The Layout of the SCSI System
A typical SCSI bus contains
Controller or host adapter
SCSI cable
SCSI device.
The primary controller for the SCSI bus is the host adapter. The controller is usually embedded on the motherboard or it is built on the SCSI device itself, which are known as Embedded SCSI Devices. At times the controller needs to be bought separately as an add-on expansion card. SCSI BIOS sits on the controller and controls the low-level operations of the bus.
The SCSI devices are connected to the controller through a SCSI cable and the arrangement is made in a chain-like progression wherein each device acts like a link in the chain and passes the SCSI signal on to any other device further down the chain. It should be remembered that each end of the chain must be terminated. Failure to do so would result in the SCSI signal reflecting back down the cable, thereby causing electrical noise that would interfere with the transfer of data.There are two types of terminating devices:
Passive -This terminator has a bunch of resistors to keep track of correct terminating voltage. This type of termination is used for basic SCSI setup that operates at a clock speed of 5 MHZ.
Active - This terminator has a voltage regulator that keeps track of the correct termination voltage. Most of them have a LED displaying the termination level. This is commonly used on Fast SCSI systems or systems with more than 3 feet between the controller and the device.
There is a variety of different cable types used that depends on the type of SCSI being used. They are:
DB-25 (SCSI-1).
50-pin internal ribbon (SCSI-1, SCSI-2, SCSI-3).
50-pin Alternative 2 Centronics (SCSI-1).
50-pin Alternative 1 high density (SCSI-2).
68-pin B-cable high density (SCSI-2).
68-pin Alternative 3 (SCSI-3).
80-pin Alternative 4 (SCSI-2, SCSI-3).
