What is LVD?
What is LVD/SE?
Mixing LVD/SE and SE devices
LVD and SCA 80 pin interfaces
LVD and running on a 50 pin
What is U2W?
What is Ultra160/m?
What does centronics or CX mean?
What is LVD?
LVD stands for Low Voltage Differential. All of your typical consumer SCSI
falls into three categories of SCSI signalling/termination
1) "Single Ended" - The most common form of SCSI
devices today use SE. Zip, Jazz, scanners, and almost all 50 pin SCSI devices
fit into this category. Ultra Wide hard drives are commonly SE also. You
usually don't see SE or single-ended written on the documentation so some
assumption is unfortunately necessary when trying to decide this. If it
doesn't specifically say LVD, Ultra2 Wide, Ultra160/m, or Differential, then
it is probably SE. SE typically needs to be (1.5 meters)5 feet or
less of total bus length.
2) "High Voltage Differential" or just "Differential" - HVD is/was
great for applications where you have devices a long ways apart from each
other or if you are in a high noise environment. You can go 25 meters on a
differential bus. This is the signalling that allows you the longest runs.
3) "Low Voltage Differential" - LVD is the newest/latest/greatest
type of signalling because it offers extended length and greater legacy
support (if LVD/SE). You can go 12 meters, which is roughly 40 ft. Not as
long as HVD, but definitely not as restricting as SE. Many new hard drives
are LVD these days.
What is LVD/SE?
LVD/SE stands for "Multimode Low Voltage Differential and Single
Ended". Most LVD devices support LVD/SE. The term multimode is very tricky
though, because it makes you think that devices can run in LVD and SE mode
at the same time. This is not the case, a LVD/SE drive must run in LVD or SE
Mixing LVD/SE and SE
When you mix LVD/SE and SE devices, you must now account for the
fact that your entire bus is running in SE mode. ANY SE DEVICES ON AN LVD/SE
BUS WILL CAUSE THE ENTIRE BUS TO REVERT TO SE MODE - ALL LVD BENEFITS ARE
LOST. When your bus reverts to SE mode, every device on the chain is treated
as SE and you will be forced to keep within the 5 foot limit of SE. There is
still hope though! Some vendors are releasing SCSI host adapters that have 2
separate SCSI channels/segments that allow you to separate your SE devices
from your LVD/SE devices. One such product is the Adaptec 2940U2W.
If you plan to run your bus in LVD mode, you will need an LVD or
LVD/SE terminator or a Twist 'n Flat cable that is terminated with one of
these types of termination. If you are content to run in SE mode, then an
Active terminator will do. KEEP IN MIND THAT ACTIVE TERMINATORS ARE FOR SE
MODE, YOU NEED AN LVD OR LVD/SE TERMINATOR TO RUN IN LVD MODE. People
commonly think you need active termination for LVD, which is incorrect.
To run your bus in LVD mode, you need to make sure you have a cable
with the proper impedance for LVD transmission. Twist 'n Flat is recommended
for this purpose. Some forms of TPO have the proper impedance for LVD, but
are not twisted, so they lose signal quality at larger runs. With a 14"
minimum stub distance for LVD, TPO is just not a viable alternative for the
educated SCSI buyer.
LVD and SCA 80 pin
One aspect of LVD that is "tricky" is the SCA interface. LVD DRIVES
WITH AN SCA INTERFACE NEED LVD COMPLIANT SCA ADAPTERS, EVEN IF THEY ARE
RUNNING IN SE MODE.
LVD and running on a 50
Many new LVD/SE drives require a special "High Byte Terminated"
68-50 pin adapter to run properly on a 50 pin bus.
What is U2W?
U2W is "Ultra2 Wide", which is a marketing term for "16 bit
Fast-40" or "80 MB/Sec.". It refers to the speed your SCSI bus is running
at. SE is limited to Ultra Wide mode which keeps it down to "40 MB/Sec.".
Only LVD and HVD can run in Ultra2 Wide mode. I have not yet seen a
commercially available Ultra2 Wide HVD model, so I assume Ultra2 Wide is
going to continue to be "LVD only".
What is Ultra160/m?
Ultra160/m is an implementation of Ultra3 Wide featured by Seagate
and Adaptec, and other vendors. It runs with a bus bandwidth of 160 MB/Sec.
Ultra160/m uses LVD also and must apply the same rules. Ultra2 Wide and
Ultra160/m devices can co-exist on an LVD bus with no speed penalties. To
get your drives running in Ultra160 mode, there is one additional
requirement. Your terminator must be Ultra160 compatible. If your terminator
is not Ultra160 compatible, your Ultra160 devices will run in Ultra2
You need to make sure you aren't attaching any SE devices to an LVD
bus if you want your bus to run in LVD mode. This includes EXTERNAL PORTS
which may be electrically attached to the internal port where all of your
LVD drives reside.
What does Centronics or CX mean?
Centronics is a style of connector which
uses "plates" instead of "pins". You will never find actual pins on a
centronics style connector. "The "Centronics" connector gets its name today
from the fact that the 36-pin connector was first used on the Centronics
printer, a popular, low-cost dot-matrix printer first offered back in the
sixties." - Walt Foley
What is HD?
HD stands for "High Density". High
Density SCSI connectors are typically "D" shaped and only have 2 rows. The
only exception is the HDI-30, which is 5 rows and square. The most common
external SCSI connectors today are HD-50 and HD-68. The HD-50 connector
usually uses clips and is called SCSI-2 by
many hardware vendors. The HD-68 connector usually uses screws
and is called SCSI-3 by many hardware vendors. The HDI-30
does not use screws or clips, and it is commonly used with PowerBooks.
What is IDC?
IDC is a term that refers to square
internal connectors, usually connected to ribbon cables. They are usually
black with no hardware. Some of these connectors have keys or knockouts. The
IDC 50 is the connector most commonly used with older SCSI devices. As a
side note, IDC 40 connectors are used with IDE drives, and IDC 34 connectors
are used with 3 1/2" floppy drives.
What is MCX?
MCX stands for "Micro-centronics".
Micro-centronics connectors are found on SCA drives and some newer high-end
SCSI devices and controllers. MCX 80 is commonly referred to as SCA.
SCA is short for "single connector attachment" and is being implemented by
many hardware manufacturers today. The VHDCI 68 (aka SCSI-5) connector is an
MCX style connector. MCX 50, 60, and 68 are commonly referred to as "RS6000"
connectors, because they were used by IBM in RISC 6000 systems way back
when. Caution is need here when looking for MCX 68 cables. When a vendor
uses the term MCX 68 without RS6000, VHDCI, .8mm, or .5, you need to ask
What is DB?
DB is the most common style of connector
used on computer systems today. Serial, Parallel, and SCSI all use DB style
connectors. SCSI only uses the DB-25 connector. A DB connector is "D" shaped
and typically uses 2 rows of large pins. The DB-50 is an exception, because
it uses 3 rows. Be careful when dealing with DB-25 ports. A parallel port
and a SCSI port look the same on some computers. (Past experience has shown
that plugging a parallel printer into a SCSI port causes damage to all of
the devices on the channel, including internal devices)
What is HVD?
Differential is different from the regular SCSI you encounter in most
computer stores today.
Signalling: High Voltage Differential uses "Differential"
signalling as opposed to "Single Ended" signalling. This makes HVD and SE
completely incompatible with each other. HVD to SE converters commonly are
very expensive. New HVD controllers are cheaper. (Your adapter AHA-2940UW
will NOT work with HVD equipment. You would need the AHA-2944UW.)
Pinout: When you go from one type of connector to the
same style, all SCSI uses the same pinout: "straight through". This means
that any SCSI cable will work (the pinout will match) as long as the
connectors are the same on both ends. (ex. 68-68 50-50 but not 68-50 or
HD50-Centronics50). When you go from 68-50 (which is the most common
adaptation) you must use adapters wired for HVD. All adapters in stores are
SE unless they state otherwise.
Impedance: Most "Ultra" rated cable has the proper impedance for SE or HVD,
even though the ratings are different. To illustrate:
Single-Ended Impedance: 90 ohms
Cable A has two
different ratings. This is because impedance changes based on which type of
signalling/testing you use. Cable A has the typical value of a Fast-20/Ultra
cable. Use Fast-20(40MB/Sec.) cable for all of your SE external cables.
Why HVD?: HVD allows a maximum bus length of 25 meters.
It is commonly used for long runs in noisy areas.
Why not HVD?: HVD was great for it's purpose in the past,
but a new technology has arrived that makes HVD obsolete. LVD aka Low
Voltage Differential is out, allowing 12 meters of maximum bus length. LVD
supports Ultra2 Wide, allowing a bus bandwidth of 80 to 160 MB/Sec.