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© June 2004 BigDumbDinosaur

SCSI: An Old Dog That Keeps Learning New Tricks

Sat Jun 19 23:39:35 2004 SCSI: An Old Dog That Keeps Learning New Tricks
Posted by BigDumbDinosaur

Did you know that the Small Computer System Interface, or SCSI, turns 25 this year?

SCSI's ancestor, SASI (Shugart Associates System Interface), was publicly disclosed by Dr. Alan Shugart in 1979 in hopes of the bus system being accepted as an industry standard. In 1981, NCR got involved with SASI and started the process that would ultimately result in SASI becoming ANSI standard X3.131-1986. Since ANSI does not permit the use of vendor names in any standards, NCR renamed SASI to SCSI and the rest, as is too often said, is history. X3.131-1986 has since become known as SCSI-1. Succeeding versions are often referred to as SCSI-2 and SCSI-3.

SASI/SCSI was designed at a time when technical excellence in computer systems was a major factor in design choices. (Today, cost seems to be the major factor -- how else to explain the success of Michael Dell and his PC's?) Anyhow, Shugart was concerned that his interface would be reliable under all conceivable conditions. Hence he developed a highly engineered bus system whose philosophy and structure have withstood the test of time. In contrast, the ATA ("advanced technology" attachment) interface commonly found in PC's was designed to be simple and cheap, an accountant-mentality bus whose performance is downright feeble when compared to SCSI.

Here are some interesting SCSI tidbits:

1) Alan Shugart, in addition to being the father of SCSI, is credited with the invention of the floppy disk drive. He later founded Seagate Technology, a company that needs no introduction to most computer enthusiasts.

2) The word "small" in Small Computer System Interface has nothing to do with the physical size of the machine. Back in the late 1970's, anything that was not a mainframe was a small computer, which term described the processing capacity. Physically speaking, there were no "small" computers back then, excepting the early micros. A minicomputer, despite the name, was anything but mini. I know: I had to manhandle those boat anchors more often than I care to remember.

3) The ANSI X3.131-1986 standard is relatively unusual in that it defines all aspects of SCSI: hardware, software, electrical signaling (including the characteristic impedance of the interconnecting cable), bus termination, and bus protocol. Later SCSI standards have separated these aspects to better accommodate technical advances and to also reflect that the software aspects of SCSI were independent of the hardware features.

4) The SCSI bus protocol has stayed remarkably constant over the years, which makes it possible to mix almost all generations of SCSI hardware and have everything work together -- although not necessarily in an optimal way.

5) SCSI is device independent. Although most think of hard drives when SCSI is mentioned, the bus protocol and command structure can be adapted to a wide variety of peripherals, e.g., document scanners, tape drives, CD burners, and even printers. If you really are a SCSI purist, Teac makes a nifty 3-1/2 inch floppy disk drive equipped to attach to the SCSI bus -- although the price might make you rethink the situation.

6) SCSI is, and always has been, a packet interface. Due to this feature, the SCSI command set is unusually consistent in operation. For example, a read data command works the same whether the target device is a disk, tape, scanner or CD-ROM. This is not the case with the PC's ATA bus. In fact, the commonly used ATAPI devices in PC's, CD-ROM's and IDE tape drives, use a protocol that is based upon SCSI.

7) SCSI can support up to seven devices (besides the host adapter) on an eight bit or "narrow" bus, or up to 15 devices (besides the host adapter) on a 16 bit or "wide" bus. Each device may be comprised of one or more "logical units," each of which may be separately addressed by the host system. The SCSI standard allows each device to support up to eight logical units (think one drive controller with eight hard drive mechanisms attached). A fully extended wide SCSI implementation could theoretically address up to 240 logical units. It is this SCSI feature that makes CD-ROM "jukeboxes" practical.

8) SCSI is a true multitasking interface that is ideally suited for use in servers. No other mass storage interface, including serial ATA (SATA), can make that claim. Also, SCSI devices are intelligent. Once the system has told the host adapter what needs to be done, all SCSI processing is handled by the host adapter and target device. The computer's CPU does not have to get involved with the bus protocol, error handling or anything of that sort. This aspect of SCSI can significantly enhance the throughput of the host machine, especially in a busy environment.

9) The X3.131-1986 standard allowed the data cable to be up to six meters (approximately 18.5 feet) in length. The high voltage differential (HVD) version could work with cable lengths up to 25 meters (over 75 feet). Compare that to the miserly 18 inches of ATA-66/ATA-100. The most current SCSI implementation, ultra-320, which uses a low voltage differential (LVD) interface, can work over 12 meters (nearly 37 feet) with a full complement (16) of devices attached. If you can live with two devices, ultra-320 can work on a 25 meter cable.

10) SCSI just keeps getting faster and faster. Ultra-160 and ultra-320 are so named to reflect their data transfer speeds: 160 and 320 megabytes per second, respectively -- sustained transfer rates, by the way, not "burst" rates. These are not the fastest forms of SCSI, however. Fiber-optic interfaces are available for those of you with really short attention spans. Also, ultra-160 and ultra-320 interfaces are electrically balanced to ground, making them highly resistant to electrical interference. This is part of the reason why they can be run at such high transfer rates.

Want to know more about SCSI? Check out https://www.pcguide.com/ref/hdd/if/scsi for more information

Got something to add? Send me email.

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"Ultra-160 and ultra-320 are so named to reflect their data transfer speeds: 160 and 320 megabytes per second, respectively..."

A tidbit I didn't mention is that during a sustained data transfer, ultra-160 or ultra-320 can actually outrun the standard 33 MHz, 32 bit PCI bus found in most PC's.


In the "what goes around, comes around" dept., the latest development in SCSI is SAS - Serial Attached SCSI. Traditional SCSI has always been a parallel interface, like the ATA interface, hence the bulky, wide (expensive) internal cables. SAS, like its SATA equivalent, is an attempt to speed up and simplify the interface even further, with speeds up to 1.5Gb/sec

So, we have SASI --> SCSI --> SAS.


I suppose we could be redundant about it and tack an "I" to the end of SAS, which would result in "serial attached SCSI interface" or (surprise!) SASI.


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