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Help Contents
SCSITutor Topics

1	Using SCSI Tutor

2	The SCSI Bus

3	SCSI Bus Termination

4	SCSI IDs

5	Advanced SCSI Programming Interface

6	SCSI Terms and Features

SCSI Definitions

Logical Units

Synchronous Negotiation

SCSI Bus Parity Checking

Single-Ended vs. Differential SCSI


Welcome to SCSITutor

SCSITutor is a graphical, easy-to-use learning tool that teaches you about SCSI.

If you have never installed and configured a host adapter before, you should study all the topics in the order they are listed on the SCSITutor Topics screen. You can run SCSITutor any time you are in Windows by double-clicking the SCSITutor icon, which appears in the Adaptec SCSI group.

SCSITutor is not intended to replace the EZ-SCSI User's Guide or your host adapter documentation. Read those documents carefully when you install and configure your host adapter and SCSI peripheral devices.


SCSITutor Tips

To select a SCSITutor topic, click the name of the topic on the SCSITutor Topics list. When you are studying a topic, press the Continue button to continue to the next screen. On the last screen of each topic, press the Return to SCSITutor Topics button to return to the Topics list.

When you see underlined text like this in a different color, click on it to view more information. This may be background information or a Glossary definition.

Use the standard features of Windows Help to

•	Search for a topic

•	Go Back to the previous screen

•	View a History (selectable list) of the screens you have accessed

•	View the Glossary of terms. The Glossary appears in a smaller secondary window, which you can scroll to find the term you want. Then click on the term to view the definition.


What Is a SCSI Bus?

A SCSI bus is an electrical pathway that connects a host adapter and one or more SCSI peripheral devices, such as a disk drive or CD-ROM drive. Special cables create this pathway between the host adapter and the peripherals.

A SCSI bus transfers data and commands at high speed to and from the peripheral device(s). The host adapter is linked to your PC's system bus, so it can transfer data from SCSI devices into the PC's main memory.

A SCSI bus must be correctly terminated in order to work. (Termination is explained in the SCSI Bus Termination topic.)


Setting up the SCSI Bus

The first step in creating a SCSI bus in your PC is to install the Adaptec host adapter board. This process is described in detail in the documentation you received with your host adapter. Be sure to follow these instructions carefully.

Next, you connect one or more SCSI devices to the host adapter—perhaps a SCSI disk drive or a CD-ROM drive. Many configurations are possible.


SCSI Bus with Internal Device

This diagram shows the simplest SCSI bus configuration: a host adapter and one internal SCSI device—in this case, a CD-ROM drive.

To create this SCSI bus configuration, you connect one end of the internal SCSI ribbon cable to the CD-ROM drive's SCSI connector. Then you connect the other end of the cable to the host adapter's internal SCSI connector.

You must also be sure that the CD-ROM drive and the host adapter are terminated.


Adding More Internal Devices

You can expand the SCSI bus configuration by adding more internal SCSI devices to your PC, as shown here.


SCSI Bus with External Device

This diagram shows a simple SCSI bus configuration with a host adapter and one external device—in this case, a SCSI hard disk drive.

To create this SCSI bus configuration, you connect one end of the external SCSI cable to the hard disk drive's SCSI connector, then you connect the other end to the host adapter's external SCSI connector.

You must be sure that the hard disk drive and the host adapter are terminated.


Adding More External Devices

You can expand the SCSI bus configuration by adding more external SCSI devices to your PC, as shown here.


Using Internal and External Devices

This SCSI bus configuration includes both an internal and an external SCSI device.

If you connect both external and internal devices to the SCSI bus you must change the termination of the host adapter.

The total length of the SCSI bus, from the first internal device on one end to the last external device on the other end, must not exceed 6 meters (19.7 feet).


Choosing SCSI Cables

Always use high-quality SCSI cables to connect the host adapter to the peripheral devices. Poor-quality cables can cause data corruption, parity errors and other problems. High-quality cabling is especially important if you use Fast SCSI data transfer rates.

The round cabling used for external SCSI devices varies widely in quality and electrical characteristics. Here are some general guidelines for selecting external cables and using them properly:

•	Use layered twisted pair cable with 25 wire pairs, as described in the SCSI-2 spec.

•	Use cables with a single-ended impedance range of 80-110 ohms.

•	Do not use cables shorter than 30 cm (11.8 inches).

•	Do not exceed the overall maximum SCSI bus length of 6 meters (19.7 feet), or 3 meters (9.8 feet) if you use Fast SCSI data transfer rates.

Adaptec sells high-quality internal and external SCSI-2 cables. You can order these cables directly from Adaptec. In the U.S. or Canada, call (800) 442-7274. If you are calling from other countries, the number is (818) 365-6264.


Troubleshooting the SCSI Bus

Check these potential problems if your SCSI devices do not work properly after you set up the SCSI bus:

•	Are all SCSI bus cables and power cables properly connected? Is pin-1 orientation correct for the internal SCSI cable?

•	Is the SCSI bus correctly terminated?

•	Do all devices on the SCSI bus have unique IDs?

•	Are all SCSI devices powered? The host adapter cannot detect a device on the SCSI bus at bootup unless the device is powered.


Termination Overview

Terminators must be installed, or enabled, on the devices at each end of the SCSI bus. Terminators must be removed, or disabled, on devices in the middle of the SCSI bus. When terminators are installed in a host adapter or other device, the device is said to be terminated.


Why is Termination Necessary?

The SCSI bus must be properly terminated so that signals and data can be transmitted successfully to and from all devices on the bus. This is roughly similar to sending vibrations back and forth over the length of a string: to do this, you must firmly hold or fasten both ends of the string.

If someone holds the middle of the string the vibrations cannot be transmitted.


Terminating Adaptec Host Adapters

Every Adaptec host adapter comes from the factory with termination enabled. Do not change this setting if your PC has only internal SCSI devices or only external SCSI devices.

You must disable host adapter termination if you have both internal and external SCSI devices. There are several different ways to do this, depending on which model of Adaptec host adapter you have:

•	Physically remove terminating resistors from their sockets on the board

•	Change a switch setting on the switch block

•	Change the termination setting in the SCSISelect utility or in an EISA configuration utility

Read the host adapter documentation to find out which method to use.


Terminating Other SCSI Devices

Disk drives, CD-ROM drives, tape drives, removable media drives and other SCSI devices must be terminated if they are the last device at either end of the SCSI bus. Termination must be disabled if a device is in the middle of the bus.

There are several ways to change the termination, depending on what kind of device you have:

•	Physically remove terminating resistors from their sockets or insert them in the sockets (internal devices)

•	Change a switch setting on the device's switch block

•	Install a cable terminator in the device's SCSI connector socket, or remove it from the socket (external SCSI drives)

Read the device documentation to find out which method to use.


What is a SCSI ID?

A SCSI ID is an identifying label—a number from 0 to 7—assigned to each device on the SCSI bus, including the host adapter itself. Each device on the SCSI bus must have a different SCSI ID.


Why Are SCSI IDs Necessary

SCSI IDs are necessary because:

•	Each device must be uniquely identified when it is trying to use the SCSI bus or when another device is sending a command to it.

•	The SCSI ID decides which device has priority when two or more devices are trying to use the SCSI bus at the same time. (The device with the highest ID wins.)


Host Adapter SCSI ID

All Adaptec host adapters are set by default to SCSI ID 7. This gives the host adapter the highest priority for using the SCSI bus.

We recommend that you do not change the default host adapter SCSI ID.


Changing SCSI IDs

Each device on the SCSI bus must have a different SCSI ID. If your Adaptec host adapter includes the SCSISelect configuration utility, you can view the SCSI IDs when you run that program. If two or more devices on the SCSI bus are set to the same ID, you must change one of them.

There are several ways to change SCSI IDs, depending on what kind of SCSI device(s) you have:

•	Change switch settings on the switch block

•	Remove or install jumpers

Read the documentation for your SCSI devices to find out how to change the SCSI ID.


Recommended SCSI ID Assignments

Follow these guidelines for a normal SCSI bus configuration:

•	Leave the host adapter SCSI ID at the default of ID 7.

•	Set the first two SCSI hard drives in your system to ID 0 and ID 1.

•	Assign SCSI IDs 2 through 6 to CD-ROM drives, tape drives, and other SCSI peripherals in your system.


ASPI Overview

Developed by Adaptec, the Advanced SCSI Programming Interface (ASPI) is an industry-standard software protocol that makes it possible to freely use different kinds of SCSI peripherals (such as disk drives and CD-ROM drives) with different kinds of Adaptec host adapters.

This is possible because each hardware-specific device driver (for example, Adaptec's CD-ROM device driver) and each host adapter-specific device driver (for example, the driver for Adaptec's AHA-2740 Family of host adapters) is written according to the same ASPI protocol standards.

Adaptec EZ-SCSI includes both hardware-specific SCSI device drivers (sometimes called ASPI modules) and host-adapter-specific device drivers (called ASPI managers).


SCSI Device Drivers

Your PC needs a different SCSI device driver for each kind of hardware device connected to your host adapter—disk drives, CD-ROM drives, tape drives, etc. Adaptec EZ-SCSI has several SCSI device drivers, including the following:

•	aspidisk.sys for disk drives

•	aspicd.sys for CD-ROM drives

•	sjiix.sys for HP scanners

The EZ-SCSI device drivers are written according to the ASPI protocol. They can communicate with supported SCSI devices through any kind of Adaptec host adapter, because the ASPI manager software for different families of host adapters is also written to the ASPI standard.


ASPI Managers

Each family of Adaptec host adapters needs a different device driver—or ASPI manager—to communicate with SCSI device drivers and SCSI devices.

Adaptec EZ-SCSI includes several different ASPI managers for the various families of Adaptec host adapters. When you run EZ-SCSI it detects what kind of host adapter is installed in your PC and configures for the appropriate ASPI manager.


SCSI Terms and Topics - SCSI Definitions
SCSI Definitions

Click on these SCSI terms to see a brief definition of each.

SCSI-1

SCSI-2

SCSI-3

Fast SCSI

Wide SCSI


Single and Multiple Logical Units (LUNs)

Most SCSI devices have one Logical Unit (LUN). It's possible for a device to have up to eight LUNs. A SCSI initiator—usually a host adapter—can address each LUN separately through the target. This diagram shows the host adapter addressing a target with a single LUN and a target with multiple LUNs.

Some of the material in this section is taken, with permission, from Zadian Technologies, Inc., training materials.


Single LUN Example

The host adapter sends a Read Data command over the SCSI bus to a CD-ROM drive (the target). The data it wants to access is not on the drive mechanism, but on the CD-ROM disk (the LUN) in the drive. The host adapter addresses the single LUN through the target.

Other examples of targets with a single LUN include: a SCSI hard disk drive and a SCSI tape drive, which holds a single tape cartridge.


Multiple LUN Example

The host adapter sends a Read Data command over the SCSI bus to a dual-cartridge removable media drive (the target). The data it wants to access is not on the drive mechanism, but on one of the cartridges (LUNs) in the drive. The host adapter addresses the LUN through the target.

Another example of a target with multiple LUNs is a "juke box" CD-ROM drive, which holds six CD-ROM disks at the same time; each disk is addressed as a different LUN.


What Is Synchronous Negotiation?

Data can be sent over the SCSI bus in synchronous or asynchronous mode. Synchronous mode is preferable because it is faster than asynchronous.

Synchronous Negotiation is what happens when the host adapter and the SCSI devices on the bus exchange messages to prepare for synchronous data transfer to begin.

Synchronous negotiation is enabled by default on most Adaptec host adapters. You should enable synchronous on all other devices on the SCSI bus, if they support it. Read the device documentation to find out how to enable synchronous.


Disabling Synchronous Negotiation

A few older models of CD-ROM drives do not support Synchronous Negotiation. You can tell that your CD-ROM drive does not support synchronous if your system does not recognize the drive after you install and configure the host adapter. If this happens, disable synchronous negotiation on the host adapter.

Then, to assure the best system performance, enable synchronous negotiation on all SCSI devices in your system that do support it. The host adapter will still negotiate for synchronous data transfer if the SCSI device initiates the request.


What is Parity Checking?

Parity checking is a SCSI feature that verifies whether data has been transmitted correctly over the SCSI bus. Parity checking is a valuable safeguard against data corruption.


Parity Checking Support

All Adaptec host adapters support parity checking. This feature is enabled by default on Adaptec host adapters.

Practically all SCSI peripheral devices, such as hard disk drives and CD-ROM drives, also support parity checking.

You can disable parity checking on Adaptec host adapters and on SCSI peripheral devices.


Using Parity Checking

To use parity checking most effectively, enable it on the host adapter and on each device on the SCSI bus. Read your peripheral device documentation to learn how to enable parity checking.

The only time when you should disable Parity Checking on the host adapter is when your system includes a SCSI device that does not support the feature. Read your peripheral device documentation to see whether the devices support Parity Checking.


What Is Single-Ended SCSI?

Single-ended SCSI is a hardware standard for connecting SCSI devices. The SCSI bus in a single-ended configuration can be up to 6 meters (19.7 feet) long, or 3 meters (9.8 feet) long if you use Fast SCSI data transfer rates.

Most SCSI devices are single-ended. All Adaptec host adapters except the AHA-1744 are designed for use with single-ended SCSI devices.

Do not use differential SCSI devices on a single-ended SCSI bus. If you do, the devices could be damaged!


What Is Differential SCSI?

Differential SCSI is a hardware standard for connecting SCSI devices. You use a differential SCSI configuration when devices on the SCSI bus must be at some distance from each other, because it allows the bus to be over four times longer than the maximum bus length for single-ended SCSI.

Differential SCSI devices use more power than single-ended SCSI and require more space on the board. They are therefore more expensive. Adaptec's AHA-1744 is designed for use with differential SCSI devices.


Glossary

A - C

ASPI

ASPI manager

asynchronous

byte

cable terminator

D - H

device driver

differential SCSI

external SCSI device

Fast SCSI

host adapter

I - L

impedance

icon

initiator

internal SCSI device

ISA bus

jumper

LUN

P - R

parity checking

peripheral device

pin-1 orientation

protocol

removable media

S

SCSI

SCSI bus

SCSI ID

SCSI-1

SCSI-2

SCSI-3

SCSISelect

single-ended SCSI

switch block

synchronous

Synchronous Negotiation

T - W

target

terminators

termination

Wide SCSI


ASPI

Advanced SCSI Programming Interface. ASPI provides an interface layer between a manager program written to a specific host adapter hardware and a device driver which is tailored to the command set of a particular kind of device such as hard disk drives. The common ASPI protocol allows the manager programs and device drivers to be switched and combined freely, depending on how a particular system is configured.


ASPI manager

A software program, written to the ASPI protocol, that manages a specific family of host adapters, such as the AHA-1510A/1520A/1522A family of non-Bus Master ISA host adapters, running under a specific operating system, such as DOS/Windows. A single ASPI manager can manage multiple input/output requests from multiple device drivers for different kinds of SCSI devices.


ASPI module

A software program, written to the ASPI protocol, that enables certain kinds of SCSI devices, such as disk drives or CD-ROM drives, to communicate with the ASPI manager.


asynchronous

A method of transferring data over the SCSI bus. In an asynchronous data transfer, bytes are sent in irregular intervals in characters, words or blocks. Start bits and stop bits are used to signal the beginning and end of bytes. Asynchronous data transfer is slower than synchronous data transfer.


byte

A unit of binary information consisting of eight bits. Each byte usually represents a character or symbol.


cable terminator

A device inserted in one of the two SCSI connectors of an external SCSI device to terminate the SCSI bus. You must insert the cable terminator if the external device is at the end of the SCSI bus.


device driver

A hardware-specific software program that acts as an interface between a computer's operating system and the hardware devices (printers, disk drives, etc.) attached to the computer.


differential SCSI

A hardware standard for connecting SCSI devices. Differential SCSI uses two wires per signal, which improves noise immunity and allows the SCSI bus to be up to 25 meters long.


external SCSI device

A SCSI device such as a hard disk drive or a tape drive installed outside the PC cabinet. External SCSI devices are connected to the SCSI bus with round cable that has layered twisted pair assemblies with proper electrical shielding.


Fast SCSI

A SCSI data transfer standard that allows a rate of up to 10 MBytes/second on the SCSI bus (up to 20 MBytes/second on wide SCSI host adapters).


host adapter

A printed circuit board that provides a SCSI bus interface to SCSI devices installed in a computer. The host adapter usually has both internal and external SCSI connectors, an onboard BIOS, terminators, and other features.


impedance

The resistance offered by an electrical circuit to the flow of an alternating current of a single frequency. SCSI cables must have the correct level of impedance, measured in ohms, so that SCSI messages and data can be successfully transmitted.


icon

A small graphical representation of an element in Microsoft Windows, such as a program, printer or disk.


initiator

A SCSI device that requests an operation such as a data transfer to be performed by another SCSI device (the target). The host adapter is normally the initiator.


internal SCSI device

A SCSI device such as a hard disk drive or a CD-ROM drive installed inside a PC. Internal SCSI devices are connected to the SCSI bus with a flat ribbon cable.


ISA bus

Industry Standard Architecture. The type of expansion bus used in most PCs. The ISA bus allows data to be sent 16 bits at a time. Sometimes called the AT bus.


jumper

A small, removable metal and plastic device used to make an electrical connection between pairs of metal pins on a jumper block. By removing or installing jumpers on some models of Adaptec host adapters you can change the settings for BIOS address, IRQ channel, and other parameters.


LUN

Logical Unit. A technical term for the device that is actually addressed when an initiator (usually the host adapter) sends a Read or Write command to a target. An example of a LUN is a tape cartridge in a tape drive.


parity checking

A SCSI feature that verifies whether data has been transmitted correctly over the SCSI bus. A check bit is added to each byte of data to make the sum of all the "1" bits either odd or even, depending on the protocol. If the sum of the bits is even when it should be odd, or vice-versa, data has been corrupted and a parity error message is displayed.


peripheral device

Any optional input or output device, such as a printer or CD-ROM drive, that connects to a PC's central processing unit. For SCSI peripheral devices this connection is made via the SCSI bus.


pin-1 orientation

The orientation of pin 1 on a SCSI cable relative to the pin 1 position on the connector into which it is plugged. The SCSI bus will not work properly if the orientation is reversed. Pin-1 orientation is not an issue with external SCSI cables, which only plug one way into the connector.


protocol

A strict set of rules that control the exchange of information between computer devices, such as between a host adapter and various SCSI devices.


removable media

Disk media such as a CD-ROM disk or a Bernoulli cartridge that can be removed from a disk drive. The media can then be stored, replaced by another removable media device, inserted in another removable disk drive, etc.


SCSI

Small Computer Systems Interface. A computer bus interface standard that enables different kinds of peripherals, such as disk drives, CD-ROM drives and tape drives, to interface with the host computer over a common SCSI bus. SCSI-1, SCSI-2 and SCSI-3 are different versions of the SCSI protocol.


SCSI bus

One or more SCSI peripheral devices and a host adapter, connected by electrical cables in a daisy-chain configuration. The SCSI bus may include both internal and external SCSI devices.


SCSI ID

A unique identifying number, ranging from 0 to 7 (or 0 to 15 for Wide SCSI), assigned to a SCSI device on the SCSI bus. The SCSI ID defines the device on the bus and determines which device has priority to use the SCSI bus when two or more devices simultaneously request access. The higher the SCSI ID number, the higher the priority.


SCSI-1

The original standard that defined the SCSI interface, including a command set and the electrical characteristics of the SCSI bus.


SCSI-2

A computer bus interface standard that adds features to the SCSI-1 standard. Included among these features are 32-bit data transfer, SCSI Linking, Command Queuing and support for a wider variety of SCSI peripheral devices.


SCSI-3

A computer bus interface standard (not yet finalized) that adds features to the SCSI-2 standard. These features include an optional P-cable Wide bus with 16 or 32 devices maximum, support for fiber channel hardware, and a serial channel interface.


SCSISelect

A built-in configuration program included with some models of Adaptec host adapters. SCSISelect enables you to configure many SCSI parameters without physically handling the host adapter board.


single-ended SCSI

A hardware standard for connecting SCSI devices. Single-ended SCSI references each signal to a common ground. This standard allows a maximum SCSI bus length of 6 meters (19.7 feet).


switch block

A small bank of switches on a peripheral device, motherboard or add-on board. Changing the switch settings modifies the configuration of the computer hardware. Some Adaptec host adapters include switch blocks.


synchronous

A mode of transferring data at high speed on the SCSI bus. Data transfers are synchronized with fixed-length, fixed-frequency strobe pulses from the clock.


Synchronous Negotiation

A SCSI feature that allows the host adapter and SCSI peripherals to communicate with each other and establish the parameters for the transfer of data in synchronous mode.


target

Or Target Device. A SCSI device that performs an operation requested by an initiator. The initiator is usually the host adapter. The target may be a peripheral device such as a disk drive performing a data transfer for an initiator.


terminators

A set of electrical resistors installed, or enabled, at each end of a SCSI bus so that data and signals can be transmitted successfully on the SCSI bus.


termination

An electrical characteristic of the SCSI bus. The SCSI bus must be terminated at both ends by installing or enabling electrical resistors. Termination must be disabled on devices in the middle of the SCSI bus.


Wide SCSI

A SCSI-2 enhancement that allows data to be transferred 16 bits or 32 bits at a time on the SCSI bus instead of 8 bits at a time. This feature allows much faster data throughput time.


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Each SCSI bus can have up to eight devices, including the host adapter. Wide SCSI host adapters can have up to 16 attached SCSI devices.


Always turn OFF and disconnect the power to your PC and attached devices before you remove the chassis cover.


The cable must be connected with the correct Pin-1 orientation. See your host adapter documentation for details.


See the SCSI Termination topic to learn about termination.


See the SCSI Termination topic to learn how to terminate SCSI devices.


See the SCSI Termination topic to learn how to terminate SCSI devices.


Wide SCSI host adapters can have up to 16 attached SCSI devices, with SCSI IDs from 0 to 15 (this includes the host adapter).


Zadian Technologies, Inc. (408-293-0800) gives training seminars on SCSI and other technical topics.


Disk partitions (D:, E:, etc.) on a hard disk drive are treated as a single LUN.


Synchronous transfer is used only for data. It cannot be used for SCSI commands, messages or status transmission.


You change the host adapter synchronous setting with either software, command line options, or jumpers. See your host adapter documentation.


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