of the wide variety of SCSI technologies, I need to define several key characteristics of SCSI devices in order to compare the different technologies:

Clock Speed: The clock speed of the SCSI bus, measured in MHz.

Bus Speed: The speed of the SCSI bus, measured in MB/s.

Bus Width: The width of the SCSI bus. Narrow SCSI is 8-bit, Wide SCSI is 16-bit.

Signaling: The type of voltage signaling used to trigger events. The three types are Single Ended (SE), Low-Voltage Differential (LVD), and High Voltage Differential (HVD).

Termination: Describes the various types of terminators needed on the SCSI bus. Termination of a SCSI bus is critical to block signal reflection noise.

Cable Type and Length: Different SCSI technologies require certain cable types and lengths.

Devices Supported: The number of devices allowed at one time on the SCSI bus chain.

SCSI technologies

There have been a wide variety of SCSI standards — each building upon the other in terms of newer technologies that increase the speed and the bandwidth of SCSI communications. The following lists each of the SCSI types that you may encounter and their technical characteristics.

SCSI: The original SCSI 1 standard introduced an 8-bit parallel bus that can perform transfers of up to 5 MB/sec.

WIDE SCSI: Also part of the SCSI 2 standard, it increased the width of the bus from 8 bits to 16 bits. This allows double the amount of information transfer, and also increases the maximum number of devices on the SCSI chain to 16 — the normal SCSI can only have a maximum of 8.

FAST SCSI: Part of the SCSI 2 standard that raised speeds to 10 MB/sec. It also introduced a different type of bus called a differential bus, as opposed to the single-ended bus of SCSI 1. The differential type of bus offered better protection from interference, and increased the bus length to 25 meters from 6 meters.

ULTRA SCSI: Another implementation in the SCSI 2 standard, Ultra SCSI technology doubles the transfer information using the same clock rate speed. This doubles the speed of the bus to 20 MB/sec.

ULTRA WIDE SCSI: This technology refers to using the same doubled transfer capability over a wide SCSI bus. Because the ULTRA SCSI bus is running over 16 bits instead of 8, the effective transfer rate is 40 MB/sec.

ULTRA 2 WIDE SCSI: Utilizing a Low Voltage Differential signaling method, this allows the ULTRA 2 WIDE SCSI bus to double its clock speed, offering an effective transfer rate of 80 MB/sec. This LVD bus can also run in single-ended mode to be compatible with older UTLRA technology. If you do run an Ultra SCSI device on an Ultra2 bus, the bus will always run at the speed of the slowest component.

ULTRA 160 SCSI: This technology improves on the transfer rate again, and offers a transfer rate of 160 MB/sec over a 16-bit bus.

ULTRA 320 SCSI: A new type of SCSI bus that uses a 16-bit wide LVD bus with a doubled clock speed to offer transfer rates up to 320 MB/sec. It is also backwards-compatible with older devices.

Table 19-3 shows a comparison of different SCSI technologies:

Table 19-3

Comparison of different SCSI technologies

SCSI cabling and termination

Because SCSI comes in a wide of variety of bus speeds, clock rates, and signaling methods, it also offers a wide variety of cable and termination requirements for the different SCSI types:

SCSI 1 50-Pin Connectors (Narrow): SCSI 1 devices use either a 50-pin “D” connector, or a 50-pin Centronics type of connector. The “D” connector is named for the shape of the shell that surrounds the pins. It is also called a DB-50, and resembles a DB25 or DB9 type serial and parallel connector. The Centronics connector (which is named after the type of printer that used this type of connector for a parallel interface) does not actually have pins, but

50 flat contacts. Internally, SCSI 1 devices use a rectangular connector of 50 pins, split into two 25-pin rows.

SCSI 2 High density 50 and 68-Pin (Wide) Connectors: Similar to the “D” shaped connectors used by SCSI 1 devices, but they are much smaller, with the pins closer together. The 68-pin versions are also referred to as Wide connectors. For internal cabling, SCSI-2 devices use high-density 50-pin or 68-pin wide connectors.

SCSI-3 68-Pin Centronics Connector (VHDCI): The Very High Density Cable Interconnect resembles the large 50-Pin Centronics connector. These connectors use 68 contacts, which sit very close to each other. Its size makes it much easier to plug two cables into the back of a host adapter.

SCA Adapters: For RAID or multi-disk systems with many SCSI hard drives, the connectors used are Single Connector Attachment adapters. Their special connections allow all signaling and power to be fed through one connector (that the hard drive plugs right into) without any cabling. This technology is most important for hot swapping drives: The ability to remove and replace drives without having to power off the system. The SCA connector utilizes

80 pins in a Centronics type of setting. The server will usually consist of a SCSI backplane with several of the connectors where the hard drives can be plugged into.

Cable lengths

Depending on the type of signaling technology that is employed, maximum cabling lengths can vary. For older single-ended signaling, the degradation rate for the signal increases dramatically as the cable length increases. For a fast bus speed on a single-ended system, the maximum cable lengths are fairly short. For LVD-type signaling, the signal degradation is much less, resulting in longer maximum cable lengths for the SCSI bus. Refer to Table 19-4 for a summary of SCSI cable lengths.

Table 19-4

Summary of SCSI cable lengths

Termination

Termination of the SCSI bus is necessary to provide a way of stopping cable signals from reflecting from the end of the bus back onto the wire and causing data corruption. The terminator must be connected to the end of the SCSI bus at the end of the