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Chapter 4 Hardware and Software Configuration 99
If you exceed these bus length restrictions, UltraSCSI devices may operate at less
than 40 Mbytes per second. Under such conditions, an UltraSCSI device may
experience errors that eventually cause it to reset and operate at 20 Mbytes per
second.
For fast/wide performance on the external SCSI bus, the maximum SCSI bus length
for daisy-chained SCSI devices is 19.7 feet (6 meters), including the Sun Fire 280R
server internal bus length of .2226 feet (.067 meter).
External UltraSCSI-compliant SCSI cables have an impedance of 90 ohm (+/- 6 ohm)
and are required for UltraSCSI interface. Sun’s implementation of UltraSCSI requires
that the total SCSI bus length be limited to no more than approximately 20 feet
(6 meters) with up to 12 Sun-compensated devices.
Due to the short bus length, an approximately 32-inch (0.8-meter) UltraSCSI-
compliant external cable is supported (Sun part number 530-2883) in addition to a
6.5-ft (2-meter) UltraSCSI-compliant external cable (Sun part number 530-2884).
External SCSI Cabling and Termination
Use the following cabling guidelines to ensure proper device cabling and
termination on the external SCSI bus:
■ In order to maintain UltraSCSI performance, all cables used must be
UltraSCSI-compliant.
■ The external SCSI bus must be correctly terminated. Most Sun devices use
autotermination. See the documentation supplied with the device.
■ If all external mass storage devices use 68-pin connectors, connect all non-Sun
devices to the system first and connect a Sun autoterminating device to the end of
the chain.
■ If external mass storage devices consist of 68-pin devices and 50-pin devices,
connect the Sun 68-pin devices to the system first and terminate the chain with a
50-pin device and its terminator. The 68-pin device connected to the 68-50 pin
adapter cable must be autoterminating in order to terminate the high-order bits.
Caution – Do not connect 68-pin devices after 50-pin devices; SCSI bus errors
will occur.