SCSI

SCSI (pronounced SKUH-zee and sometimes colloquially known as "scuzzy"), the Small Computer System Interface, is a set of American National Standards Institute (ANSI) standard electronic interfaces that allow personal computers (PCs) to communicate with peripheral hardware such as disk drives, tape drives, CD-ROM drives, printers and scannersfaster and more flexibly than previous parallel data transfer interfaces. SCSI standards are generally backward-compatible. That is, if an older peripheral device is attached to a newer computer with support for a later standard, the older device will work at the older and slower data rate. In personal computing, SCSI interfaces have been replaced, for the most part, by Universal Serial Bus (USB). In the enterprise, SCSI is still used in server farmsfor hard drive controllers.

There are several components used in SCSI storage systems:

• Initiator. An initiator issues requests for service by the SCSI device and receives responses. Initiators come in a variety of forms and may be integrated into a server’s system board or exist within a host bus adapter. ISCSIconnectivity typically uses a software-based initiator.
• Target. A SCSI target is typically a physical storage device (although software-based SCSI targets also exist). The target can be a hard disk or an entire storage array. It is also possible for non-storage hardware to function as a SCSI target. Although rare today, it was once common for optical scanners to be attached to computers through the SCSI bus and to act as SCSI targets.
• Service delivery subsystem. The mechanism that allows communication to occur between the initiator and the target; it usually takes the form of cabling.
• Expander. Only used with serial-attached SCSI (SAS); allows multiple SAS devices to share a single initiator port.

PCI

PCI (Peripheral Component Interconnect) is an interconnection system between a microprocessor and attached devices in which expansion slots are spaced closely for high speed operation. Using PCI, a computer can support both new PCI cards while continuing to support Industry Standard Architecture (ISA) expansion cards, an older standard. Designed by Intel, the original PCI was similar to the VESA Local Bus. However, PCI 2.0 is no longer a local bus and is designed to be independent of microprocessor design. PCI is designed to be synchronized with the clock speed of the microprocessor. PCI is now installed on most new desktop computers, not only those based on Intel's Pentium processor but also those based on the PowerPC. PCI transmits 32 bits at a time in a 124-pin connection (the extra pins are for power supply and grounding) and 64 bits in a 188-pin connection in an expanded implementation. PCI uses all active paths to transmit both address and data signals, sending the address on one clock cycle and data on the next. A PCI bus lets you change different peripherals that are attached to the computer system, so it allows the use of different sound cards and hard drives. 

APPLICATION

Usually, there are three or four PCI slots on a motherboard. With PCI, you can unplug the component you want to swap and plug in the new one in the PCI slot. Or, if you have an open slot, you can add another peripheral like a second hard drive to dual boot your computer, or a special sound card if you deal with music a lot.

Computers might have more than one type of bus handling different traffic types. The PCI bus used to come in both 32-bit and 64-bit versions. PCI runs at 33 MHz or 66 MHz.