SCSI was designed to be a highspeed bus that could be used to connect multiple devices to a single connection. SCSI uses an integrated processor which directs and controls the multiple devices, allowing them to effectively use the same resources. Devices are connected to a SCSI "chain" with a special cable and communicate with a common protocol called ASPI. SCSI is a high speed communication and it comes in many versions, ranging from 5MB/sec to 320MB/sec bandwidth.

All types of SCSI run on the same logical bus, and can interface with the same SCSI equipment. SCSI isn't limited to hard drives or storage, SCSI can be used to interface with almost any other type of equipment or computers because of the advanced processors that are used in the controllers.

Bus	Name	8-bit (50 wire, Narrow)	16-bit (68 wire, Wide)
5MHz	SCSI 1	5MB/s	(N/A)
10MHz	Fast SCSI, SCSI 2	10MB/s	20MB/s
20MHz	Fast-20, Ultra SCSI	20MB/s	40MB/s
40MHz	Fast-40, Ultra-2 SCSI	40MB/s	80MB/s
80MHz	Ultra-3 SCSI, Ultra160	(N/A)	160MB/s

SCSI started with a narrow 8-bit bus using a 50 pin connector. This connection could only transfer 1 bit at a time. SCSI later progressed into a wide 16-bit bus which could transfer 2 bits at a time. The 16-bit bus uses either a 68 pin or an 80 pin cable. The 80 pin cable is used to allow the use of hot swappable devices.

SCSI does not use a parallel interface because that will limit it's speed. Earlier versions did, but were later converted to serial to allow for advancements. All SCSI formats are able to transfer data in parallel, but a frequency decrease must to take place. This is because with higher bus frequencies, signal delays can become a major concern. Each wire on the chain passes the signal at slightly different speeds. This can cause the signals to overlap and become unsynchronized. Earlier versions of SCSI were capable of transferring a full 8-bits for narrow devices and 16-bits for wide devices. This was later switched to a serial signal of one ore two bits at a time while the bus frequency increased.

SCSI devices are fully multithreaded, each device is capable of accessing the bus and for filling the requests whenever the bus is being unused. This means that when another device like a hard drive is accessing information, another device like a CD-ROM could be transferring data through the bus. This is called multithreading. Newer SCSI controllers are build with fast controllers and large buffers over 32MB that allow the SCSI controller to make efficient use of the bus. SCSI is also able to use peer-to-peer connections, meaning that devices are able to communicate with each other independent of the SCSI controller.

A new technology much like SCSI that is used in some high end servers. It is capable of speeds of 800 to 1600Mbps, and distances from 30m using copper wires or up to 10km using fibre optical cable.

SCSI & Fibre Channel | IDE | USB, Parallel, Serial, Floppy, & FireWire