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206 AppendixB
SCSI Connections
SCSI Restrictions
Terminators
The terminators for Ultra2 Wide Low-Voltage Differential SCSI (LVD SCSI) and Ultra
Narrow Single-Ended SCSI (NSE SCSI) are different. The NSE SCSI terminator has 50
pins and will fit on an NSE SCSI connector. The LVD SCSI terminator has 68 pins and will
fit on a LVD SCSI connector. Note that these SCSI terminators were shipped with your
system.
CAUTION
It is necessary to put a terminator on unused SCSI connectors on the back
panel of the system. However, if SCSI devices are attached to the SCSI bus
via these connectors, the external device connected to the SCSI bus must be
terminated with a SCSI terminator. If you do not already have the SCSI
terminators, you must order terminator A1658-63012 (for the 50-pin
terminator; NSE SCSI) or A4986-63008 (for the 68-pin terminator; LVD
SCSI) from Hewlett-Packard.
Number of Devices Per SCSI Bus
The number of SCSI devices per bus is limited (see Table B-1. on page 203). Before adding
another SCSI device, determine if the system can support the additional device.
This workstation offers the following types of SCSI buses, each with their own
configuration constraints:
• Ultra Narrow Single-Ended SCSI bus
• Ultra2 Wide Low-Voltage Differential SCSI bus
Considerations for Selecting SCSI Devices
SCSI devices supported by the Hewlett-Packard Company for the Ultra Narrow
Single-Ended (NSE) and Ultra2 Wide Low-Voltage Differential (LVD) SCSI buses have
been qualified as SCSI devices which meet NSE and LVD specifications. Contact the local
HP sales representative for a current list of SCSI devices supported by HP for use on the
Ultra Narrow Single-Ended and Ultra2 Wide Low-Voltage Differential SCSI buses.
If devices other than those supported by HP are connected to the Ultra Narrow
Single-Ended or the Ultra2 Wide Low-Voltage Differential SCSI bus on this system, they
must adhere to cabling, termination and bus length restrictions discussed in this chapter
to assure functionality of the SCSI busses.
Because Ultra2 Wide Low-Voltage Differential SCSI is a relatively new technology, there
can be products which are advertised as LVD compliant, but may not meet LVD
specifications. Therefore, additional caution must be exercised when choosing Ultra2 Wide
Low-Voltage Differential SCSI bus on this system. For example, the cabling inside one of
these devices in question may not have a differential impedance between 110 ohms and
135 ohms and it may be constructed in a way such that the device has considerable
crosstalk on critical signals, such as Reset, Select and Busy. Failure to strictly adhere to
the Ultra2 Wide Low-Voltage Differential SCSI bus specifications will impact the
performance of the LVD bus. The Ultra2 Wide Low-Voltage Differential interface is a much