Bus extenders are used to increase cable lengths for distance-limited bus protocols. They are similar to extension cords. The bus extender form factor determines the amount and type of supporting technology that the bus requires.

Specifications

There are four main form factors for bus extenders:

• Cable: the bus is either a simple cable that connects one bus to another, or is a device that connects two cables
• Board level unit: the bus extender is a card or blade added to a chassis, which allows the system to function
• Rack mount: the bus extender is a stand-alone unit that must be mounted into a telecom rack
• Stand-alone: the unit does not need to be mounted and comes equipped with interfaces and an independent power supply

Other unlisted, unspecified, or proprietary form factors for bus extenders are also available.

Selecting bus extenders requires an understanding of bus protocols and standard conventions for transmitting data and addresses. Bus extenders use many different bus protocols. Examples include IDE and EIDE, GPIB or HPIB, IEEE 1394 or FireWire^® (Apple Computer, Inc.), InfiniBand (IB) ISA and EISA, PCI, PCI-X, and Compact PCI (cPCI, PXI.) Integrated drive electronics (IDE) is a standard electronic interface used between a computer motherboard's data paths or bus and the computer's disk storage devices. The IDE interface is based on the IBM PC Industry Standard Architecture (ISA) 16-bit bus standard, but it is also used in computers that use other bus standards. General purpose interface bus (GPIB) is designed to connect computers, peripherals, and laboratory instruments so that data and control information can pass between them. It is also known as IEEE 488, or HPIB, and is electrically equivalent to IEC 625 bus. IEEE 1394, a companion to the universal serial bus (USB), provides high-speed serial connections. InfiniBand (IB) architecture is an interconnect technology that creates a centralized I/O fabric allowing for greater server performance, design density, reliability, and performance scalability. Peripheral component interconnect (PCI) is a local bus system designed for high-end computer systems. It transfers 32 or 64 bits of data at a clock speed of 33 MHz. Common PCI variants include PCI Express, PCI extended (PCI-X), and CompactPCI (cPCI, PXI).  

Features

Bus extenders also use bus protocols such as SCSI, SAS, VME, VSB, and VXI. Small computer systems interface (SCSI) is an intelligent I/O parallel peripheral bus with a standard, device-independent protocol that allows many peripheral devices to be connected to the SCSI port. A single SCSI bus can drive up to eight devices or units: the host adapter or controller, and seven other devices. Each device is assigned a different SCSI ID, ranging from 0 to 7. SCSI formats include SCSI-1, SCSI-2, Wide SCSI, Fast SCSI, Fast Wide SCSI, Ultra SCSI, SCSI-3, and Ultra2 SCSI or Wide Ultra2 SCSI. VersaModule Eurocard (VME) is a 32-bit bus used in industrial, commercial, and military applications. VME64 is an expanded version that provides 64-bit data transfer and addressing. VME Subsystem Bus (VSB) is an auxiliary bus used with a primary 32-bit bus called a VersaModule Eurocard (VME), made for commercial, industrial, and military uses. VME extensions for instrumentation (VXI) is a standard for many electronic platforms that defines electrical and mechanical backplane characteristics. Other unlisted, specialized, or proprietary bus protocols for bus extenders are also available. 

Image credit: 

National Instruments