Ultra3 SCSI Low Voltage Differential (LVD) Drives

Explanation of the SCSI interface known as Ultra3 SCSI Low Voltage Differential (LVD).

Ultra3 SCSI Low Voltage Differential Drives

Ultra3 SCSI (160 Mbytes/sec) Low Voltage Differential (LVD) drive data transfer rates have been doubling about every two to three years. Ultra3 SCSI, also known as Ultra160 SCSI, is the latest iteration of this technology explosion. Ultra160 represents the fifth generation of the small computer systemsinterface (SCSI). Using double transition clocking, Ultra160 has again doubled the transfer speeds of Ultra2 SCSI and quadrupled the speed of earlier Wide Ultra SCSI technology. Ultra160 represents the fastest data transfer rates of any parallel I/O interface currently available and provides for better data reliability using cyclic redundancy checking (CRC) and system configuration verification using domain validation. Double transition clocking uses both edges of the SCSI request/acknowledge signal to clock data. CRC is mandatory for these increased data transfer rates to ensure that all data reaches the intended destination correctly. Domain validation does a system configuration check during the boot cycle or when a new device is added to the system. The host adapter negotiates with each device and determines the maximum speed available for each. This means Ultra160 and Ultra2 LVD devices can operate on the same bus simultaneously at their appropriate maximum transfer rates. As with Ultra2 LVD, an Ultra160 device will multi-mode down to single-ended mode if a non-LVD capable device is introduced to the bus segment. Beginning with the Ultra 160 drive families, and continuing with all future drive development, SCSI-1 features and formats will no longer be supported.

Seagate’s high-performance Cheetah and Barracuda Ultra2 and Ultra3 LVD drives (drive models with a suffix of LC, LCV, LW or LWV) are ushering in the future by utilizing the super-fast Ultra2 and Ultra3 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 longer cable lengths offered by LVD. With the capability to multi-mode, Seagate Ultra3 and Ultra2 LVD drives are backward compatible with single-ended Narrow and Wide SCSI devices, but not High Voltage Differential (HVD), and provide cost savings, rapid bus rates and increased reliability.

Frequently Asked Questions (FAQ)

What is Ultra3 SCSI LVD?

The ANSI standards T10 committee is revising the SCSI Parallel Interface (SPI-3). This revision to SPI-3 is the basis of the Ultra160 SCSI technology. All changes must be incremental and existing SCSI protocols are maintained for backward compatibility. Three new components of Ultra160 SCSI are Double Transition Clocking, CRC, and Domain Validation. Low Voltage Differential (LVD) devices are defined under the LVD subset of the SCSI-3 standard. Industry-wide, the terms Ultra3 SCSI LVD and Ultra160 SCSI are used interchangeably.

What transfer rates can I expect from my Ultra160 SCSI drive?

Ultra160 SCSI provides SCSI bus maximum burst data rates of 160 Mbytes/sec. That is double the Ultra2 LVD drives (80 Mbytes/sec), quadruple 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. More realistically, sustained data transfer rates of 30-50 Mbytes/sec can be expected. 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 moves quickly and effortlessly.

What are the benefits of Ultra160 SCSI?

Ultra160 SCSI increases the maximum burst transfer rates to 160 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 multi-mode.  Multi-mode means when all requirements are met, the drive will operate in LVD mode, but when all requirements are not met, it will degrade to single-ended mode. 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 Ultra160 SCSI mode?

- There are three basic requirements for Ultra160 SCSI:

- Ulra160 SCSI Controller

- LVD Termination

- Ultra160 SCSI Device

- Ultra160 SCSI is a SCSI interface and the following SCSI restrictions must still be met: must have a unique SCSI ID, the physical ends of a SCSI bus must be terminated, and termination power must be correctly supplied.

I just purchased and installed an Ultra160 SCSI controller and Seagate Ultra160 drive. How do I set this up under Windows 95/98?

Most SCSI controllers are plug-n-play and Windows will install and configure them automatically at boot-up. Otherwise, go to Control Panel and select the Add New Hardware icon.  On the second screen of the Add New Hardware Wizard, select Yes to let Windows detect new hardware. If Windows does not detect the adapter, run the Add New Hardware Wizard again. This time, select No on the second screen, then select SCSI Controllers. Choose the Have Disk option and insert the diskette or CD provided with the controller.

Are Ultra160 SCSI drives compatible with my Ultra2 LVD, Wide SCSI, and High Voltage Differential devices?

Ultra160 SCSI is fully compatible with the existing installed LVD and single-ended SCSI base, SCSI-2 and later. 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 (i.e., multi-moding). Ultra160 SCSI is a subset of the SCSI-3 standard. Ultra160 and Ultra2 LVD devices can operate at their respective maximum data transfer speeds simultaneously on the same bus segment. Ultra160 LVD devices will work on SCSI-2 LVD and single-ended bus segments. Conversely, legacy 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 vintage 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, all devices on the bus will respond at Ultra mode. 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 Ultra160 SCSI drives?

The 16-bit Wide SCSI connection (68-pin) is required to achieve the maximum LVD bus data rates (80 – 160 Mbytes/sec). As mentioned above, the future of the 8-bit Narrow SCSI bus (SCSI-1) is limited. All Seagate Ultra2 and Ultra3 SCSI designs will incorporate 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 Ultra160 SCSI drives?

Due to the specifications of Ultra2 and Ultra3 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 limited to “Drive supplies the Bus” and “None”. Check the documentation that came with your SCSI controller for correct settings.

My Ultra160 SCSI drive will only operate in single-ended mode. What should I check?

Try the following:

- Are all requirements for LVD being met (such as LVD controller, LVD termination, and an LVD device)?

- Try a different slot for the controller. Not all PCI slots are created equal.

- Ensure the terminator is LVD compliant.

- Ensure there is not a single-ended device on the LVD bus segment.

- Ensure the drive is not “locked” into single-ended mode with a misplaced jumper.

- If an Ultra160 device is operating at Ultra2 LVD speeds, check SCSI controller configurations to ensure the maximum data transfer rate is selected.

For further information see: www.scsita.org.