Frequently Asked SCSI Questions
 

 

What is LVD?

What is LVD/SE?

Mixing LVD/SE and SE devices

LVD termination

LVD Cables

LVD and SCA 80 pin interfaces

LVD and running on a 50 pin cable

What is U2W?

What is Ultra160/m?

What does centronics or CX mean?

What is HD?

What is IDC?

What is MCX?

What is DB?

What is HVD?

 


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 signalling. Most 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 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.

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Footnote:
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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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 more questions.


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?

High Voltage Differential is different from the regular SCSI you encounter in most computer stores today.

1) 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.)

2) 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.

3) Impedance: Most "Ultra" rated cable has the proper impedance for SE or HVD, even though the ratings are different. To illustrate:

Cable A

Differential Impedance: 135 ohms
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.

4) Why HVD?: HVD allows a maximum bus length of 25 meters. It is commonly used for long runs in noisy areas.

5) 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.