LVD, SE, HVD, SCSI compatibility - or lack of it
Jim Stein -
here - February 2000
Many people who use SCSI regularly but are
not involved with it on a day-to-day basis have some confusion relative to the "new"
LVD SCSI, especially with regard to its compatibility with the "older"
differential (HVD) and single ended (SE) SCSI varieties. I will try to clarify
the situation a little.
What is SE, LVD, HVD?
First, however, let me try to define what is meant
by SE, HVD and LVD SCSI. These terms refer to the manner in which the SCSI
interface places signals on the SCSI cabling. The original SCSI specifications
defined only the single ended (SE) interface. I have heard this referred to as
"regular" SCSI, however, "regular" is not a SCSI term.
Anyway, the SE interface drives each signal line against ground. Narrow (8-bit)
SCSI has 18 signal lines plus a few others for term power and some reserved
pins. SE signals are susceptible to noise and do not offer very long cable
lengths. Slow SCSI has a maximum recommended cable length of 6 meters (20
feet); Fast SCSI is 3 meters (10 feet); and Ultra SCSI is only 1.5 meters (5
feet). The introduction of differential SCSI (now called HVD for High Voltage
Differential) overcame these cable problems.
The HVD interface drives two lines for each SCSI signal one line is
the inverse of the other line and the SCSI signal is the difference (hence, the
term differential) between these two lines. This results in a more robust
signal that is not as susceptible to noise as the SE interface and offers SCSI
cable lengths of 25 meters (82 feet) regardless of the speed of the bus. The
maximum data throughput of the SE and HVD interface is Ultra SCSI (also called
Fast-20) offering 20 Mbytes/sec throughput for narrow SCSI and 40 Mbytes/sec
throughput for wide SCSI. The disadvantage of the HVD interface is that it is
more expensive than the SE interface.
When the members of the X3T10 SCSI Committee looked for ways of again
doubling the data throughput of SCSI, they realized that the 5 volt logic used
in SE and HVD SCSI would not allow 40 Megatransfers/sec data rates. Enter LVD
SCSI. LVD uses 3.3 volt logic and common sense tells us that you can change a
signal through a 3.3 volt range at a faster rate than through a 5 volt range.
Making the interface a differential interface (thus the term Low Voltage
Differential) retained the advantages of noise rejection and longer cable
lengths that differential signals offer.
This lower voltage offers another advantage. The lower voltage and lower
current requirements of LVD SCSI drivers means lower heat dissipation. That
means that the differential drivers can be included on the LVD SCSI interface
ASIC which results in an interface with a smaller parts count, lower parts cost,
a requirement for less real estate on the pcb and increased reliability.
Those of us who have been around long enough know that every new variety of
SCSI has always been backward compatible. But LVD SCSI is not compatible with
the signal levels of previous forms of SCSI. Some clever individual or
individuals on the X3T10 Committee came up with the idea of "multimode"
LVD (called LVD/MSE in the SCSI specs for Multimode Single Ended) which is an
LVD interface that switches from LVD to SE when it is connected to a SE bus.
Now the manufacturers of SCSI peripherals need to produce only the one LVD
variety of SCSI drive, instead of two varieties, SE and HVD. That saves them
money and it is passed on to their customers.
Oh-Oh. My New LVD Drive is Wide SCSI, but My Host Adapter
is SE Narrow!
By the way, all the LVD hard drives have the wide
SCSI interface. You can place these multimode LVD (or LVD/MSE) wide devices on
a SE narrow SCSI bus by the use of an adapter to change the 68 pin high density
connector on the hard drive to a 50 pin connector for the cable to your SCSI
bus. The unused pins on the LVD drive should be terminated. This can cause
some heartburn if the LVD drive is not the last device at the end of the SCSI
cabling. If it is the last device on the cabling, you can get a 68 pin to 50
pin adapter with the upper byte terminated. Note that LVD drives do not have
By the way, save some more grief and do not buy an LVD drive with the SCA
SCSI connector. The SCA connector is designed for drives that plug into SCSI
backplanes. In order to use it in a cabled system, you will have to get an SCA
to 68-pin (or 50-pin) adapter. There are a number of suppliers of this kind of
What if I Connect a SE Drive to an LVD Bus?
In a paragraph above it was noted that a multimode
LVD device is compatible with a SE SCSI bus because the interface will switch
from LVD to SE when it is connected to the SE bus. It works the other way also.
If you have a multimode LVD bus and you connect a SE peripheral to it, the
entire bus will switch to the SE mode. The data throughput to all LVD
devices will be cut at least in half and there will be a drastic reduction in
the maximum allowable SCSI cable length. We have yet to see an LVD SCSI host
adapter or peripheral device that is not multimode LVD, but I am sure they exist
somewhere, so be sure to check the SCSI device before you buy it and not just
assume it is multimode LVD.
How About LVD-HVD Compatibility?
So now we have shown compatibility between LVD and
SE SCSI. What about HVD SCSI? I am sorry to have to tell you that there is no
more direct compatibility between LVD and HVD SCSI than there is between HVD and
SE SCSI. It does not exist. Placing an LVD device on an HVD bus will cause the
LVD device to shut down. Placing an HVD device on an LVD bus will cause the
entire LVD bus to shut down.
But the Only SCSI Port Available on My Computer is HVD!
By now you probably know that the hard disk drive
manufacturers are no longer producing HVD disk drives (or, for that matter,
SE-only hard drives). There are a large number of higher-end computers in the
field that have only HVD SCSI ports. What do you do when your HVD hard drives
die and you cannot buy replacements? The only solution is to purchase a type of
device that the EPI document of the SCSI-3 specs calls a SCSI "Expander"
(another new term). The particular expander you need for this is an HVD to LVD
Converter. This allows the HVD host to communicate with the LVD drive(s) at the
full data throughput of the HVD host.
How do I Preserve Data Throughput to my LVD Devices When I
Must Place A SE Peripheral on my Multimode LVD Bus?
Let me introduce a SCSI concept that you may not be
familiar with. That is the SCSI "domain" and the SCSI bus "segment".
The SCSI domain is all the devices including cabling and terminators that are
connected to one SCSI port. This domain may include one, two or more SCSI bus
segments. A bus segment is defined as the cabling and peripheral devices
between two adjacent terminators. Each bus segment may be a different variety
of SCSI and each segment is allowed the SCSI specified cable length. That is,
one bus segment may be LVD with up to 12 meters (40 feet) of cable and the next
bus segment may be SE with up to 6 meters (20 feet) of cable (or HVD with up to
25 meters [82 feet] of cable).
In order to create another bus segment, we need a SCSI bus expander. This
expander is different than the one mentioned in one of the above paragraphs.
That one was an HVD to LVD Converter. In this case, we need an LVD/MSE to
LVD/MSE Expander (Because nearly all LVD devices are multimode LVD, the terms
LVD and LVD/MSE are just about synonymous). We can connect the LVD bus to one
port of this expander and we can connect the SE peripheral to the other port of
the expander. Now, the LVD bus will continue to operate as an LVD bus and the
SE bus will be a SE bus. The LVD host adapter can "talk" to the LVD
devices at their maximum data rate and it can talk to the SE devices at their
slower data rate. Everyone is happy. ...Paralan profile
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