Technical The Internal Components SCSI Interface

Small Computer System Interface

The SCSI (Small Computer System Interface), SCSI-2, SCSI-Wide, SCSI Ultra and so on define a bus that can connect one or more host computers with a variety of peripheral devices. The SCSI specification defines a logical command set for each generic peripheral type as well as the physical and electrical characteristics of the bus.

Each device connected to the SCSI bus must have an onboard controller. Any device can communicate directly with any other device over the SCSI bus. The bus therefor, must allows for arbitration, temporary disconnection and subsequent reconnection. By disconnecting peripherals from the bus during long delays, such as disk seeks, SCSI allows multithreaded operations, therefore there can be more than one input/output at the same time, including between the same two devices. These operations can also be queued in the local controller and executed in the proper order.

A SCSI I/O subsystem can better handle performance for example as a multi-user, multitasking system such as Linux. The SCSI bus can also be a single connection for a variety of peripherals, such as hard disk drives, CD-ROM drives and high speed tape backup drives. SCSI is also host independent, so it is possible to easily move SCSI devices from say an Apple Mac to a PC.

SCSI Standards

The ANSI Committee that developed the SCSI-2 specification ensured backward compatibility with the original SCSI specification. With SCSI-2 the host and peripherals still perform all message, command and status phases in the 8 bit asynchronous mode. 16 and 32 bit wide fast synchronous transfers are an option and is not implemented unless specified. SCSI-3 systems double the number of signal lines in the cable and the signal can be carried at a much higher speed. Then there is SCSI Ultra Wide. Like the IDE specification, the SCSI standard is still evolving.

SCSI Compared to IDE

Most PC systems use an enhanced IDE interface or ATA-2, which costs less but provides a similar performance to 16 bit fast SCSI-2 systems. EIDE supports faster data transfer rates than before and up to four bus mastering devices. However in a high performance networked files server, the comparison ends.
SCSI is not supported by DOS on the PC, so a device driver must be installed. There are however, motherboards that support some SCSI functionality such as booting from SCSI hard disks or a SCSI CD-ROM drive.

A SCSI port can daisy chain peripherals from a single card, supporting up to seven devices per port and the interface card itself. If you needed to install separate cards for each CD-ROM writer, scanner and so on, as well as hard disk drives, CD-ROM reader, scanner and fast tape backup, you can run out of bus space really quickly and possibly start experiencing conflicts with interrupts and other bus signals.
SCSI reduces the load on the processor and can actually do all the work internally only taking time from the main processor to notify when the job is done. Simultaneous hard disk and processor activity, such as playing demanding games or decompressing/compressing the latest videos offers greater performance on a SCSI system, not to mention your operating system speed.

Optical Technology

EIDE Interface