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SCSI configuration
contents of this section
summary of SCSI terminology
setting up the SCSI bus
SCSI ID numbers
identifying SCSI IDs
setting the SCSI ID
SCSI termination
termination power
plug and play adaptor cards
SCSI cables
cable length
cable quality
what is SCSI?
the standards
SCSI in hp surestore tape drives
using wide and narrow devices together
using SE and LVD, ultra and ultra 2 SCSI devices together
summary of SCSI terminology
Note: this section discusses SCSI tape drive devices in general, although specific
reference is made to the particular SCSI requirements of HP Surestore DLT1 tape
drives where appropriate.
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).
HP Surestore DLT1 drives are Ultra SCSI-2 wide devices designed to operate on
a low voltage differential SCSI bus (LVDS) with a burst transfer rate of 40 MB/sec.
Using the drive on a single-ended SCSI bus restricts performance. Do not use on
the same bus as other single-ended devices as this will switch the LVDS host
adapter into single-ended mode and restrict performance.
See SCSI Cables for more information about cable lengths.
setting up the SCSI bus
Each device on a SCSI bus, including the SCSI host bus adapter (HBA), must be
configured with a unique ID (identifier).
Note: HP recommends that a dedicated host bus adapter is used for the tape
drive. A suitable adapter is available from HP as an accessory (see Ordering
Information).
SCSI ID numbers
SCSI IDs are numbered as follows:
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
Surestore 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).
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 HP Library & Tape Tools. HP
Library & Tape Tools 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 drives, set the SCSI ID by attaching or
removing jumpers at the rear of the drive.
On external HP Surestore DLT1 drives, the ID is displayed on the
rear panel and can be set by using a small screwdriver or a ball-point
pen to press the little buttons above and below the number.
In either case, see the Getting Started Guide 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.
There are two main types of termination, active and passive. Active terminators
reduce interference and allow faster data throughput. On devices with high
transfer speeds, such as HP Surestore DLT1 drives, active termination is
required.
Correct termination can be achieved in four 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
other external devices, the terminator simply attaches to the spare
SCSI connector of the last device.
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.
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 adaptor 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.
SCSI cables
Cables matter in SCSI systems. There are two factors to consider:
cable length
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.
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 HP Surestore DLT1 internal drives, you need a SCSI ribbon cable with the
correct termination. The drives have a 68-pin wide, high-density SCSI connector.
If you are using an HP Surestore DLT1 drive on an internal bus with other
peripherals that run at Ultra2 speeds, it is important that a 68-pin LVD-compatible
ribbon cable is used.
The cable provided with HP Surestore DLT1 external tape drives will attach to a
computer with a wide LVDS SCSI connector (68 pins). If your server or host bus
adapter is equipped with a very high density (VHD) wide SCSI connector, you will
need to order a 68-pin HD-to-VHD converter or 68-pin HD-to-VHD cable. See
Ordering 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 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.
the standards
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 Fibre 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
HP Surestore DLT1 drives are Ultra SCSI-2 wide devices designed to operate on
a low voltage differential SCSI bus (LVDS) with a burst transfer rate of 40 MB/sec.
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.
Using HP Surestore DLT1 drives on a narrow bus will restrict performance.
Narrow devices used on a wide bus will not normally affect the performance of
any wide devices connected
using SE and LVD, ultra and ultra 2 devices together
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.
Using HP Surestore DLT1 drives on a single-ended SCSI bus restricts
performance. Do not use on the same bus as other single-ended devices as this
will switch the LVDS host adapter into single-ended mode and restrict
performance to SE speeds and SE cable length.
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.
© 2000, Hewlett-Packard Company
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