Serial ATA and SCSI rematch

Magic pirates Road ridge - ATA and Serial SCSI rematch





Speaking from the application level, ATA and SCSI is much larger than complementary relationship between competition and compete with the existing value should be no distinction; But if tapping the technical details of what what the weak strong, the discussion of precedence is not no significance.

History: Winning the basis of

The advent of SCSI earlier than ATA, specifications and more perfect, but it does not mean that the application of new technologies is always walking in front of SCSI. This more typical example is the double along the transmission (DT, principle equivalent to DDR) and the CRC checking, introduced in 1996, Ultra ATA/33 (also known as Ultra DMA/33) already in place, a full two years earlier than the SCSI (Ultra3 SCSI).

Some features are equally matched. Still CRC checksum, for example, initially only the data with (Data Phase) to enjoy this treatment, then ATA and SCSI invariably be enhanced to include command and status on all transfers, including content protection (below) - from standard setting to the product of, Serial ATA 1.0 and Ultra320 SCSI are almost the same in time.



CRC-wide changes

However, the basic underlying framework, inherent in the ATA how hard they try to shrink the gap with SCSI. LVD (Low Voltage Differential, low-voltage differential), an uncomplicated but requires double data lines (transmission, respectively positive and negative voltage signal) technology, a decision SCSI and parallel ATA (Parallel ATA, PATA) for development important factors. LVD enhanced signal interference, anti-fading ability, help to extend the connection distance, to ensure the reliability of high-frequency transmission. LVD's point of view standing problem, it is easy to understand why SCSI do not rush to adopt the double along the transmission and the CRC checksum.

If you do not use double along the transmission, PATA to achieve 33MB / s bandwidth, operating frequency will reach 16.6MHz, but PATA's 40 data lines when the operating frequency approximation 11.1MHz had not guarantee transmission quality, while the double along the transmission to Ultra ATA/33 without raising operating frequency (8.3MHz) can achieve the same purpose, CRC checksum of the introduction is beneficial to maintaining data integrity. However, with bandwidth re-doubled (66MB / s), 11.1MHz was eventually break, so Ultra ATA/66 forced to move out of the ground 40 to increase the design, although results can not be compared with LVD, but finally able to extend the life of to Ultra ATA/133 (running frequency 33.3MHz).

In contrast Ultra2 Wide (U2W) SCSI, 80MB / s of bandwidth do not need to double along the transmission, because the LVD cable sufficient to meet the operating frequency of 40MHz, there is no CRC check data quality is not bad. Along with a pair of Ultra160 SCSI transfer and CRC check, so operating frequency remains unchanged, cables do not have to change; Ultra320 SCSI operating frequency increased to 80MHz, with a special cable required in theory, but the structure of cable no substantive changes, therefore good quality LVD cable can still be used (only connections from the may be some impact).

Distance from the connection, the number of devices to multi-task operating capacity, SCSI built on the underlying architecture is the PATA advantage of not catching, LVD is just one example.







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