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This chapter discusses the following topics:
• Shared SCSI bus configuration requirements (Section 3.1)
• SCSI bus performance (Section 3.2)
• SCSI bus device identification numbers (Section 3.3)
• SCSI bus length (Section 3.4)
• SCSI bus termination (Section 3.5)
• UltraSCSI hubs (Section 3.6)
• Configuring UltraSCSI hubs with RAID array controllers (Section 3.7)
3.1 Shared SCSI Bus Configuration Requirements
A shared SCSI bus must adhere to the following requirements:
• Only an external bus can be used for a shared SCSI bus.
• SCSI bus specifications set a limit of 8 devices on an 8-bit (narrow) SCSI
bus. The limit is 16 devices on a 16-bit SCSI bus (wide). See Section 3.3
for more information.
• The length of each physical bus is strictly limited. See Section 3.4 for
more information.
• You can directly connect devices only if they have the same transmission
mode (differential or single-ended) and data path (narrow or wide). Use
a SCSI signal converter to connect devices with different transmission
modes. See Section 9.1 for information about the DWZZA (BA350) or
DWZZB (BA356) signal converters or the DS-BA35X-DA personality
module (which acts as a differential to single-ended signal converter
for the UltraSCSI BA356).
• For each SCSI bus segment, you can have only two terminators, one
at each end. A physical SCSI bus may be composed of multiple SCSI
bus segments.
• If you do not use an UltraSCSI hub, you must use trilink connectors
and Y cables to connect devices to a shared bus, so you can disconnect
the devices without affecting bus termination. See Section 9.2 for more
information.
• Be careful when performing maintenance on any device that is on a
shared bus because of the constant activity on the bus. Usually, to
perform maintenance on a device without shutting down the cluster, you
must be able to isolate the device from the shared bus without affecting
bus termination.
• All supported UltraSCSI host adapters support UltraSCSI disks at
UltraSCSI speeds in UltraSCSI BA356 shelves, RA7000 or ESA10000
3–2 Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware