SUMMARY: Differential Terminators: HVD or LVD?

From: Jon Godfrey <>
Date: Wed Feb 11 2004 - 13:08:55 EST
Thanks to the several folks who replied, and the obligatory mention to the
persons from Goldman Sachs, Citigroup, and Texas Instruments for their "I'm
on vacation and you're not!" replies.  Honorary mention to John Whelan and
his weird send-me-another-mail anti-spam tool called STEVEN.

Everyone was in consensus, but Darren Dunham's reply was the most verbose:

   "HVD is a completely different voltage setup than SE or LVD.  You cannot
cross any equipment between the two other than cables.

   "The HVD terminator is different.

   "Most LVD equipment will "throttle-back" to SE speeds and length
requirements if SE equipment is used on the bus.

   "A D1000 requires an HVD controller, proper cables, and an HVD
terminator.  Most sun terminators for this equipment say simply
"differential".  At the time they were most popular, Sun sold no LVD
equipment, so the possibility for confusion arises.

Thanks Darren!

I also found a few descriptions on the web that served useful from

Q. 11. What is HVD SCSI?
Answer: This is the "old" differential SCSI using TTL voltage levels that
was originally defined in SCSI-2, offering 25 meter (82 ft) cable length. It
was functionally replaced by LVD (Low Voltage Differential) SCSI in the
SPI-2 document of SCSI-3 and obsoleted in the SPI-3 document of SCSI-3. HVD
and LVD SCSI are not directly compatible but can be interconnected by the
use of a SCSI expander called an LVD to HVD Converter.

Q. 13. What is LVD SCSI?
Answer: LVD, which stands for Low Voltage Differential, was introduced in
the SPI-2 document of SCSI-3. It is also called Ultra 2 or Fast-40 SCSI. It
uses 3 volt instead of 5 volt logic level and is not directly compatible
with the "old" differential (HVD) SCSI. LVD again doubles SCSI data
throughput to 40 Megatransfers/sec. Cable lengths are 12 m (40 ft). Single
initiator-single target applications may use up to 25 m (82 ft) of cable.
The "multimode" implementation of LVD is backward compatible with
single-ended SCSI. However, connecting one single-ended peripheral to a
multimode LVD bus will cause the entire bus to switch to the single-ended
mode with the single-ended limitations on data throughput and cable length.
LVD can be interconnected with HVD by the use of a SCSI expander called an
LVD to HVD Converter.

Thanks again!

-----Original Message-----
From: Jon Godfrey [] 
Sent: Wednesday, February 11, 2004 9:07 AM
Subject: [Maybe Spam] Differential Terminators: HVD or LVD?

Is there a difference in terminator requirements as it relates to a D1000?
Are there specific types of differential terminators, or does the HVD and
LVD only refer to the chain type leaving just un-typed differential

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Received on Wed Feb 11 13:08:35 2004

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