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In the beginning, SCSI lacked the capability to run hard disks off the SCSI bus. To boot from these drives and use a variety of operating systems was a problem that resulted from the lack of an interface standard. The standard IBM XT and AT ROM BIOS software was designed to talk to ST-506/412 hard disk controllers. The software easily was modified to work with ESDI because ESDI controllers are similar to ST-506/412 controllers at the register level. (This similarity at the register level enabled manufacturers to easily design self-booting, ROM-BIOS-supported ESDI drives.) The same can be said of IDE, which completely emulates the WD1003 ST-506/412 controller interface and works perfectly with the existing BIOS as well. SCSI is so different from these other standard disk interfaces that a new set of ROM BIOS routines is necessary to support the system so it can self-boot. The newer IBM PS/2 systems that come with SCSI drives have this support built into the motherboard BIOS or as an extension BIOS on the SCSI host adapter.
Companies such as Adaptec and Future Domain have produced SCSI cards with built-in ROM BIOS support for several years, but these BIOS routines were limited to running the drives only under DOS. The BIOS would not run in the AT-protected mode, and other operating systems included drivers for only the standard ST-506/412 and ESDI controllers. Thus, running SCSI was impossible under many non-DOS operating systems. This situation has changed significantly, however; IBM now supports many third-party SCSI host adapters in OS/2, especially those from Adaptec and Future Domain. For compatibility reasons, I usually recommend using SCSI adapters from these two companies, or any other adapters that are fully hardware-compatible with the Adaptec and Future Domain adapters.
ANSI SCSI standards
The original SCSI standard (ANSI X3.131-1986) was approved in 1986, SCSI-2 was approved in January of 1994, and a new revision called SCSI-3 is being developed. One problem with the original SCSI-1 document was that many of the commands and features were optional, and there was little or no guarantee that a particular peripheral would support the expected commands. This problem caused the industry as a whole to define a set of 18 basic SCSI commands called the Common Command Set (CCS), which would become the minimum set of commands supported by all peripherals. CCS became the basis for what is now the SCSI-2 specification.
In addition to formal support for CCS, SCSI-2 provided additional definitions for commands to access CD-ROM drives (and their sound capabilities), tape drives, removable drives, optical drives, and several other peripherals. In addition, an optional higher speed called FAST SCSI-2 and a 16-bit version called WIDE SCSI-2 were defined. Another feature of SCSI-2 is command queuing, which enables a device to accept multiple commands and execute them in the order that the device deems to be most efficient. This feature is most beneficial when you are using a multitasking operating system that could be sending several requests on the SCSI bus at the same time.
Most companies indicate that their host adapters follow both the ANSI X3.131-1986 (SCSI-1) as well as the x3.131-1994 (SCSI-2) standards. Note that because virtually all parts of SCSI-1 are supported in SCSI-2, virtually any SCSI-1 device is also considered SCSI-2 by default. Many manufacturers advertise that their devices are SCSI-2, but this does not mean that they support any of the additional optional features that were incorporated in the SCSI-2 revision.
For example, an optional part of the SCSI-2 specification includes a fast synchronous mode that doubles the standard synchronous transfer rate from 5MBps to 10MBps. This Fast SCSI transfer mode can be combined with 16-bit Wide SCSI for transfer rates of up to 20MBps. There was an optional 32-bit version defined in SCSI-2, but component manufacturers have shunned this as too expensive. In essence, 32-bit SCSI was a stillborn specification. Most SCSI implementations are 8-bit standard SCSI or Fast/Wide SCSI. Even devices which support none of the Fast or Wide modes can still be considered SCSI-2.
The SCSI-3 standard is still being defined and is still a long way off from being approved. However, portions of this specification, although not final, are being sold in products today. One of these developments is the new Fast-20 mode, which is also called Ultra-SCSI. This essentially is quad-speed SCSI, and will run 20MBps on an 8-bit standard SCSI bus, and 40MBps on Wide (16-bit) SCSI.
Table 2.4 shows the maximum transfer rates for the SCSI bus at various speeds and widths, as well as the cable type required for the specific transfer widths.
|Table 2.4 SCSI Data-Transfer Rates.|
|Bus Width||Standard SCSI||Fast SCSI||Fast-20 (Ultra) SCSI||Cable Type|
|16-bit (Wide)||10MBps||20MBps||40MBps||P (68-pin)|
The A cable is the standard 50-pin SCSI cable, whereas the P cable is a 68-pin cable designed for 16-bit. Maximum cable length is 6m (about 20 feet) for standard speed SCSI, and only 3m (about 10 feet) for Fast or Fast-20 (Ultra) SCSI.
So-called SCSI-1 adapters have no problems with SCSI-2 peripherals. In fact, as was stated earlier, virtually any SCSI-1 device can also legitimately be called SCSI-2 (or even SCSI-3). You can't take advantage of Fast, Fast-20, or Wide transfer capabilities, but the extra commands defined in SCSI-2 can be sent by means of a SCSI-1 controller. In other words, nothing is different between SCSI-1 and SCSI-2 compliant hardware. For example, I am running a Seagate Barracuda 4GB Fast SCSI-2 drive with my standard IBM SCSI-1 host adapter, and it runs fine. Most adapters are similar, in that they actually are SCSI-2 compatible, even if they advertise only SCSI-1 support.
Because the SCSI-2 standard was not actually approved before January 1994, any devices that claimed to be SCSI-2 before that time were not officially in compliance with the standard. This is really not a problem, however, because the SCSI-2 document had not changed appreciably since it was nearly approved in 1990. The same thing is currently happening with advertisers listing devices as "SCSI-3". The SCSI-3 specification is not yet approved, although certain areas are being worked out.
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