LVD FAQ and more.

There has been a large influx of LVD/SE controllers, hard drives, and cables onto the SCSI market. Some people were expecting and even anticipating the 80MB/Sec. SCSI movement. It's twice as fast so why not be happy about it? The answer to that question and many more lie within this document.

LVD? What is it?
LVD/SE? What does that mean?
Can I mix LVD/SE and SE devices?
What kind of termination do I use with LVD?
What kind of cables should I use with LVD?
My LVD drive doesn't work with my SCA adapter. Why?
Can I run a LVD drive on a 50 pin chain?
What is U2W?
What is Ultra160/m? Is it LVD?
Why do my drives stop working when I plug in my scanner?

What is LVD?

LVD stands for Low Voltage Differential. All of your typical consumer SCSI falls into three categories of SCSI signaling/termination

1) "Single Ended" - The most common form of SCSI signaling. Most devices today use SE. Zip, Jaz, 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 signaling that allows you the longest runs.

3) "Low Voltage Differential" - LVD is the newest/latest/greatest type of signaling 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 mode.

Mixing LVD/SE and SE devices.

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.

LVD termination

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.

LVD Cables

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 interfaces

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 pin cable

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 (80MB/Sec.) mode.

Be careful!

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.

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