The Primary Buses There are generally two buses within a computer. The first is the internal bus (sometimes called the front-side bus, or FSB for short). The internal bus allows the processor to communicate with the system's central memory (the RAM). In computer architecture, a bus (related to the Latin 'omnibus', meaning 'for all') is a communication system that transfers data between components inside a computer, or between computers.
In computing, a bus is defined as a set of physical connections (for example, cables, printed circuits, etc.) that can be shared by multiple hardware components in order to communicate with one another. The purpose of buses is <boldWto reduce the number of pathways needed for communication</bold> between the components, by carrying out all communications over a single data channel. This is why the metaphor of a data highway is sometimes used.
PCI-X (PCI eXtended) is a computer bus and expansion card standard that enhanced the PCI Local Bus for higher bandwidth demanded by servers. It is a double-wide version of PCI, running at up to four times the clock speed, but is otherwise similar in electrical implementation and uses the same protocol. Computer bus is a subsystem that transfers data between components inside a computer or between computers. Modern computer buses can use parallel and bit-serial connections, and can be wired in either a multidrop (electrical parallel) or daisy chain topology.
Characteristics
If only two hardware components communicate over the line, it is called a hardware port, (such as a port or parallel port).
There are different types of computer buses. A bus is characterized by the amount of information that can be transmitted at once. This amount, expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously. A 32-wire ribbon cable can transmit 32 bits in parallel. The term width is used to refer to the number of bits that a bus can transmit at once.
Additionally, the bus speed is defined by its frequency (expressed in Hertz), the number of data packets sent or received per second. Each time that data is sent or received is called a cycle.
It is possible to find the maximum transfer speed of the bus, or the amount of data that it can transport per unit of time, by multiplying its width by its frequency. For example, a bus with a width of 16 bits and a frequency of 133 MHz has a transfer speed equal to:
Architecture
In reality, each bus is generally constituted of 50 to 100 distinct physical lines, divided into three subassemblies:
- The address bus: sometimes called the memory bus, it transports memory addresses that the processor wants to access in order to read or write data. It is a unidirectional bus.
- The data bus: it transfers instructions coming from or going to the processor. It is a bidirectional bus.
- The control bus, or the command bus: it transports orders and synchronization signals coming from the control unit and traveling to all other hardware components. It is a bidirectional bus, as it also transmits response signals from the hardware.
The Primary Buses
There are generally two buses within a computer.
- The internal bus: sometimes called the front-side bus, or FSB for short, it allows the processor to communicate with the system's central memory (the RAM).
- The expansion bus: sometimes called the input/output bus, it allows various motherboard components (namely, USB, serial, and [contents/415-serial-port-and-parallel-port parallel ports], cards inserted in PCI connectors, hard drives, CD-ROM and CD-RW drives, etc.) to communicate with one another. However, it is mainly used to add new devices using what it’s called expansion slots connected to the input/output bus.
Motherboard Chipset
What is “chipset”? A chipset is the component that routes data between the computer's buses, so that all the components that make up the computer can communicate with each other. The chipset originally was made up of a large number of electronic chips, hence the name.
It generally has two components. The first is the NorthBridge (also called the memory controller) that is in charge of controlling transfers between the processor and the RAM; this is the reason why it is located physically near the processor. It is sometimes called the GMCH, for Graphic and Memory Controller Hub.
The second is the SouthBridge (also called the input/output controller or expansion controller) that handles communications between peripheral devices. It is also called the ICH (I/O Controller Hub). The term bridge is generally used to designate a component that connects two buses:
It is noteworthy that, in order to communicate, two buses must have the same width. This explains why RAM modules sometimes have to be installed in pairs (for example, early Pentium chips, whose processor buses were 64-bit, required two memory modules each 32 bits wide).
| Standard | Bus width (bits) | Bus speed (MHz) | Bandwidth (MB/sec) |
|---|---|---|---|
| ISA 8-bit | 8 | 8.3 | 7.9 |
| ISA 16-bit | 16 | 8.3 | 15.9 |
| EISA | 32 | 8.3 | 31.8 |
| VLB | 32 | 33 | 127.2 |
| PCI 32-bit | 32 | 33 | 127.2 |
| PCI 64-bit 2.1 | 64 | 66 | 508.6 |
| AGP | 32 | 66 | 254.3 |
| AGP (x2 Mode) | 32 | 66x2 | 528 |
| AGP (x4 Mode) | 32 | 66x4 | 1056 |
| AGP (x8 Mode) | 32 | 66x8 | 2112 |
| ATA33 | 16 | 33 | 33 |
| ATA100 | 16 | 50 | 100 |
| ATA133 | 16 | 66 | 133 |
| Serial ATA (S-ATA) | 1 | 180 | |
| Serial ATA II (S-ATA2) | 2 | 380 | |
| USB | 1 | 1.5 | |
| USB 2.0 | 1 | 60 | |
| FireWire | 1 | 100 | |
| FireWire 2 | 1 | 200 | |
| SCSI-1 | 8 | 4.77 | 5 |
| SCSI-2 - Fast | 8 | 10 | 10 |
| SCSI-2 - Wide | 16 | 10 | 20 |
| SCSI-2 - Fast Wide 32 bits | 32 | 10 | 40 |
| SCSI-3 - Ultra | 8 | 20 | 20 |
| SCSI-3 - Ultra Wide | 16 | 20 | 40 |
| SCSI-3 - Ultra 2 | 8 | 40 | 40 |
| SCSI-3 - Ultra 2 Wide | 16 | 40 | 80 |
| SCSI-3 - Ultra 160 (Ultra 3) | 16 | 80 | 160 |
| SCSI-3 - Ultra 320 (Ultra 4) | 16 | 80 DDR | 320 |
| SCSI-3 - Ultra 640 (Ultra 5) | 16 | 80 QDR | 640 |
Image: © Signs and Symbols - Shutterstock.com
Alternatively known as an address bus, data bus, or local bus, a bus is a connection between components or devices connected to a computer. For example, a bus carries data between a CPU and the system memory via the motherboard.
- Types of computer buses.
Why is a computer bus called a bus?
You can think of a computer bus like public transportation or a school bus. These types of buses are capable of transporting people from one destination to another destination. Like these buses, a computer bus transmits data from one location or device to another location or device.
A computer bus maintains a strict schedule, 'picking up' data and 'dropping it off' at a regular interval. For example, if a bus operates at a frequency of 200 MHz, it completes 200 million data transfers per second.
Computer bus overview
The bus contains multiple wires (signal lines) with addressing information describing the memory location of where the data is being sent or retrieved. Each wire in the bus carries a bit(s) of information, which means the more wires a bus has, the more information it can address. For example, a computer with a 32-bit address bus can address 4 GB of memory, and a computer with a 36-bit bus can address 64 GB of memory.
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The illustration below shows the different types of computer buses and how they connect devices on the motherboard.
Types of computer buses
A bus is either a parallel or serial bus, and either an internal bus (local bus) or an external bus (expansion bus).
Internal bus vs. external bus
An internal bus enables the communication between internal components, such as a video card and memory. An external bus is capable of communicating with external components, such as a USB or SCSI device.
Parallel bus vs. serial bus
A computer bus can transmit its data using either a parallel or serial method of communication. With a parallel bus, data is transmitted several bits at a time. However, with a serial bus, the data is transferred one bit at a time.
Bus speeds
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A computer or device's bus speed is measured in MHz, e.g., an FSB may operate at a frequency of 100 MHz. The throughput of a bus is measured in bits per second or megabytes per second.
Examples of computer buses
| A-F | G-N | O-P | P-Z |
|---|---|---|---|
AGP | HyperTransport | PATA | SATA |
Most popular computer buses
Today, many of the buses listed above are no longer used or are not as common. Below is a listing of the most common buses and how they are used with a computer.
- eSATA and SATA - Computer hard drives and disc drives.
- PCIe - Computer expansion cards and video cards.
- USB - Computer peripherals.
- Thunderbolt- Peripherals connected through a USB-C cable.
Related pages
ADB, AGP, AMR, AT Bus, Back-side bus, Channel, CNR, EISA, Hardware terms, HyperTransport, IDE, Input/output bus, Internal data bus, ISA, MCA, Motherboard terms, Multiplier, NuBus, PCI, PCI Express, PCMCIA, SBus, SCSI, SMBus, USB, Vitesse-Bus, VLB, XT