SCSI is a set of standards for physically connecting and transferringdata betweencomputers andperipheral devices, such as ahard drive orCD orDVD drive. SCSI stands forSmall Computer System Interface, although the "small" is historic; SCSI is used on both large and small computers. SCSI is usually pronounced as "scuzzy".
Few of these peripherals (other than hard drives and tape drives) were made after the 20th century. Later ones usedUSB or in some casesFirewire.
SCSI goes from abus slot on themotherboard. There is a card called host adapter on themotherboard. From this card is a manageddata cable. This cable connects individual SCSI peripherals. Each peripheral has its ownsequencer which is functional independent. Operation of the data cable is controlled by the host adapter. This makes it possible to installinternal orexternal components. TheCPU is not loaded by transferring data.
Each peripheral has its own sequencer and host adapter. They can be used for communication with devices by a defined "computer" language. They could also cooperate with other peripherals of SCSI.Hard disks,optical drives, tape adapter units,ZIP drives,scanners andprinters used SCSI.
SCSI is not restricted by conditions (for example: bydata capacity). One of the main advantages of SCSI is a possibility todaisy-chain devices. There is a device on the bus SCSI which controls this.
For example, hard disk defines “head's of operations” etc. At the same moment the other device could send data to other devices, (scanner). Operations of other device are not cancelled with origin connection. After transferringdata toscanner device could continues with transferring data to hard disk. SCSI is faster because of this fact even maximal speed of transferring is not so different toEIDE.
Each peripheral of SCSI has to beidentified. That is the reason why every peripheral must have a uniqueID. This number is set by a bond or shunt on the case of components. There cannot be two devices on the same bus with the same ID.
Buses have to be finished withresistances, calledterminators. On the last devices it is necessary to install terminators.
The host adapter has to be in connection to assign system resources - addressesI/O,IRQ,DMA, reservedmemory.
SCSI has existed for a long time. That is why there are many kind of SCSI. They are different indata width (8 or 16 bits) and working frequency (from 5 MHz to 80 MHz). With 8-bit data width, the parallel SCSI bus can be connected to 8 devices (used a 50 pin connector); with 16 bits, the SCSI bus can be connected to 16 devices.
In addition to the parallel SCSI interface, there are also other interfaces that use the SCSI protocol but use different cables.