Summary of SCSI Terminology

Various terms are used when describing SCSI devices. This section provides a brief summary. See What is SCSI for a more detailed discussion of the different SCSI specifications.

There are three main factors that affect performance and cable length:

• The speed of the data bus, which may be Fast, Ultra or Ultra 2.
• The width of the data bus, which may be Narrow or Wide.
• The voltage level of the interface, which may be single-ended (SE) or low voltage differential (LVD).

The following table summarizes transfer speeds. See SCSI Cables for more information about cable lengths.

Setting Up the SCSI Bus

SCSI ID Numbers

Each device on a SCSI bus, including the SCSI host bus adapter (HBA), must be configured with a unique ID (identifier).

• For wide SCSI buses, the ID will be a number from 0 through 15, so a typical wide SCSI HBA can accommodate up to fifteen other devices.
• For standard, narrow SCSI buses, the ID will be a number from 0 through 7, so a typical narrow SCSI HBA can accommodate up to seven other devices.

SCSI IDs are usually set on the device itself (sometimes via configuration software), but some newer devices are capable of selecting an unused ID automatically when powered-up (these are known as "SCAM" devices). HP DAT drives are not SCAM compliant.

SCSI ID 7 is reserved for the HBA because it has the highest priority on the bus. On wide buses, the priority runs from 7 (highest) to 0, then 15 down to 8 (lowest).

Note: As a general rule, avoid putting tape devices on the same bus as any hard disks.

Identifying SCSI IDs

If your computer already has devices connected to the SCSI bus, you will need to know their IDs to avoid any conflict with the new tape drive. Here are some methods of finding out the information:

• The fastest and easiest way is to run TapeAssure from this CD-ROM. TapeAssure will check your SCSI configuration, including the SCSI controller itself and any devices currently attached to it.
   
• Most computers display a list of SCSI devices and IDs during the boot-up process. This usually scrolls past very fast. If you press the [Pause] key, you should be able to halt the scrolling and view the list.
   
• If you have an Adaptec host adapter in the computer, it may have come with a Windows utility called SCSI Interrogator (SHOWSCSI.EXE) which displays information about connected devices.
   
• If you have Windows NT installed, select SCSI Adapters from the Control Panel, select a device in the Devices tab and click on Properties to view information about the device, including its SCSI ID.
   
• If you have Novell NetWare installed, use its LIST DEVICES command.
   

If none of these is available to you, try the following sources of information:

• The details of all installed devices and settings may have been written down and stored with your computer's documentation (for new computers, this is often done by the supplier).
   
• Your HBA's documentation should tell you which settings it uses.
   
• Look at each device to find out its ID. This is usually easy with external devices. With internal devices, you will probably need the help of the device's documentation to identify the SCSI ID setting, which is usually set with jumpers.

Setting the SCSI ID

• On internal HP SureStore DAT drives, set the SCSI ID by attaching or removing jumpers at the rear of the drive.
   
• On external HP SureStore DAT drives, the ID is displayed on the rear panel and can be set by pressing the little buttons above and below the number.  

In either case, see the Installation Guide that came with your tape drive for more details. Note that host adapters check SCSI IDs only at power-on, so any changes will not take effect until the host system is power-cycled.

SCSI Termination

Terminators are essential, as they provide the correct voltages on the SCSI bus and prevent unwanted signal reflections from interfering with data transfers. The rule is:

There must be termination at both physical ends of the bus and only at the ends.

Correct termination can be achieved in three ways:

• Using the host bus adapter: Often the HBA forms one end of the SCSI bus and provides termination. If you have both internal and external devices attached to the same SCSI bus, the HBA will be in the middle of the cable and thus its termination must be disabled. See the host bus adapter's documentation for details of how to do this.
   
• Using a terminated SCSI cable: Usually the internal SCSI cable in a PC-based server has a number of SCSI connectors along its length and a terminator at the end farthest from the host bus adapter. In this case, you must ensure that any devices attached to the cable have termination removed or disabled.
   
• Using a terminator attached to the last device on the bus: For external devices, the terminator simply attaches to the spare SCSI connector of the last device.
   

There are two main types of termination, active and passive. Use active terminators wherever possible, as they reduce interference and allow faster data throughput. On devices with high transfer speeds, such as the HP SureStore DAT40, DAT40x6,DAT24 or DAT24x6, active termination is required.

The HP SureStore DAT40 and DAT40x6 can be used on both single ended and low voltage differential buses. Always use the supplied multi-mode terminator for proper operation. The terminator detects the type of bus and automatically supplies the correct termination. See Using SE and LVD, Ultra and Ultra2 SCSI Devices Together for more information.

Termination Power

SCSI terminators require power to function, which is supplied along the SCSI cable from one or more of the connected devices. Most host adapters provide power by default, as do HP SureStore DAT drives.

If you are using long SCSI cables and some of the devices do not provide termination power, it is best to connect these to the middle of the cable, so that devices which do provide termination power are nearer the ends of the cable, and thus closest to the terminators themselves.

Plug-and-play Adapter Cards

Most plug-and-play adapter cards have an auto-termination feature which, when the adapter is in the middle of the bus, can detect the terminators situated at either end of the SCSI bus and automatically turn off the on-board termination.

VHD Connectors

SCSI Cables

Cables matter in SCSI systems. There are two factors to consider:

Cable Length

• For single-ended SCSI with Ultra devices the maximum permissible length is 3 meters for four or fewer devices, and 1.5 meters for more than four devices.  (For single-ended SCSI with Fast devices there is a maximum permissible length of 6 meters for a single SCSI bus.) 

• For LVD SCSI there is a maximum permissible length of 25 meters for a single device. With multiple devices the maximum combined internal/external length is 12 meters.

• If you have a combination of LVD and SE devices on the bus, the maximum cable length reverts to the SE specification. See Using SE and LVD, Ultra and Ultra2 SCSI Devices Together for more information. 

• For best performance, keep lengths to a minimum, but avoid very short overall lengths (less than 0.5 meters). 
  

Cable Quality

It is important to use good quality cables. Generally speaking, cable quality affects performance and reliability. This is particularly true for external, shielded cables.

For optimum performance with the HP SureStore DAT40 and DAT40x6, always use the supplied cable. This cable is specified for use with LVD systems up to Ultra2 speeds. This means that you can use Ultra2 devices on the same bus as the HP SureStore DAT40 and DAT40x6, and all devices will perform at their maximum speeds. See Using SE and LVD, Ultra and Ultra2 SCSI Devices Together for more information.

Look after your SCSI cables. In particular, take care when connecting or disconnecting not to damage the high-density connectors. Avoid putting excessive twists in external shielded cables, as this can cause premature failure.

What Is SCSI?

The Standards

The Small Computer Systems Interface (SCSI) is popular because it offers a fast and flexible method of connecting a variety of devices to a host computer. The SCSI standards define both the physical connections between the devices (cables and connectors) and the protocols devices use to communicate with each other.

There have been three general standards:

• SCSI-1, which is now obsolete
   
• SCSI-2, which is very common, and still a current standard
   
• SCSI-3, which is an emerging set of linked standards that define much more than the simple bus systems used by the earlier versions. SCSI-3 includes Fiber Channel, Wide SCSI, FireWire (IEE 1398), Low Voltage Differential (LVD or LVDS), and Fast Serial SCSI. Some of these standards are now being used on PC platforms.
   

Another variant is ATAPI, which uses an EIDE physical bus to pass SCSI protocols to connected devices. Technically, this is not part of the SCSI-3 standards, although some operating systems (notably Windows NT) manage ATAPI devices as if they were fully SCSI.

SCSI is a backward-compatible standard, so that SCSI-2 and SCSI-3 devices can almost always be made to work together.

SCSI is a bus interface: all the devices are connected to a single cable (some of this may be inside and some outside the host computer's case). The connection to the host itself is known as the Host Bus Adapter (HBA). You can have several HBAs in a single computer, each with its own SCSI bus: this is a common arrangement in high-performance servers. Some host bus adapters (such as the Adaptec 3940W) have more than one SCSI bus available on a single card.

SCSI in HP SureStore Tape Drives

The HP SureStore DAT40 drive and HP SureStore DAT40x6 are ultra wide SCSI-2 compatible devices. They can be used with both low voltage differential and single-ended host bus adapters. However, if one device is single-ended, all devices will be treated as single-ended. The HP SureStore DAT40 drive and HP SureStore DAT40x6 autoloader are not compatible with high voltage differential SCSI devices.

All other Hewlett-Packard SureStore DAT drives are single-ended, fast SCSI-2 compatible devices.

Using Wide and Narrow Devices Together

Narrow SCSI devices can transfer data one byte at-a-time (and are sometimes called "8-bit SCSI" devices). They can conform to either the SCSI-2 or SCSI-3 protocols. They have a 50-pin connection to the SCSI bus.

Wide SCSI devices can transfer two bytes of data simultaneously ("16-bit SCSI"). They usually have a single, 68-pin connection to the SCSI bus. (This physical arrangement is part of the SCSI-3 specification.) They may support either SCSI-2 or SCSI-3 protocols. Wide and narrow devices can simultaneously be connected to the same bus without problem, provided certain rules are followed.

Narrow devices used on a wide bus will not normally affect the performance of any wide devices connected.

Narrow Host Adapters

Wide SCSI devices can be connected with a suitable converter, but you must use IDs in the range 0-6. Note that there will inevitably be a reduction in performance of the wide devices, as wide data transfers cannot take place. However, in all other respects they should work normally.

Wide Host Adapters

If the host adapter will only be connected to narrow devices, the bus is effectively narrow, so narrow cables and terminators can be used (with a suitable converter). If there are any other wide devices connected, the following rules apply:

• Wide cabling must be used between the HBA and the other wide devices (there must be no intervening narrow cable).
   
• Both the wide and narrow parts of the bus must be correctly terminated.
   

If all the devices are internal:

• Use wide ribbon cable throughout.
   
• Use wide-to-narrow converters for the narrow devices.
   
• Use wide terminators.
   

If any of the narrow devices are fitted externally:

• On the narrow side of the last external wide device (or on the HBA's external socket), use a wide-to-narrow converter cable that terminates the wide part of the cable correctly (known as "high-order line termination" or the "high byte termination"). A suitable cable is provided with the HP SureStore DAT8e, DAT24e, and DAT24x6e products. It is also available separately.
   
• On the last narrow device, use a narrow terminator.

SE and LVD define how the signals are transmitted along the cable.

With single-ended (SE) SCSI, each signal travels over a single wire and each signal's value is determined by comparing the signal to a paired ground wire. Signal quality tends to decrease over longer cable lengths or at increased signal speed.

With low voltage differential (LVD) signaling, signals travel along two wires and the difference in voltage between the wire pairs determines the signal value. This enables faster data rates and longer cabling with less susceptibility to noise than SE signaling and reduced power consumption.

If you combine SE and LVD devices on the same bus, the host bus adapter will automatically switch to single-ended mode and optimum performance for any device on the bus will be at SE speeds and SE cable length. This means that the performance of any Ultra2 devices will drop to that of Ultra devices. All other devices will operate at their optimum speed.

If you connect only LVD devices, the bus will operate in low voltage differential mode and Ultra2 speeds will be enabled. You can use a combination of Ultra and Ultra2 devices. Each device will operate at its optimum speed.