LVD SCSI - Low Voltage SCSI Cables
LVD SCSI - Low Voltage SCSI Cables
We have a long tradition of manufacturing Low Voltage Differential (LVD) SCSI cables, Auto-sensing Low-Voltage / High-Voltage Terminators and Transceivers. Please E-mail or call our expert Sales Support group with your specific LVD requirements.
Low Voltage Differential SCSI Fundamentals
Low Voltage Differential, LVD SCSI is defined under the Ultra subset of the SCSI-3 standard. Industry wide, the terms LVD and Ultra SCSI are used interchangeably. LVD provides greater I/O bandwidth, improved device connectivity, data reliability, and greater cable lengths.
The LVD SCSI Advantages
LVD provides SCSI bus data rates of 80 Mbytes/sec. That's double the fastest SCSI-2 standard (40 Mbytes/sec), and immensely ahead of the SCSI-1 standard used prior to 1992 in which SCSI bus rates were as slow as 3 Mbytes/sec.
Besides increasing the maximum burst transfer rates to 80 Mbytes/sec, LVD SCSI provides
differential data integrity, extends the SCSI bus cable lengths to 25 meters (12 meters with 16 devices) and provides easy system configuration for up to 16 devices. This is a dramatic increase from Ultra SCSI single-ended cable restrictions of 3 meters and maximum burst transfer rates of 40 Mbytes/sec.
LVD is a subset of the SCSI-3 standard. LVD devices will work on SCSI-1 and SCSI-2 bus segments. Conversely, older SCSI-1
and SCSI-2 single-ended devices will work on an LVD bus. What's important to remember is that the Low-Voltage Differential, LVD transceiver technology offers full compatibility with older SCSI devices in the existing installed base.
LVD SCSI Cables
LVD bulk cable, meeting the spec for shielded external LVD cables, is required for low voltage external applications. External LVD cables meet the maximum rated impedance of 135 Ohms as well as conform to the Time Delay, Time Delay Skew, and Attenuation specs. External LVD cables can be built with 28AWG and 30AWG cable, enabling the use of any 68-pin
LVD bulk cable, meeting the spec for internal ribbon LVD cables, is required for low voltage internal applications. Internal LVD cables meet the maximum rated impedance of 135 Ohms as well as conform to the Time Delay, Time Delay Skew, and Attenuation specs.
Applications for SCSI cables and LVD SCSI Cables
SCSI cables can be grouped into three categories of SCSI signaling/termination:
1) "Single Ended" – A large number of 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. 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 in 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.
4) 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 seems to imply that devices can run in LVD and SE mode at the
same time. This is not the case… an 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.
LVD SCSI terminators
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 longer runs. With a 14" minimum stub distance for LVD, TPO is just not a viable alternative for the knowledgeable SCSI buyer.
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