LVD - Ultra2 SCSI Low Voltage Differential Drives

Explanation of the SCSI interface known as Low Voltage Differential (LVD).

Disc drive data transfer rates double about every three years. Due to the backward compatibility specifications of the SCSI standard, drives with data rates of 10 Mbytes/sec work well on an Ultra SCSI bus with a bandwidth of 40 Mbytes/sec. The Seagate Barracuda and Cheetah drives have recorded sustained transfer rates as fast as 20 Mbytes/sec. Multiple drive applications require the 80 Mbytes/sec bandwidth provided by LVD. LVD also provides for easy integration of up to 16 devices on the SCSI bus using 12 meters of cable, providing the data integrity of differential at approximately the same cost as the older single-ended designs and the parallel SCSI bus performance needed for Seagate’s high-performance Barracuda and Cheetah drive families.

Seagate’s high-performance Cheetah and Barracuda LVD drives - drive models with a suffix of LC or LW (see product list - are ushering in the future by utilizing the super-fast LVD Ultra2 SCSI parallel interface capabilities. Users of video, database servers, RAIDs, workstations and high-end desktop applications benefit from the greater I/O bandwidth, device connectivity, data reliability, and cable lengths offered by LVD. With the capability to multi-mode, Seagate LVD drives are backward compatible with single-ended Narrow and Wide SCSI devices, but not High Voltage Differential (HVD or Differential), and provide cost savings, rapid bus rates and increased reliability.

Frequently Asked Questions (FAQ)

What transfer rates can I expect from my LVD drive?

LVD provides SCSI bus burst data rates of 80 Mbytes/sec. That's double the fastest SCSI-2 standard (40 Mbytes/sec), and light years ahead of the SCSI-1 standard used prior to 1992 in which SCSI bus rates were as slow as 3 Mbytes/sec. This increased bandwidth means optimal performance for server environments where rapid response is required and random access and large queues are the norm. When using applications such as CAD and CAM, digital video and any RAID environment, the increased bandwidth is immediately noticeable as information is moved quickly and effortlessly.

What are the benefits of LVD Drives?

LVD increases the maximum burst transfer rates to 80 Mbytes/sec, 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.

The lower voltage requirements of LVD versus HVD allow for the integration of the differential drivers and receivers into the drive's on-board SCSI controller. Seagate LVD drives can multimode, meaning they will operate in LVD mode when all requirements are met and degrade to Single Ended mode when all requirements are not met. The older Ultra HVD design requires separate and costly high-voltage components. The low voltage and current components of LVD allow the differential transceivers to be implemented onto the drive's on-board SCSI controller, eliminating the need for separate and costly external high voltage differential components. As a result, LVD provides the integrity of high voltage differential designs at the system cost of single-ended I/O.

What is required to attain LVD mode?

There are four basic requirements for LVD:

  1. 32-Bit Bus mastering
  2. LVD SCSI controller
  3. LVD Termination
  4. LVD Device

In addition, LVD uses a standard SCSI protocol interface and the following SCSI restrictions must still be met: a unique SCSI ID, the physical ends of a SCSI bus must be terminated, and termination power must be correctly supplied.

Who makes LVD SCSI controllers? How can I contact them?

The following is a partial listing of companies currently offering Ultra2 SCSI Controllers:

Are LVD devices compatible with my legacy SCSI and Differential devices?

LVD is fully compatible with the existing installed single-ended SCSI base. A unique DiffSens circuit determines the type of SCSI bus the device is being used on, LVD or single-ended, and configures the drive operation to the appropriate bus capability, otherwise known as multi-moding.

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. Compatibility is an important SCSI feature, but when using SCSI devices of different vintages on the same SCSI bus, all peripherals on that bus will respond to the earliest version SCSI specification. If an Ultra device is installed on an LVD bus with LVD devices, all devices on the bus will respond at Ultra mode.

High Voltage Differential (HVD) devices require a special controller and are not compatible with LVD or single-ended devices; therefore, when DiffSens senses an HVD device, it disables the operation.

What connectors are found on Seagate LVD drives?

The 16-bit Wide SCSI connection (68-pin) is required to achieve the maximum LVD bus data rates of 80 Mbytes/sec. Because of this requirement, the future of the 8-bit Narrow SCSI bus is limited, and Seagate has no plans to produce 8-bit Narrow LVD devices. All Seagate designs will incorporate LVD using the standard 68-pin Wide SCSI connector or the 80-pin single connector SCA-2 when multiple drives and easy integration are required.

How do I configure termination and termination power on Seagate LVD drives?

Do to the specifications of Ultra2 SCSI, Seagate LVD drives have no provision for setting termination on the drive. Termination must be provided by an external source. Similarly, termination power is hard locked to "Drive supplies the Bus" and can not be changed. Although compliant with LVD controllers, these settings may not be fully compatible with standard Single-Ended SCSI controllers.

My LVD drive will only operate in Single-Ended mode. What should I check?

Consider the following sequentially:

  1. Are all requirements for LVD being met: LVD controller, 32-bit bus mastering, LVD termination, and an LVD device?
  2. Try a different slot for the controller, not all PCI slots are created equal.
  3. Double check that the Terminator is LVD compliant.
  4. Ensure there is not a Single-Ended device on the LVD bus segment.

Check that the drive is not "locked" into SE mode with a misplaced jumper.