SCSI:restrictions. HP (Hewlett-Packard) N/A

SCSI connections

SCSI restrictions

SCSI restrictions

This section describes the SCSI restrictions that apply to your workstation in the following areas:

• Cables

• Terminators

• Number of Devices Per SCSI Bus

• Considerations for Selecting SCSI Devices

NOTE The Ultra Narrow, single-ended SCSI bus supports only 7 devices because address 7 is reserved by the system. The Ultra2 Wide, Low-Voltage

Differential SCSI bus supports 15 devices because address 7 is reserved by the system.

cables

Only SCSI cables approved by HP can be used to connect your workstation and any SCSI devices. HP offers the following SCSI cables for Ultra Narrow single-ended SCSI devices:

Table B-2. Ultra Narrow Single-Ended SCSI Cables

Cable Number Cable

Length

Description

C2955A

C2908A

0.5m

1.0m

50-pin HDTS

C2956A

C2957A

1.5m

2.0m

1. High Density Thumb Screw (HDTS)

1

to 50-pin HDTS

204 Appendix B

SCSI connections

SCSI restrictions

HP offers the following SCSI cables for connecting externally connected devices to the system Ultra2 Wide Low-Voltage Differential port:

Table B-3. Ultra2 Wide Low-Voltage Differential SCSI Cables

Cable Number Cable

Length

Description

C2978A

C2979A

C2911B

C2924B

0.5m

1.5m

1.0m

2.5m

68-pin HDTS

1

to 68-pin HDTS

C2361A

C2362A

C2363A

C2365A

1.0m

2.5m

10.0m

5.0m

68-pin VHDCI

2

to 68-pin HDTS

1. High Density Thumbscrew (HDTS)

2. Very High Density Cabled Interconnect (VHDCI)

CAUTION SCSI cables approved by HP are designed to function within the SCSI tolerances for HP devices. Use of other cables can result in significant problems with system operation.

Always use the shortest possible cable(s) for your configuration.

NOTE See “determining scsi bus length” later in this Appendix to determine the total length of your cables.

Appendix B 205

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

206 Appendix B

SCSI connections

SCSI restrictions higher performance SCSI bus than previous SCSI buses and is more sensitive to adherence to the electrical specifications.

Low-voltage differential signaling specifications are described in the SCSI Parallel

Interface-2 (SPI-2) document and in the Electrical Characteristics of Low Voltage

Differential Signaling (LVDS) Interface Circuits TIA/EIA-644 document. Both of these documents can be obtained from:

Global Engineering Documents

15 Inverness Way East

Englewood, CO 80112-5704

(800) 854-7179 or (303) 397-7956

Appendix B 207