SCSI Guide
Terms/Names and maximum Data Transfer Rate
Terms Name MHz Bus Width MB/s MBit/s
SCSI-1 SCSI-1 5 8 5 40
Fast SCSI SCSI-2 10 8 10 80
Fast-Wide SCSI SCSI-2 /
10 16 20 160
Ultra SCSI SCSI-3 20 8 20 160
Ultra-Wide SCSI SCSI-3 20 16 40 320
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.
However, You can not plug in a Wide ScSI Drive into an old Narrow SCSI Server, since the connectors are too wide.

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.

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The 2 most important Consideration on a SCSI Bus are:
  • Properly Terminate the Bus
  • Make sure every device has a unique SCSI ID

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.

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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.
Normal assignation:
Device SCSI ID Binary
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
STA Terms


SCSI-1 6 25 12
Fast SCSI 3 25 12
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
Ultra2 SCSI (1) (1) 12
Wide Ultra2 SCSI (1) (1) 12
(1) Single-ended and high-powered differential are not defined at Ultra-2 speeds.
Max Number of devices per external bus
in function of bus speed, with, length and symmetry/voltage
STA Terms Max Bus Speed
Bus Width
Max Bus Length
Single Ended
Max Bus Length
Max Bus Length
Max. Device
SCSI-1 5 8 6 25 12 8
Fast SCSI 10 8 3 25 12 8
Fast Wide SCSI 20 16 3 25 12 16
Ultra SCSI 20 8 1.5 25 12 8
Ultra SCSI 20 8 3 25 12 4
Wide Ultra SCSI 40 16 - 25 12 16
Wide Ultra SCSI 40 16 1.5 - - 8
Wide Ultra SCSI 40 16 3 - - 4
Ultra2 SCSI 40 8 (1) (1) 12 8
Wide Ultra2 SCSI 80 16 (1) (1) 12 16
(1) Single-ended and high-powered differential are not defined at Ultra-2 speeds.
Obsolete SCSI Terms
STA Term Previously Known As
SCSI-1 SCSI-1 (X3.131-1986)
Fast SCSI SCSI-2 (X3.131-1994)
Ultra SCSI Fast 20, SCSI-3, Double-speed (SCSI)
Ultra2 SCSI Fast 40, SCSI-3