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SCSI Explained

If you are thinking of getting a new server or a workstation where the main requirement is for heavy duty file access, you must consider using a Small Computer Systems Interface (SCSI, pronounced scuzzy).


Why SCSI

Most servers spend a considerable amount of their time reading and writing to storage devices.

Most workstations use the IDE or EIDE interface. Probably the reason is that IDE drives are cheaper than SCSI type devices. For most workstation usage, the IDE interface is more than adequate. But if high speed disk access is crucial, the choice should be SCSI. Other articles in our newsletter have referred to the fact that you can't measure a system's performance based on the CPU speed alone. This is especially true in the case of a server when tasks are not normally CPU bound, but are file I/O bound.

Are all SCSI interfaces created equal? Of course not! As time marches on, improvements have been made to the original SCSI design. The terminology that has been used to define the improvements does not exactly represent the changes. There are terms like Fast SCSI, Fast Wide SCSI, Ultra SCSI, Wide Ultra SCSI, Ultra2 SCSI and Wide Ultra2 SCSI. From these titles can you tell what is the best? My guess is probably not.

Should you know the difference? My guess is yes, if you are planning to acquire new equipment or update old equipment. So we have finally arrived at the purpose of this article. That is to define the differences and to provide you with the knowledge that will help you understand.


Bus Width and Speed

The definitions below are listed in order from the slowest to the fastest type. In simple terms, there are two measurements to consider: Bus Width and Bus Speed. One other consideration is the number of SCSI devices that can be configured in one machine.

  • Bus width is measured in bits and is either 8 or 16 bits. Easiest way to understand this is to relate to lanes on a highway. A 16 lane highway will allow for more traffic than an 8 lane highway. Of course the 16 lane highway will cost more.
  • Bus speed is measured in megabytes (MB). The rating represents how many megabytes can be accessed in a second. A higher rating represents faster access.

SCSI Chart
Type Alternate Name Bus Width (bits) Bus Speed
(MB's/sec)
Max.
Devices
Bus Length
(Meters)
Single HVD LVD
SCSI-1 Narrow SCSI 8 5 8 6 25
Fast SCSI Narrow Fast SCSI 8 10 8 3 25
Ultra SCSI Narrow Ultra SCSI 8 20 8 1.5 25
Ultra2 SCSI Narrow Ultra2 SCSI 8 40 8 25 12
Fast Wide SCSI 16 20 16 3 25
Wide Ultra SCSI 16 40 16 1.5 25
Wide Ultra2 SCSI 16 80 16 25 12
Ultra3 SCSI Ultra160 SCSI 16 160 16 12
Ultra320 SCSI 16 320 16 12

Other SCSI Benefits

Besides the speed issues, another important feature of SCSI interfaces is that backwards compatibility has been incorporated in all new improvements. A rare situation for computer technology.

Thus, if you have older SCSI devices, they will still work as you upgrade to newer technology. Different SCSI technologies can be mixed on the same machine. Even though a new SCSI method may be used, older SCSI devices will not run any faster and newer SCSI devices will only run as fast as the SCSI controller allows.

SCSI devices provide the capability of interfacing with a wide variety of devices. Supported peripherals are tape drives, optical drives, hard disk drives, scanners, printers, disk array subsystems (RAID) and CD-ROM drives.


>What is LVD
  • The terms LVD and Ultra2 SCSI are used interchangeably.
  • It is a subset of the SCSI-3 standard.
  • It provides SCSI bus data rates of 80 Mbytes/sec.
  • It provides differential data integrity
  • It extends the SCSI bus cable lengths to 25 meters (12 meters with 16 devices)
  • LVD was defined in the original SCSI standards.

The increased bandwidth of 80 Mbytes means optimal performance where rapid response is required and random access and large queues are the norm. When using applications such as CAD and CAM, digital video and any RAID environment, the increased bandwidth is immediately noticeable as information is moved quickly and effortlessly.

The lower voltage requirements of LVD allow for the integration of the differential drivers and receivers into the drive's onboard SCSI controller. The older Ultra HVD design requires separate and costly high-voltage components.

LVD is fully compatible with the existing single-ended SCSI base. A unique circuit determines the type of SCSI bus the device is being used on, LVD or single-ended, and configures the drive operation to the appropriate bus capability.

LVD devices will work on SCSI-1and SCSI-2 bus segments. Older SCSI-1 and SCSI-2 single-ended devices will work on an LVD bus.

When using SCSI devices of different vintage on the same SCSI bus, all peripherals on that bus will respond to the earliest version SCSI specification.


Connector Guide
Sample Type Pins Used By Comments
IDC50-M 50 Narrow: SCSI-1 & 2, Ultra SCSI

Internal

8 bit

IDC50-F
HD68-M 68 Ultra2 LVD and Ultra Wide SCSI3

Internal/External.

About 1 7/8" wide

HD68-F
CN50-M 50 SCSI 1 & 2

External.

Also called Centronics C50

CN50-F
HD50-M 50 SCSI 2 & 3

External

About 1 3/8" wide

HD50-F
DB25-M 25 SCSI-1

External

Used by older Macs, Zip drives and scanners

DB25-F
HDI30-M 30

External

Apple PowerBooks

DB50-M 50 SCSI-1 Used on old Sun Sparcstations
DB50-F
DB37-M 37 SCSI-1
DB37-F
VHDCI-M 68 Ultra SCSI 2 & 3

Popular on RAID cards.

0.8mm

VHDCI-F
HPCN50 50 Used in Japan on Digital cameras
HDCN60 60 Used on old IBM rs6000's

Cable Guide
SCSI Type Code Pins Comments
SCSI -1 A 50 External - Centronics C50
ble
SCSI-2 A 50 External - High Density D50M
Internal
SCSI-3 P 68 External - High Density D68M
Internal - High Density
External - VHDCI connector
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