Bus expanders are computer boards or cable systems that connect to other interface boards, using only one I/O slot while remaining compliant with the bus standard. Bus expanders allow increased bus loads across all buses while maintaining the integrity of legacy firmware and software. They may also be used to extend the maximum distance between devices. The goal with any network is to create a system that does not require any additional configuration due to the expander. There are many designs for bus expanders that support this type of performance. Additionally, bus expanders should not affect the speed of systems on the expander’s side of the system, or reduce its transfer rate. However, propagation delays are possible through expanders when data must be transferred between the systems.
When a bus expander connects two or more networks, signal transfer experiences speed degradation (time delay) through the expander when traveling from one side to the other. This occurs because the network is being asked to function with more peripherals and at a higher system drain than originally designed. While this type of performance delay is a normal and expected function of the enlarged system, higher-grade expanders may help to minimize the delay.
When selecting among available types of bus expanders, there are three main criteria to consider: the bus protocol required, the form factor of the bus expander, and the number of ports or slots. Common bus protocols include IDE, EIDE, GPIB, HPIB, IEEE 1394, ISA, EISA, PCI, PCI-X, PCI Express, cPCI, PCMCIA, SCSI, SAS, VME, VSB, and VXI. Form factor is the style of housing for bus expanders.
The form factor determines the amount and type of supporting technology necessary for the bus to interface with the network system. Bus expanders are available in four main form factors: cable, where the bus is either a simple cable that connects one bus to another, or is a device that connects two cables; board level unit, where the expander is a card or blade added to a chassis, which allows the system to function; rack mount, where the expander is a stand-alone unit that must be mounted into a telecomm rack; or stand-alone, where the unit does not need to be mounted and comes equipped with interfaces and an independent power supply.
When referring to bus expanders, the number of ports or slots refers to the expandability of the device. This is also refereed to as the number of terminating connections, or the range of acceptable ports or slots that can be added to the system when bus expanders are used.
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Bus adapters connect peripherals to computers that do not provide native support for a peripheral's interface. Some bus adapters are housed in cables that connect one bus to another. Others are board-level cards, blades, or stand-alone units with interfaces and power supplies.
Bus extenders are used to increase cable lengths for distance-limited bus protocols.