Scsi cables, Cable length, Cable quality – HP StoreEver DAT Tape Drives User Manual
Page 68: With internal devices, With external devices, With removable devices, Note on se and lvd interfaces
device to one of the SCSI connectors for the second device and attach the terminator to the spare
SCSI connector for the second device.
Refer to your tape array documentation for detailed instructions.
SCSI cables
Cables matter in SCSI systems. There are two factors to consider: cable length and cable quality.
Cable length
•
For single-ended SCSI with Fast devices there is a maximum permissible length of 6 meters
for a single SCSI bus.
•
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 LVD SCSI the maximum length for a single device is 25 meters. For 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 note on SE and LVD interfaces 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.
•
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.
With internal devices
For HP StorageWorks DAT internal devices, you need a SCSI ribbon cable with the correct termination.
The drives have a 68-pin wide, high-density SCSI connector. A suitable cable is supplied with the tape
drive. If you are using an HP StorageWorks DAT drive on an internal bus with other peripherals that run
at Ultra160 speeds, it is important that a 68-pin LVD-compatible ribbon cable is used. Do not connect
your tape drive to lower-rated SCSI or to narrow SCSI.
With external devices
The cable provided with all HP StorageWorks DAT external tape drives and autoloaders will attach to a
computer with a wide, very high density (VHD), LVDS SCSI connector.
If your server or host bus adapter is equipped with a high density (HD) wide SCSI connector, you will need
to order a 68-pin VHD-to-HD converter or 68-pin VHD-to-HD cable. See
With removable devices
For HP StorageWorks DAT removable tape drives, appropriate cables and terminators are provided
with the tape array.
Note on SE and LVD interfaces
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.
68
SCSI configuration guide