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Wide Ultra 2 SCSI: Low Voltage Differential

What is Differential SCSI?
A differential SCSI bus has a maximum length of 25 meters. This is one of the differences from the 3 meters for a single-ended fast-SCSI bus. The idea behind differential signals is that each bus signal has its own return wire. So, each signal is carried on a pair of wires. The first in this pair carries the same type of signal the single-ended SCSI carries. The second in this pair, however, carries its logical inversion. The receiver takes the difference of the pair (thus the name differential), which makes it less susceptible to noise and allows for greater cable length.

What is Ultra2 SCSI?

  • Ultra2 SCSI doubles the data rate of Ultra SCSI
    - 40 Mbytes/sec for 8-bit
    - 80 Mbytes/sec for 16-bit
  • Improves connectivity by increasing Ultra SCSI cable lengths by 4x
    - Enables 12m for 15 devices on Wide Ultra2 SCSI bus
    - 25m for point-to-point connection

What type of technology is used?

  • Low Voltage Differential (LVD) transceiver is integrated into the SCSI chip using CMOS technology
  • Low Voltage Differential SCSI Enables faster data rates and longer cabling with less susceptibility to noise
  • Reduced power consumption through low voltage differential signal swings

The LVD Advantage

  • Faster speeds Eliminates skew
  • Quadruples distances
  • Allows for 16 devices
  • Improves reliability
  • Backward compatibility

Ultra2 SCSI System Benefits

  • LVD reduces output signal levels
  • LVD increases receiver sensitivities
  • Low power drivers allow integration
    - Reduces the power dissipated by the chip
    - Lowers the amplitude of noise reflections
    - Provides higher transmission rates
    - Increases multipoint connections across
  • longer cables Offers the signal quality of differential SCSI
    - Provides the signal integrity and data reliability expected from high voltage differential

Summary
The industry is transitioning toward Ultra 2. Reasons:

  • LVD is a long term standard
  • Preserves existing S/W investment
  • Higher performance with greater system integrity
  • Approximates single-ended SCSI cost More devices - 15
    - EPI - LUN bridges
    - 225 devices
  • Longer distance than single-ended
    - Point-to-Point 25m, 15 devices 12m

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