|Terms/Names and maximum Data Transfer Rate|
|Fast-Wide SCSI||SCSI-2 /
|Ultra2 SCSI (LDV)||(SCSI-4)||40||8||40||320|
|Ultra2-Wide SCSI (LDV)||(SCSI-4)||40||16||80||640|
|Ultra3-Wide SCSI (LDV)||(SCSI-4)||80||16||160||1280|
|Fast-Wide SCSI is actually SCSI-2,
however many cable manufacturers refer to the 68-pin plugs as SCSI-3. This is by
definition of the STA incorrect!
SCSI-4 definition is not yet final
Most SCSI 1-4 Devices can be mixed on the same bus, however, the slowest device/controller determines the bus speed!
You can not mix single ended and differential or LVD devices/controllers on the same SCSI bus! Please note, that Fast-Wide SCSI-2 is also sometimes called SCSI-3 (in particular when referring to the 68 HP connector).
You can also
mix Wide and Narrow, Fast-SCSI2 and Ultra SCSI Compaq Pluggable drives.
|Different types of common connectors|
|Name||Common Name||Number of Pins||Application||Bus Width||SCSI Standards||Comment|
|D-Sub 25||Apple||25||External||8 narrow||SCSI 1+2||Primarily for Microtest Discport and Apple Computers|
|50 Centronics||SCSI-1||50||External||8 narrow||SCSI 1+2+3||Most common for Scanners, Tapes and external CD-ROM's and other slower periphereals|
|50 HP||SCSI-2||50||External||8 narrow||SCSI 2+3||External Raid-Disks and high performance Tape Libraries|
|68 HP||SCSI-3||68||External||16 wide||SCSI 2+3||External Raid-Disks and high performance Tape Libraries|
|50 pin Ribbon||normal||50||Internal||8 narrow||SCSI 1+2+3||Standard internal Narrow SCSI Disks, CD-ROM's, Tapes and optical disks|
|68 pin Ribbon||wide||68||Internal||16 wide||SCSI 2+3||Wide SCSI Disks and other high performance drives|
|The words "narrow" stand for 8 bits
and "wide" is 16 bit. The majority of the millions of SCSI devices in use today
use single-ended transceivers (Asymmetrical, ground referenced). Differential (symmetrical
+-) devices, which provide for greater bus lengths, are typically used in applications
which can properly bear the added expense.
A new transceiver technology, Low-Voltage Differential (LVD), will combine the best features of single-ended and high-powered differential transceivers. LVD will also enable higher speeds. To ease the migration to LVD, most new SCSI devices will support Universal transceivers which include both single-ended and LVD transceivers.
|The 2 most important Consideration on a SCSI
You must have a terminator (preferably active) on either end of the bus. They should be on the very extremes of the bus, and not somewhare in the middle or near the end. Most intermittend problems are due to loose connectors, improper termination or inferior cable quality. Please refer to the picture for some sample configurations.
|It basically does not matter what ID you assign to each device, as long as they are unique. It is good practice to number them in sequence they are attached on the bus.|
|Disk C:||0||0 0 0|
|Disk D:||1||0 0 1|
|Disk E:||2||0 1 0|
|Disk F:||3||0 1 1|
|Tape Drive||4||1 0 0|
|CD-ROM||5||1 0 1|
|Disk H:||6||1 1 0|
|Host Adapter (SCSI Controller)||7||1 1 1|
|The normal setup for the first controller
(programmed mode) is: IO x330, IRQ 11 and DMA 5
Most devices support parity and are using synchronous transfer mode (rather than async, where every byte is acknowledged).
Some devices support LUN (logical Unit Numbering). With this technique you can assign upto 8 devices per SCSI ID. Thus the total possible devices comes to 56 per normal SCSI Bus. This usually requires A CONTROLLER in the cabinet where the CD-ROM's are mounted. Not all SCSI Controllers are LUN capable, however most are. Also the Software must be able to use LUN adressing. The most common use for LUN is a small SCSI CD-Changer (like the Pioneer DRM-604).
|Maximum Bus Length|
|Fast Wide SCSI||3||25||12|
|Ultra SCSI||1.5-3||Up to 25||Up to 12|
|Wide Ultra SCSI||Up to 3||Up to 25||Up to 12|
|Wide Ultra2 SCSI||(1)||(1)||12|
|Max Number of devices per external bus|
|in function of bus speed, with, length and symmetry/voltage|
|Fast Wide SCSI||20||16||3||25||12||16|
|Wide Ultra SCSI||40||16||-||25||12||16|
|Wide Ultra SCSI||40||16||1.5||-||-||8|
|Wide Ultra SCSI||40||16||3||-||-||4|
|Wide Ultra2 SCSI||80||16||(1)||(1)||12||16|
|Obsolete SCSI Terms|
|STA Term||Previously Known As|
|Fast SCSI||SCSI-2 (X3.131-1994)|
|Ultra SCSI||Fast 20, SCSI-3, Double-speed (SCSI)|
|Ultra2 SCSI||Fast 40, SCSI-3|