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A bus is a subsystem that is used to connect computer components and transfer data between them. For example, an internal bus connects computer internals to the motherboard. A “bus topology” or design can also be used in other ways to describe digital connections.
A bus may be parallel or serial. Parallel buses transmit data across multiple wires. Serial buses transmit data in bit-serial format.
A bus was originally an electrical parallel structure with conductors connected with identical or similar CPU pins, such as a 32-bit bus with 32 wires and 32 pins. The earliest buses, often termed electrical power buses or bus bars, were wire collections that connected peripheral devices and memory, with one bus designated for peripheral devices and another bus for memory. Each bus included separate instructions and distinct protocols and timing.
Parallel bus standards include advanced technology attachment (ATA) or small computer system interface (SCSI) for printer or hard drive devices. Serial bus standards include universal serial bus (USB), FireWire or serial ATA with a daisy-chain topology or hub design for devices, keyboards or modem devices.
Computer bus types are as follows:
System Bus: A parallel bus that simultaneously transfers data in 8-, 16-, or 32-bit channels and is the primary pathway between the CPU and memory.
Internal Bus: Connects a local device, like internal CPU memory.
External Bus: Connects peripheral devices to the motherboard, such as scanners or disk drives.
Expansion Bus: Allows expansion boards to access the CPU and RAM.
Frontside Bus: Main computer bus that determines data transfer rate speed and is the primary data transfer path between the CPU, RAM and other motherboard devices.
Backside Bus: Transfers secondary cache (L2 cache) data at faster speeds, allowing more efficient CPU operations.
As it evolves, the bus can be discussed based on different engineering models. For example, there is the parallel bus and the serial bus, as mentioned above, and the different types of buses that you would encounter on the motherboard of a computer, for example, a system bus, address bus or input-output bus.
We can also talk about buses in the form of data transfer rates. Here the “bus speed” or rate may be listed in megahertz, or in megabytes per second. For example, 100 MHz is said to correspond to perhaps 6400 MB per second in some architectures.
In general, the kinds of speed achieved by modern processors include bus speeds of typically under 10,000 MB or 10 GB per second.
Then there’s also the designation of a bus specific to where it is located on the circuit board. The front-side bus is typically considered the fastest bus on the motherboard.
In terms of other uses for the term “bus”, the network bus topology differs from the electrical bus setups mentioned in connection to circuit board and electrical design.
In a network, the “bus” is a digital structure that will transmit data in either parallel bus or serial bus format along a set of nodes.
Bus design has been integral in electrical engineering, and also, as mentioned, in networking. As connections evolve, the central concept of the bus will remain relevant.