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LVD Drives

Low Voltage Hard Disk Installation in Onyx and Challenge Computers

It has become extremely difficult to obtain the Seagate Wide Differential (WD) hard disks traditionally used in the Onyx and Challenge series of computers. 

Recently a number of organizations have attempted to use the newer Low Voltage Wide (LW) or Low Voltage Differential drives, which are easily obtainable, as a replacement for the older WD drives.  

The LW and LVD drives are not direct replacements for the older WD drives. This is because the older drives use a high voltage wide differential logic defined under the Small Computer Standard Interface (SCSI) 2 ULTRA specification, while the newer LVD drives use a low voltage differential logic defined under the SCSI 3 ULTRA2 specification. The 2 logics are not compatible.

Differential or High Voltage Differential (HVD)

Differential (D, ND, WD, WDC) drives use a logic signal system. It uses a paired plus and minus signal level to reduce the effects of noise on the Small Computer Standard Interface (SCSI) bus. Any noise injected into the signal would be present in both a plus and minus state, thereby being canceled. 

Due to changing definitions, Differential is now often referred to as High Voltage Differential (HVD). 

When the LVD drivers are used the Onyx/Challenge WD SCSI bus the differences in the interface logic disables the entire SCSI Bus.

Possible Solutions: 

There are 2 possible solutions.  The first lower cost solution is to use 3rd generation LW or LVD drives set up as a single ended device and placed on a single ended controller bus. 

This means modifying the jumpers on the drive mounting sled and moving the connection cable to the single ended port. 

Most WD drive sleds are setup for differential drives (left picture). To set them up for single ended drives set them up to look like the right illustration and plug the drive cable into Channel A.


On occasion the Onyx/Challenge SCSI channels will be reversed. The easiest way to determine which channel is set for single ended is to look at the settings on any installed CD-ROMs or tape drives.

The second solution is to use a translation device manufactured by Rancho Technology, Inc.  These translation devices are available to support HVD to LVD drives as well as HVD to single ended devices.  

The Rancho Technology, Inc translators will support up to 16 drives per translator but are not so inexpensive as to support use of 1 translator for each LW drive installed in the HVD SCSI channel.  The most cost effective approach to using the translators is to convert an entire SCSI drive box/channel to support LW drives.

Hummingbird Computing, Inc has extensive experience in implementing both approaches and can provide integration support to help you extend the life of your existing SGI Challenge and Onyx systems.