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| Net World Directory: Computer hardware-Bus |
In computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers. A computer bus is a method of transmitting data from one part of the computer to another part of the computer.
One of the first complications was the use of interrupts. A key distinguishing characteristic between various host adapter models is the type of system bus the card is designed for. The term is almost certainly derived from the electrical engineering term "bus bar" - a substantial, rigid power supply conductor to which several connections are made.
Unlike a point-to-point connection, a bus can logically connect several peripherals over the same set of wires. The computer bus will connect all devices to the computer CPU and main memory. Early computers performed I/O by waiting in a loop for the peripheral to become ready.
SCSI host adapters have been made for all of the common PC I/O buses, including ISA, EISA, VLB, MCA and PCI. Each bus defines its set of connectors to physically plug devices, cards or cables together. The computer bus consists of two parts, the address bus and a data bus. This was a waste of time for programs that had other tasks to do.
You obviously need to choose a host adapter that matches the system bus(es) in your machine. Early computer buses were literally parallel electrical buses with multiple connections, but the term is now used for any physical arrangement that provides the same logical functionality as a parallel electrical bus. Also, if the program attempted to perform those other tasks, it might take too long for the program to check again, resulting in lost data. Until recently, motherboards that featured both PCI and ISA slots were common, giving you a choice.
Engineers thus arranged for the peripherals to interrupt the CPU. As is the case with most interfaces that have a significant impact on performance, an ISA-based card is generally a bad idea since this will greatly limit the performance of the bus.
You cannot efficiently use even something like Fast SCSI through an ISA-bus-based host adapter, because ISA cannot handle more than about 8 MB/s of data throughput. (Or rather, you can use it, but you won't get all the performance possible.) Of course, by today's standards Fast SCSI isn't all that fast, and higher-performance transfer modes are even less suitable for ISA.