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termination and covers radial configurations with the DWZZH UltraSCSI
hubs and non-UltraSCSI RAID array controllers.
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.
3–2 Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware