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Ultra160/m SCSI: The Next Industry Standard

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On September 14, 1998, seven vendors, representing a broad cross-section of the computer system and storage industry, announced support for evolutionary changes to the Small Computer System Interface (SCSI) that increase performance, reliability, and manageability. Ultra160/m SCSI doubles transfer rates from 80 to 160 Megabytes per second, improves manageability by automatically testing the interface’s performance level and increases reliability by adding Cyclical Redundancy Checks (CRC). When Ultra160/m SCSI is used with low-voltage differential (LVD) signaling, cable lengths of 12 meters are maintained providing full backward compatibility.

The SCSI technology continues to evolve and its evolution is successfully meeting the increasing demand for I/O bandwidth. The SCSI interface has the stability, ease of connectivity, large installed base and a 15 year heritage offering full backward compatibility. The new implementation of SCSI boosts performance, reliability, and manageability even more.

What is Ultra160/m SCSI?

The ANSI standards T10 committee is revising the SCSI Parallel Interface (SPI-3). This document is the basis of the Ultra160/m SCSI technology. Evolutionary changes have been made to the existing SCSI protocol (SPI-2) to increase performance, manageability, and reliability. All changes are incremental, and existing SCSI protocols are maintained for backward compatibility. Three new underlying components of Ultra160/m SCSI are Double Transition Clocking, Cyclical Redundancy Checks (CRC), and Domain Validation.

What technology Enables Ultra160/m SCSI?

Ultra160/m SCSI doubles transfer rates to 160 Megabytes per second by using both edges of the request/acknowledge signal to clock data. This creative solution provides designers with the choice of improving speed, reliability or connectivity. It allows system designers to choose bus bandwidths up to 160 MB/second using existing Ultra2 SCSI cable plants. Alternatively, this technology lets designers maintain Ultra2 SCSI speeds (80 MB/second) and improve reliability by lowering clock speed, allowing more margin for ASICs and cables.

Other Ultra160/m SCSI improvements include automatic tests of the interface’s performance level for increased manageability and the addition of CRC for reliable data transmission. When Ultra160/m SCSI is used with LVD signaling, cable lengths of 12 meters are maintained providing full backward compatibility.

Double Transition Clocking

Double transition clocking changes the digital protocol to use both edges of the SCSI request/acknowledge signal to clock data. Data transfer rates can be doubled simply by increasing the speed of only the data lines. For example, request/acknowledge signal on Ultra2 SCSI runs at 80 MHz, while data runs at only 40 MHz, or 80 MB/second on a 16-bit wide bus. By using both edges of the same 80 MHz request/acknowledge signal, the data rate can be increased to 80 MHz, or 160 MB/second on a 16-bit wide bus.

Choosing the Speed Advantage with Double Transition Clocking

Double Transition Clocking doubles the Ultra2 SCSI data transfer rates from 80 MB/second to 160 MB/second. Interface bandwidth is an essential ingredient for Windows NT and UNIX workstations, video and web servers, and storage area networks (SANs).

Choosing the Reliability Advantage with Double Transition Clocking

For a given transfer rate Double Transition Clocking keeps the maximum clock rate at half the rate of single edge clocking. This provides more timing margin for ASICs, cables, motherboard traces, high capacitance devices, extra connectors, etc. Longer pulses reduce the likelihood of problems by increasing timing margins and tolerance to noise. Double Transition Clocking reduces the maximum frequency of the clock lines (REQ/ACK) without slowing the data rate. Slower clocks should also reduce EMI issues for system designers.

Cyclical Redundancy Checks (CRC)

The Ultra160/m SCSI reliability enhancements include the addition of a Cyclical Redundancy Check on customer data. CRC provides extra data protection for marginal cable plants, external devices, and is one of the best ways to assure data protection during hot plugging. CRC offers higher levels of data reliability by ensuring complete integrity of transferred data. It dramatically reduces undetected error rates by using the same proven CRC that is utilized by FDDI, Ethernet, and Fibre Channel interface.

The Ultra3 SCSI CRC detects:

  • All single bit errors
  • All double bit errors,
  • All odd number of errors
  • All burst errors up to 32-bits long
  • And has a ~2-32 rate of undetected random error patterns

Domain Validation

The third component of Ultra160/m SCSI is Domain Validation. This technology intelligently tests storage networks including cables, backplanes, terminators, expanders, bridges etc. Domain Validation ensures that the network is operating at the required specifications. If reliability is at risk, the transfer proceeds without a hitch at a lower speed—much the way today's modem and fax transmissions connect despite variations in equipment. Domain Validation should increase end-user satisfaction and decreases total cost of ownership by reducing service calls for under performing systems. In addition these tests could save on call center support resources and help alleviate end-user frustration

In the past new devices such as HBAs (host bus adapters) and HDDs (hard disk drives) did not always work smoothly with legacy configurations. Domain Validation helps assure that Ultra160/m SCSI devices operate smoothly in existing legacy systems. This testing is done automatically without changing controller settings, setting BIOS parameters, or fumbling with manuals.

Ultra160/m SCSI: The Need for Speed

The rule of thumb for the past 15 years has been that bus bandwidth should be at least 4 times the maximum throughput of a drive. The Ultra160/m SCSI bus bandwidth should stay comfortably ahead of the internal transfer rates of the next generation 10K HDDs expected in 1999. These HDDs can saturate the Ultra2 SCSI bus with as few as three drives.

Ultra160/m SCSI: The Low Risk Interface Upgrade

SCSI has a 15-year heritage of maintaining full backward compatibility and excellent forward compatibility. About every two years small evolutionary changes are made to this standard to improve speed, reliability and manageability. The low risk upgrade to Ultra160/m SCSI is the next natural transition to protect your investment. The implementation of Ultra160/m SCSI should give customers peace of mind because the changes are mostly digital and straightforward to simulate and implement


These new interface improvements address the most critical requirements in data storage environments. Ultra3 SCSI provides fast data delivery and is both a simple and cost effective solution for system OEMs to implement. From a system designer’s perspective, the big benefit of the new interface is the flexibility it offers to readily optimize a platform for a particular market. It enables end users to continue capitalizing on the low-cost connectivity advantages of the SCSI interface. Ultra160/m SCSI maintains complete backward compatibility with all earlier SCSI systems. Watch for individual companies’ announcements for full details on products and availability.

A lot of work is being done on this technology and good progress is being made. Systems that incorporate Ultra3 SCSI devices are expected to ship in 1999.

See Also