Lesson 3: SCSI Drives

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The Small Computer System Interface (SCSI) has become the mass-storage device of choice for large network installations. SCSI was first introduced in Chapter 8, Lesson 2: Hard Disk Drives, and has many advantages over standard IDE and EIDE drives. SCSI is the favored drive for high-end workstations, network and Internet servers, and the Macintosh line of personal computers. In many installations, the advantages far outweigh the slight extra effort in configuration. In this lesson, we explore the advantages and uses of a SCSI system.

After this lesson, you will be able to:

  • Define the advantages and disadvantages of a SCSI system.
  • Determine whether a SCSI system is best for your client.
  • Set up a SCSI system.

Estimated lesson time: 30 minutes

SCSI was introduced in 1979 as a high-performance interface, allowing connection of both internal and external devices. Because it runs on virtually any operating system, it was adopted by the ANSI Standards Committee and is now an open standard in its third generation.

At its core, SCSI is a simple design. A single card, the host adapter, (or a chip set on the motherboard) connects up to 15 devices. These devices can be attached inside or outside the PC using standard cables and connectors. SCSI is the only interface that can connect such a wide variety of devices. Communication between the devices and the host adapter is done without involving the CPU or the system bus until data must be passed to one or the other.

This design frees expansion slots and reduces the number of interrupts and memory addresses needed, while cutting down the number of drivers required. Less-robust solutions, such as IDE and EIDE, are little more than switching stations, relying on the PC's CPU to manage the data bus. SCSI host adapters are true subsystems with advanced commands that can order and route data to improve performance.


In the late 1970s, Shugart Associates developed an interface to handle data transfers between devices, regardless of the type of device. The interface operated at the logical—or operating system—level instead of at the device level. This new interface was called the Shugart Associates System Interface, or SASI—the precursor to SCSI.

In June 1986, the ANSI X3.131-1986 standard known as SCSI-1 was formally published. This was a very loose definition, with few mandates. As a result, manufacturers of SCSI products developed a variety of competing designs.

SCSI-1 supported up to seven devices on a chain (plus the host adapter). Each device transferred data through an 8-bit parallel path. Compatibility of SCSI drives was nearly impossible because many SCSI devices had their own custom commands on top of the limited SCSI standard.

You might encounter older SCSI adapters, drives, and peripherals that are based on the original SCSI-1 standard. In reality, this standard amounts to little more than a few agreed-upon commands. The wide range of proprietary drivers, operating system interfaces, setup options, and custom commands made true compatibility a real problem and gained SCSI a bad reputation on the PC platform. It was, however, popular with Apple and UNIX developers, who could work with a limited range of devices.

In most cases it is best to upgrade any SCSI-1 devices to SCSI-2. If circumstances require you to work on an early SCSI product, you will have to contend with both hardware and driver issues. Check the Web site of your SCSI device's manufacturer for possible new drivers.


The limited acceptance, but great potential, of SCSI-1 led to a more robust standard with a range of commands and a layered set of drivers. The result was a high-performance interface that began to take over the high-end market. It was the interface of choice for fast hard disk drives, optical drives, scanners, and fast tape technology.

One of the most important parts of the SCSI-2 specification is a larger (and mandatory) standard command set. Recognition of this command set (18 commands) is required for a device to be SCSI-2 compliant. The Common Command Set (CCS) made compatibility of multivendor devices possible. The CCS also introduced additional commands to more easily address devices such as optical drives, tape drives, and scanners.

SCSI-2 also supports:

  • Wide (16-bit) SCSI.
  • Fast SCSI.
  • Fast/Wide (combines fast and wide features).
  • Ultra (32-bit) SCSI SI-2.
  • Backward compatibility with SCSI-1.

Fast SCSI-2

This standard uses a fast synchronous mode to transfer data, doubling the data transfer speed from 5 MB/s to 10 MB/s. Wide SCSI doubles that again.

Plug and Play SCSI adapters first arrived with the advent of the SCSI-2 standard. Today, all new SCSI host adapters are Plug and Play. The SCSI-2 standard took a long time to gain final approval, requiring agreement by many vendors. As a result, you might run into products labeled "Draft SCSI-2." In almost all cases, you can get these products running on any SCSI-2 or later system if you get the appropriate drivers from the vendor or the maker of the host adapter or operating system.


To speed up the pace of development, the SCSI Committee approved a "fast track" system for the SCSI-3 standard. It let a subcommittee handle most of the work, and new subsections were adopted without waiting for the publication of the entire SCSI definition.

That bright idea, plus the advent of the PCI bus and mature Plug and Play operating systems, has made it easy to install components and given users excellent control and flexibility. All SCSI-3 cards have ways to support existing SCSI-2 devices. Some of the highlights of current state-of-the-art SCSI technology on the desktop include the following seven features:

High-Performance Products

The success and stability of the SCSI standard makes it an ideal platform for developing high-performance products. SCSI's robust, reliable interface and advanced commands allow manufactures to build "best-of-breed" products to take advantage of its power. The fastest hard disk drives and CD-ROM devices traditionally show up first, sporting a SCSI interface. The most advanced scanners are SCSI-based, and many optical products come only in SCSI versions. Even when non-SCSI versions reach the market, they generally under-perform their SCSI siblings.

Plug and Play Installation

Well-designed SCSI cards are recognized and drivers are installed automatically with Plug and Play operating systems such as Windows 98, Windows NT, 2000, and the Macintosh OS. Most SCSI-based peripherals provide Plug and Play setup. The first time the system is booted up after they are added to a SCSI chain, the system notices the new device and asks for the product's setup disk.

Simple Expansion

Adding external devices is as simple as connecting an industry standard cable and power cord. If users decide to add additional host adapters, they can share the same drivers, reducing system overhead.

Advanced Management

SCSI products generally offer a range of tools to tune the bus and devices attached to it. For example, many host adapters have firmware that provides the ability to format and inspect hard disk drive reliability and define custom settings for each device on the chain. Operating-system utilities are provided to check the status of a device and enable advanced features.

SCAM Support

This SCSI acronym stands for "SCSI configured auto-magically." Most new SCSI products are SCAM enabled, meaning that the user does not have to worry about setting the ID numbers for them, because they will configure themselves, using an open ID position on the SCSI chain.

Even if a hard disk drive is SCAM enabled, you might have to set an ID on multidrive PCs, because the host adapter will need it to determine which drive is the normal boot device.


This command allows a SCSI device handling a large amount of data or performing complex operations to disengage from the host adapter's bus while performing the task, allowing other devices free access until it is finished.

Tag Command Queuing

SCSI devices with this feature can reorder how blocks of data are moved on the bus to speed transfer. The way it functions can be compared to letting a shopper with only a few items move to the head of the checkout line, to reduce the average wait time per shopper.

SCSI and IDE Compared
Devices per channel 7/15 per chain 2 per chain
Maximum potential throughput for major classes of SCSI and IDE 160 MB per second (Ultra 160)
80 MB per second (Ultra2)
40 MB per second (Wide SCSI)
66 MB per second (UDMA)
33 MB per second (UDMA)
16.7 MB per second(Fast ATA)
Connection types Internal and external Internal only
True bus mastering Yes No
Operate more than one I/O device at a time? Yes No
Advanced commands (such as tag command queuing, connect/disconnect) Yes No

Noise and SCSI

Any electrical signal other than data is called noise. Due to the many signals and electrical devices present, the interior of a computer is a noisy place. Computer manufacturers do many things to contain the noise inside the case, including adding shielding and grounding. Anything inside, or directly connected to, a computer is either a contributor to or a victim of the noise.

Because of the high data transfer speed, products using the SCSI-2 and later standards can be very sensitive to noise. Cables tend to act as antennae for noise. For this reason, proper cabling and minimizing of cable length are needed to maintain low noise in a SCSI system. Any noise spread through either the electrical power cables or the data cable is called "common-mode" noise.

A single-ended device communicates through only one wire per bit of information. This one wire is measured, or referenced, against the common ground provided by the metal chassis. Single-ended devices are vulnerable to common-mode noise (they have no way of telling the difference between valid data and noise). SCSI-1 devices are all single-ended.

Some SCSI-2 and SCSI-3 devices are differential-ended. These products employ two wires per bit of data—one wire for the data and one for the inverse of the data. The inverse signal takes the place of the ground wire in the single-ended cable. By taking the difference of the two signals, the device is able to reject common-mode noise in the data stream.

Under no circumstances should you try to connect single-ended and differential-ended devices on the same SCSI chain. You might fry the single-ended device and, if the differential-ended device lacks a security circuit to detect your mistake, you will probably smoke it as well.

Troubleshooting a Device Conflict

Determine which is the offending device by taking the following measures:

  • Load only the device drivers for the SCSI devices.
  • If the problem still occurs, use the F8 key to determine which driver conflicts. (Press F8 when starting MS-DOS or Windows 95 or 98—this will allow step-by-step confirmation of the startup process.)
  • If the device driver is an executable file, try running it with the "/?" option. This will usually show a variety of command-line switches for the device driver (for instance, MOUSE.EXE /?). For more details on MS-DOS commands programs and switches, see Chapter 15, "Software: MS-DOS and Windows 3.x."

Here are some ways to correct the problem after you've found it:

  • Look in the manuals or "readme" files of both devices. The problem might be a common one with a known solution.
  • Try a variety of switches to see if any of them solves the problem.
  • Attempt to find an updated driver for one or both of the devices (the Internet is a good place to look).
  • If none of those solutions fixes the problem, you might be forced to choose between the devices or go to a multiple boot configuration.

Memory Management

SCSI host adapters typically have their own ROM chips. For MS-DOS systems, put the appropriate "X=" statements in the EMM386.EXE line of the CONFIG.SYS and the appropriate EMMEXCLUDE= statement in the SYSTEM.INI file. (For more details about configuring these files, see Chapter 15, "Software: MS-DOS and Windows 3.x.") A missing or erroneous "exclude" statement can cause intermittent lock-up problems.

Costs and Benefits of SCSI

Initially, the cost of a SCSI system and SCSI devices is greater than IDE. However, there are several environments in which a SCSI system might justify the increased cost. Some ideal uses for SCSI include:

  • File servers.
  • Workstations (both graphical and audio).
  • Multitasking systems.
  • Systems moving large amounts of data between peripheral devices.
  • Systems with a large number of peripheral devices.
  • Systems requiring fault tolerance (mostly file servers).

The Future of SCSI

SCSI continues as the device of choice for systems in which speed and compatibility are important. The ability of the SCSI format to provide fast and efficient fault tolerance for network systems through the use of RAID (redundant array of independent disks) will keep it as the drive of choice for networks. Although it is not required, the SCSI drive is generally preferred over IDE by Windows NT system designers for its performance and flexibility. SCSI continues to be more expensive than IDE, but SCSI's ability for RAID, hot plugging (changing drives without shutting down a system), and machine independence will keep it popular for workstations and servers.

Setting Up a SCSI Subsystem

There are several steps in setting up a SCSI-based system or adding a new SCSI peripheral to an existing system. Performing these steps in the proper order, without shortcuts, is the key to a fast, easy installation.

Start with the Host Adapter

SCSI cards come in a wide variety of sizes, shapes, and configurations. Some offer one connection, others have four. Options include secondary or even tertiary channels—RAID, cache RAM, and so forth. Be sure that the card will be able to service the devices planned for it. Set any jumpers first, then install the SCSI adapter card in the appropriate expansion slot.

Set the SCSI IDs, Termination, and Peripheral Cabling

Write down the ID for each device—including the host adapter—as it is assigned. After the IDs are set, verify termination for each end of the chain. Finally, attach the cables—first to the host adapter, then to the closest internal device—and move outward on the chain. Repeat the process for the external devices.

External devices usually use some form of switch to set the ID. Most allow setting IDs from 0 through 7 only. You might need to adjust that with internal devices that often allow a wider range of ID numbers. Cable types include: 50-pin Centronics type, SCSI-2 D-Shell 50, and 68-p type connectors. Make sure the last device in the chain is properly terminated.

Internal SCSI devices are installed inside the computer and are connected to the host adapter through an interior connector on the host adapter. Check the connection diagram to be sure the fitting is the right one for that type of device. The options are a 50-pin ribbon cable (similar to a 40-pin IDE cable) and two similar 68-pin cables. Be sure to use the right type of 68-pin cable: one is for ultra-low voltage differential (LVD), and the other is for single-ended (SE) drives. They are NOT interchangeable.

SCSI devices connected incorrectly (for instance, with the cable plugged backwards) can be damaged! Be sure the red or blue strip on the cable is facing toward pin 1. Some SCSI devices allow only a proper connection.

Power Up One Device at a Time

A good practice is to connect the power to one device, power up, and check for problems. Then power additional devices one at a time and make sure everything is working and without conflict.

Load Operating System Drivers and SCSI Software

Finally, load any software required to allow the operating system to recognize the new hardware and take full advantage of its features.

Using a cable with enough connectors enables you to easily link multiple internal devices. You can have up to eight (numbered 0 through 7) devices, or 16 (numbered 0 through 15, depending on the host adapter and the devices) on a single SCSI chain. Don't forget that one position in each SCSI chain is taken up by the host adapter. Figure 9.3 shows a SCSI chain.

click to view at full size.

Figure 9.3 SCSI Chain

The exact number of devices will vary depending on a number of conditions. The host adapter must support the number selected, the installer must be able to set proper IDs, and the cables and connectors must be compatible. Older adapters allow only seven total IDs, and the card will use one, leaving you with six devices. Some SCSI devices have limited ID options. Many older products have only seven possible settings; some scanners or optical products are factory-set to an ID. Given the range of cable options and performance considerations, you might have to limit the number of devices on a single chain to get maximum performance.

Setting SCSI IDs

A simple SCSI chain works like a network, and—like a network—each device requires its own unique address. But unlike a network, setting an address on a SCSI chain is simple. A SCSI device can have any ID number in a range recognized by the host adapter, as long as no other device on the same chain has been set to the same number.

In SCSI numbering conventions:

  • The host adapter is typically set to SCSI ID 7. (This is a de facto standard, not a requirement.)
  • There is no mandated order for the use of SCSI IDs, but the SCAM feature will use a preestablished pattern of IDs if one is available.
  • The ID of a bootable hard disk drive may be preset by the host adapter manufacturer. Most manufacturers use SCSI ID 0, although a few are configured to SCSI ID 6.

Setting a SCSI ID for a device is accomplished using jumpers or switches located on, or inside, the SCSI device. Typically, all internal SCSI hard disk drives use jumpers to set their IDs. External devices usually (but not always) have switches. Some SCSI devices have automatic ID and termination, using SCAM.

Some external devices will offer a limited number of choices. This lack of choices could cause some problems when the chain is full. You might then have to adjust other drive IDs in order to find a unique ID for the new drive.

If you plan to utilize a SCSI drive as your C drive (this is required if you want to boot into MS-DOS from this drive), it must be configured as a bootable drive. This is done by either specifying the host adapter as the "bootable" SCSI ID or setting the host adapter to emulate a standard AT-style controller.

Logical Unit Numbers (LUN)

It is possible to have a single SCSI ID support more than one device. Logical unit numbers (LUNs) can be used to provide a unique identifier for up to seven subunits per ID number. These are used primarily in hard disk drive arrays to create one large logical drive out of several smaller physical drives. LUNs require highly specialized software and are most often found in network servers running NetWare, Windows NT, or UNIX.


Whenever you send a signal through a wire, some of that signal will reflect back up the wire, creating an echo. To terminate a device simply means to put a terminating resistor on the ends of the wire. The purpose of this terminator is to prevent the occurrence of this echo. Two kinds of termination are used in SCSI technology: active and passive. Most older (and all SCSI-1) devices use passive termination. Proper termination of a SCSI device requires special consideration. Older hardware can be damaged by improper termination but, more often, lack of proper termination will result in a boot failure or the failure of the system to recognize a device that has been connected to the SCSI chain.

On most devices within a computer, the appropriate termination is built into that device. On other devices, including SCSI chains and some network cables, termination must be set during installation. The only absolute termination rule is that both ends of the chain must be terminated and that devices that are not on either end must not be terminated. Most SCSI devices come equipped with some form of termination. For most internal products, jumpers can be set to enable termination and connectors can be attached to cables that lead to one of the two SCSI connectors on an external device. Internal Ultra-SCSI 80 and Ultra-SCSI 160 drives do not have termination options on the actual devices. Their termination is handled by a termination block on the end of the cable.

Most new SCSI host adapters are equipped with autotermination circuitry, which polls the chain and sets the proper termination at their ends (or middles). On older cards, you might have to set jumpers. Check the manual for any SCSI device you are installing for instructions on how to set termination and ID before powering it up.

Lesson Summary

The following points summarize the main elements of this lesson:

  • SCSI was introduced in 1979 as a system-independent means of mass storage.
  • A SCSI chain is a series of devices that work through a host adapter.
  • SCSI chains can have up to 8 devices, including the host adapter (or 16, depending on the configuration) connected together.
  • SCSI chains must be terminated on both ends.
  • SCSI is used with many different types of peripherals, including printers, scanners, hard disk drives, and tape units.
  • Bus mastering is a method used by SCSI to transfer data independently of the CPU.
  • RAID uses several SCSI hard disk drives to provide improved performance and fault tolerance for data storage.

Microsoft Corporation - A+ Certification Training Kit
Microsoft Corporation - A+ Certification Training Kit
Year: 2000
Pages: 127

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