How works SCSI?

A computer is full of buses, that are simply freeways within the equipment which they take the information from a site to another one. For example, when you connect a reproducer MP3 or a digital camera to a computer, probably you are using a port USB. Port USB is good taking to the electricity and the data required for small electronicses that make things like creating and storing to photos or archives of music. But that bus is not sufficiently great to support to everything a computer, a servant or many devices simultaneously.
For this reason, you need something more powerful like SCSI. Originally, SCSI was “Small Computer System Interface” that means something like system of interface for small computers, although has surpassed the term of “small”. It is a fast bus that can connect many devices in a computer at the same time, including hard disks, scanners, printers, etc. Other technologies like SATA, have replaced SCSI in new systems, although it is continued using habitually.

Basic principles of SCSI
Technology SCSI is based on a previous system of called bus SASI (Shugart Associates System Interface), which was developed originally in 1981. In 1986 the national institute of American standards (ANSI), bring to light SCSI, which was a modified version of SASI. SCSI uses a controller to send and to receive data and power supply.
SCSI has several advantages. He is quite fast, reaching 320 Mbps. It has been time in the market already and has been verified and tried carefully, reason why it has reputation of being trustworthy. Like the ports TIE and Firewire, allows to put many objects in a single bus. SCSI works with the majority of the operating systems.
Types of SCSI
There are three basic specifications for SCSI:

• SCSI-1 - The original specification of this model developed in 1986, is already obsolete. It had a bus of 8 bits and one speed of 5 clock of MHz.
  
• SCSI-2 - Developed in 1994, this specification included CCS (Common Command Set), that is 18 considered commandos an absolute necessity to support a device SCSI. It also had the possibility of doubling to the speed of clock and the one of the bus, and of increasing the number of devices up to 15 units. It could store and also prioritize commandos of the computer.
  
• SCSI-3 - It is of 1995 and it included a series of smaller standards in a general scope. These standards where is parallel interface SCSI, are the form in which devices SCSI communicate to each other.
  

Cables, devices and controller
A controller SCSI coordinates between all the other devices in bus SCSI and the computer. Also call adapter of host, the controller can be a card that you connect to a groove available or another method. The BIOS of the SCSI is also a controller. This is a small flash memory or ROM that contains software necessary to accede and to control the devices of the bus.
Each device SCSI must have a unique identifier so that it can work correctly. For example, if the bus can support 16 devices, their IDs must have a rank from 0 to 15. Own controller SCSI must use of these IDs, normally highest, leaving space for other 15 devices in the bus.
Internal devices are connected to a controller SCSI with a tape cable. External devices SCSI are connected to the controller by means of a heavy round cable, and in chain. With this method, each device connects in line with the following one. Therefore, the external devices normally have two connectors SCSI - one to connect to the previous device in the chain, and the other to be able to connect the following device.
The own cable normally wears three layers:

• The internal layer - protecting Is the layer more and contains the data that are sent.
• The average layer - It contains the threads that the commandos of control send to the device.
• The external layer - It includes threads with transport parity information, which assures that the data are correct.

iferentes variations SCSI use different connectors, which often are incompatible to each other. These connectors normally use 50, 68 or 80 pins. Once all the devices are installed in the own bus and have their IDs, each end or completion of the bus must be closed. If bus SCSI to be left open, the electrical signals sent by the bus could create returns and interfere with the communication between devices and controller SCSI. The solution is to put a “completion” in the bus, closing each one of the ends with a resistance. If the bus supports as much internal devices as external, then the last device in each series must be finished.