SCSI was used mainly to connect on / off line storage such as disks / tape.  Advantages were the greater data rate of SCSI systems over contemporaneous IDE systems, and that more devices could be attached.  Disadvantages were expense and the following complexities:

SCSI termination and ID should be set before mounting a device in a PC; to do so afterwards can be impossible.

SCSI Standards

The following are the more commonly used standards:

SCSI Standard Bus
Max #
Data Rate
Max Cable (m) Cables & Connectors
(internal & external)
(internal & external)
I 8-bit 7 5 6 Flat ribbons / IDC 50-pins
(like IDE cables, only wider)

Round shielded ending in
Centronics C50s
On end device:
   Initially resistor arrays
   Later jumper settings

Centronics C50 block
II 8-bit 7 5 6
II Fast 8-bit 7 10 3
III Ultra 8-bit 7 20 1.5 - 3
II Fast Wide 16-bit 15 20 3 Flat ribbons
Round shielded ending in ...
68-pin D-connectors
Jumpers on end device
SE block on end connector

SE 68-pin D block
III Ultra Wide 16-bit 15 40 1.5 - 3
III Ultra2 Wide LVD 16-bit 15 80 12 Interleaved, laminated, or
twisted-pair ribbons / round

Round shielded ending in ...
68-pin D-connectors
LVD termination usually
built in to cable end

LVD 68-pin D block
III Ultra160 LVD 16-bit 15 160 12
III Ultra320 LVD 16-bit 15 320 12

All non Low / High Voltage Differential (LVD / HVD) SCSI standards are collectively termed Single Ended (SE).

Mixing devices of different SCSI standards on the same bus is not recommended.  Generally, it results in the whole bus reverting to the performance of the slowest device, particularly:

SCSI Termination

A SCSI cable is part of a 'bus' system, each end of which must be 'terminated' to avoid data pulses being reflected back and corrupting data flow.  This is usually achieved by one of:

Note that it's easy to overlook that altering a working system may alter its termination requirements as a side effect.

The Controller

The controller will normally be just at one end of an internal cable, and therefore at one end of the main bus, and therefore will need to apply termination to it.

In this context, the 'main' bus is the one on the primary connector:

However, where there is an external connector on the backplane of the controller card, this usually continues the main bus out of the controller.  In this situation, where there are external devices so connected, the controller will now be in the middle of the bus, and therefore should not apply termination, while the external cable will have to be terminated in the normal way.

The controller may also be at one end of an internal cable on another connector.  On some controllers, this might be an entirely separate secondary bus needing normal termination, but with Adaptec's AHA-2940UW we have also seen that this controller can be at the end of the high 8-bits of the main bus, and so should terminate these, while at the same time being in the middle of the low 8-bits and the control bits, and so should not terminate those.

Fortunately, the factory default of 'automatic termination' will allow most modern controller cards to sense all the above situations and apply appropriate termination.  You can usually check the setting during boot up by pressing a keystroke as the card boot message appears (for example <Ctrl-A> for Adaptec controllers), which takes you into the card's configuration menu.  It can be informative to check this, but it's not usually recommended to change factory defaults without good reason.  Before doing so, check the card's manual &/or manufacturer's website for advice.

The Far Cable Ends

From the table above, the appropriate method of termination for the bus standard should be applied, either by connecting a termination block to the connector farthermost from the controller, or by enabling termination on the device farthermost from the controller  if required, a search of the device's manual or of the manufacturer's website should reveal how to enable termination on it.

Where, as with wide SCSI II, there is a choice in method, and there is a termination block available, it is probably easiest to use that, as then you don't have remember to enable/disable termination on devices every time you make changes to your hardware.

Intermediate Connectors & Devices

There should be no termination on any intermediate connector or device.

SCSI ID Numbers

Every device on any SCSI bus must have a unique ID, including the controller, usually 7  this default may be alterable in the configuration menu, although it's generally not recommended.

Other devices can be 0-6 on 8-bit buses and 0-6,8-15 on 16-bit buses, usually set by jumpers.  A device's manual or the manufacturer's website should explain how to set its ID, but usually absence/presence of a jumper indicates a binary 0/1, a common system being (but note there can be 2 interacting banks of jumpers, the other being elsewhere on the device) ...

                     SCSI ID
    SCSI Data        8 4 2 1   Power
  \ :: ... :: /  : : : : : :  O O O O

... with the following ID settings ...

     8 4 2 1       8 4 2 1       8 4 2 1       8 4 2 1
  0  : : : :    4  : | : :    8  | : : :   12  | | : :
  1  : : : |    5  : | : |    9  | : : |   13  | | : |
  2  : : | :    6  : | | :   10  | : | :   14  | | | :
  3  : : | |    7  : | | |   11  | : | |   15  | | | |

Rarely, if both controller and device support SCSI Configured Automatically (SCAM), its ID need not be configured.

Where ≤ 7 SCSI HDs are used in different PCs in a limited environment, a home or small office, it can make sense to give them all unique IDs, so that if necessary each can be mounted in any PC without need to reassign its ID.

SCSI Boot Devices

PC Boot Process details booting from HD.  A dedicated BIOS on the SCSI controller may be required to boot via it, and may also provide utilities to low-level format and verify SCSI HDs.

Depending on the make of motherboard and system BIOS, SCSI disks may be included under a catchall such as 'Hard Disk' and a further option chooses the order in which to try them, or a SCSI controller may be explicitly designated in the BIOS as a boot device.

In the former case, the controller will most probably try to boot only the designated device.

In the latter case, the controller will try to boot the device with SCSI ID 0, though this default can often be changed in the configuration menu.  If it fails to boot from the designated device, it may then try the others in ascending ID order until it succeeds or totally fails.

If the controller fails to provide a bootable partition, then booting will usually revert to the system BIOS, which will then attempt to boot via whichever device it is configured to try next.

The first bootable HD found becomes the primary HD (0, 1, or 0x80, depending on the counting system).  Usually other SCSI disks will be added successively in ascending SCSI ID order.

Hard Disk LED

The front panel HD LED can be connected to some controller cards, but doing so will result in only SCSI HD activity being monitored, whereas the LED should also monitor non-SCSI HD / CD / DVD activity.  This fix requires a couple of suitable diodes and a soldering iron:

  From SCSI LED Connector +ve: ---|>|--
  From Mobo LED Connector +ve: ---|>|--
]- To HD LED +ve
  From Mobo LED Connector -ve: ----------- To HD LED -ve