| Comparing
SCSI Specifications |
| Specification |
 |
Bit Width |
|
MBps |
|
Max. # of Devices |
| SCSI-1 |
|
8 |
|
5 |
|
8 |
| Fast SCSI (SCSI-2) |
|
8 |
|
10 |
|
8 |
| Fast Wide SCSI (SCSI-2) |
|
16 |
|
20 |
|
16 |
| Ultra SCSI |
|
8 |
|
20 |
|
8 |
| Ultra Wide SCSI (SCSI-3) |
|
16 |
|
40 |
|
16 |
| Ultra 2 SCSI (LVD) |
|
8 |
|
40 |
|
8 |
| Ultra Wide 2 SCSI (LVD) |
|
16 |
|
80 |
|
16 |
| Ultra Wide 3 SCSI (LVD) |
|
16 |
|
160 |
|
16 |
| |
|
|
|
|
|
|
| NOTE: LVD isn't defined
in the original SCSI standards. After Ultra2, all new
speeds are Wide only, meaning they use either a 16-
or 32-bit bus, as opposed to Narrow, which uses 8 bits. |
SCSI’s evolution
In the long history of SCSI adaptations and improvements, the line sometimes blurs between where one Small Computer System Interface (SCSI) ends and another begins. Although there are official standards and specifications proposed and approved, it's often the manufacturers who drive the industry. This creates some confusion when talking about the different SCSI levels.
SCSI-1, the original SCSI standard approved in 1986 by the American National Standards Institute (ANSI), supports transfer rates of up to 5 MBps (megabytes per second). By today's standards, SCSI-1 is slow. Worse yet, it supports a very short bus length. When SCSI-1 was introduced, however, it represented a significant improvement over what was available to users at the time. Still, compatibility was a problem since many vendors offered their own unique SCSI options.
The SCSI-2 standard incorporated SCSI-1 as its subset. In development since 1986, SCSI-2 gained its final approval in 1994 (long after the industry began work on SCSI-3) and resolved many of the compatibility issues SCSI-1 faced. With SCSI-2, users were able to construct more complex configurations using a mix of peripherals. Additionally, it provided a choice between single-ended and differential interfaces—SCSI-1 only offered a single-ended interface.
But the most noticeable benefit of SCSI-2 was its speed. Also called Fast SCSI, SCSI-2 typically supports bus speeds up to 10 MBps but can go up to 20 MBps when combined with Fast and Wide SCSI connectors.
What are Fast and Wide? Fast SCSI enables faster timing on the bus (from 5 to 10 MHz, for instance), thereby providing for greater speed on the overall bus. Wide SCSI uses an extra cable to send data that's 16 or 32 bits wide, which allows for double or quadruple the speed over the bus. In contrast, standard Narrow SCSI interfaces are eight bits wide.
Ultra2 SCSI
The development of Ultra2 SCSI Low-Voltage Differential (LVD) in the late 1990s coincided with ever-increasing market demands for both speed and versatility. With businesses committing greater amounts of critical data to computers, even more storage capacity and faster data movement are required. Therefore, the interface bus linking disk drives and computers must transfer a greater volume of data at higher speeds.
Ultra2 SCSI (LVD) provides that capacity and delivers the performance demanded by many data-intensive applications. Found on many newer workstations and servers, Ultra2 SCSI (LVD) is a good fit with the newest SCSI technology, too. It offers forward compatibility and has room for growth, particularly in its supports for faster microprocessors and drivers.
The main benefits of Ultra2 SCSI (LVD) are as follows:
- It can achieve rates up to 80 MBps (in the Wide version) and support cable lengths up to 39 feet (11.8 m). Earlier SCSI bus versions required repeaters to transfer data at this distance.
- It provides faster drive buffer-to-buffer transfers, freeing the bus for additional devices.
- It's compatible with lower levels of SCSI hardware. Ultra2 SCSI (LVD) requires few SCSI software or protocol changes.
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