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SCSI cables
Cables matter in SCSI systems. There are two factors to consider: cable length and cable quality.
Cable length
• For LVDS SCSI the maximum length for a single device is 25 meters. For multiple devices, the maximum combined in-
ternal/external length is 12 meters.
• If you have a combination of LVDS and SE devices on the bus, the maximum cable length reverts to the SE specification.
This is 3 meters for four or fewer devices, and 1.5 meters for more than four devices. See also "Note on SE and LVD in-
terfaces" on page 48.
• 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
LTO tape drives have a 68-pin wide, high-density SCSI connector. A suitable cable with the correct termination is supplied
with the tape drive. If you are using an LTO drive on an internal bus with other peripherals that run at Ultra2 speeds, it is
important that a 68-pin LVDS-compatible ribbon cable is used. Do not connect your tape drive to lower-rated SCSI or to
narrow SCSI.
With external devices
Your tape drive requires a 68-pin VHDCI-to-HD68 SCSI cable to connect to the host server.
Note on SE and LVDS interfaces
SE and LVDS 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 (LVDS) 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 use LVDS SCSI devices on the same bus as single-ended devices this will switch the LVDS SCSI host adapter into
single-ended mode and restrict cable length.
If you connect only LVDS SCSI devices, the bus will operate in low voltage differential mode and Ultra160 and higher speeds
will be enabled. If you use a combination of Ultra160 and Ultra2 devices, each device will operate at its optimum speed.
48
SCSI configuration guide
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