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In the late 1970s, Shugart Associates developed an interface to handle data transfers between devices, regardless of the type of device. The interface operated at the logical-or operating system-level instead of at the device level. This new interface was called the Shugart Associates System Interface, or SASI-the precursor to SCSI.
In June 1986, the ANSI X3.131-1986 standard known as SCSI-1 was formally published. This was a very loose definition, with few mandates. As a result, manufacturers of SCSI products developed a variety of competing designs.
SCSI-1 supported up to seven devices on a chain (plus the host adapter). Each device transferred data through an 8-bit parallel path. Compatibility of SCSI drives was nearly impossible because many SCSI devices had their own custom commands on top of the limited SCSI standard.
You might encounter older SCSI adapters, drives, and peripherals that are based on the original SCSI-1 standard. In reality, this standard amounts to little more than a few agreed-upon commands. The wide range of proprietary drivers, operating system interfaces, setup options, and custom commands made true compatibility a real problem and gained SCSI a bad reputation on the PC platform. It was, however, popular with Apple and UNIX developers, who could work with a limited range of devices.
In most cases it is best to upgrade any SCSI-1 devices to SCSI-2. If circumstances require you to work on an early SCSI product, you will have to contend with both hardware and driver issues. Check the Web site of your SCSI device's manufacturer for possible new drivers.
The limited acceptance, but great potential, of SCSI-1 led to a more robust standard with a range of commands and a layered set of drivers. The result was a high-performance interface that began to take over the high-end market. It was the interface of choice for fast hard disk drives, optical drives, scanners, and fast tape technology.
One of the most important parts of the SCSI-2 specification is a larger (and mandatory) standard command set. Recognition of this command set (18 commands) is required for a device to be SCSI-2 compliant. The Common Command Set (CCS) made compatibility of multivendor devices possible. The CCS also introduced additional commands to more easily address devices such as optical drives, tape drives, and scanners.
SCSI-2 also supports:
- Wide (16-bit) SCSI.
- Fast SCSI.
- Fast/Wide (combines fast and wide features).
- Ultra (32-bit) SCSI SI-2.
- Backward compatibility with SCSI-1.
This standard uses a fast synchronous mode to transfer data, doubling the data transfer speed from 5 MB/s to 10 MB/s. Wide SCSI doubles that again.
Plug and Play SCSI adapters first arrived with the advent of the SCSI-2 standard. Today, all new SCSI host adapters are Plug and Play. The SCSI-2 standard took a long time to gain final approval, requiring agreement by many vendors. As a result, you might run into products labeled "Draft SCSI-2." In almost all cases, you can get these products running on any SCSI-2 or later system if you get the appropriate drivers from the vendor or the maker of the host adapter or operating system.
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