Types of Computer Buses
Types of Buses in Computer Architecture
Inside computers, there are many internal components. In order for these components to communicate with each other, they make use of wires that are known as a ‘bus’.
A bus is a common pathway through which information flows from one computer component to another. This pathway is used for communication purpose and it is established between two or more computer components. We are going to check different computer bus architectures that are found in computers.
Different Types of Computer Buses
Functions of Buses in Computers
Summary of functions of buses in computers
1. Data sharing - All types of buses found in a computer transfer data between the computer peripherals connected to it.
The buses transfer or send data either in the serial or parallel method of data transfer. This allows for the exchange of 1, 2, 4 or even 8 bytes of data at a time. (A byte is a group of 8 bits). Buses are classified depending on how many bits they can move at the same time, which means that we have 8-bit, 16-bit, 32-bit or even 64-bit buses.
2. Addressing - A bus has address lines, which match those of the processor. This allows data to be sent to or from specific memory locations.
3. Power - A bus supplies power to various peripherals connected to it.
4. Timing - The bus provides a system clock signal to synchronize the peripherals attached to it with the rest of the system.
The expansion bus facilitates easy connection of more or additional components and devices on a computer such as a TV card or sound card.
Computers have two major types of buses:
1. System bus:- This is the bus that connects the CPU to the main memory on the motherboard. The system bus is also called the front-side bus, memory bus, local bus, or host bus.
2. A number of I/O Buses, (I/O is an acronym for input/output), connecting various peripheral devices to the CPU. These devices connect to the system bus via a ‘bridge’ implemented in the processors' chipset. Other names for the I/O bus include “expansion bus", "external bus” or “host bus”.
Expansion Bus Types
These are some of the common expansion bus types that have ever been used in computers:
- ISA - Industry Standard Architecture
- EISA - Extended Industry Standard Architecture
- MCA - Micro Channel Architecture
- VESA - Video Electronics Standards Association
- PCI - Peripheral Component Interconnect
- PCI Express (PCI-X)
- PCMCIA - Personal Computer Memory Card Industry Association (Also called PC bus)
- AGP - Accelerated Graphics Port
- SCSI - Small Computer Systems Interface.
The 8 Bit and 16 Bit ISA Buses
This is the most common type of early expansion bus, which was designed for use in the original IBM PC. The IBM PC-XT used an 8-bit bus design. This means that the data transfers take place in 8-bit chunks (i.e. one byte at a time) across the bus. The ISA bus ran at a clock speed of 4.77 MHz.
For the 80286-based IBM PC-AT, an improved bus design, which could transfer 16-bits of data at a time, was announced. The 16-bit version of the ISA bus is sometimes known as the AT bus. (AT-Advanced Technology)
The improved AT bus also provided a total of 24 address lines, which allowed 16MB of memory to be addressed. The AT bus was backward compatible with its 8-bit predecessor and allowed 8-bit cards to be used in 16-bit expansion slots.
When it first appeared the 8-bit ISA bus ran at a speed of 4.77MHZ – the same speed as the processor. Improvements done over the years eventually made the AT bus ran at a clock speed of 8MHz.
Comparison Between 8 and 16 Bit ISA Bus
8-Bit ISA card (XT-Bus)
16-Bit ISA (AT –Bus card)
8-bit data interface
16-bit data interface
4.77 MHZ bus
36-pin AT extension connection
MCA (Micro Channel Architecture)
IBM developed this bus as a replacement for ISA when they designed the PS/2 PC launched in 1987.
The bus offered a number of technical improvements over the ISA bus. For instance, the MCA ran at a faster speed of 10MHz and supported either 16-bit or 32-bit data. It also supported bus mastering - a technology that placed a mini-processor on each expansion card. These mini-processors controlled much of the data transfer allowing the CPU to do other tasks.
One advantage of MCA was that the plug-in cards were software configurable; this means that they required minimal intervention by the user when configuring.
The MCA expansion bus did not support ISA cards and IBM decided to charge other manufacturers royalties for use of the technology. This made it unpopular and it is now obsolete technology.
The EISA Bus
EISA (Extended Industry Standard Architecture)
This is a bus technology developed by a group of manufactures as an alternative to MCA. The bus architecture was designed to use a 32-bit data path and provided 32 address lines giving access to 4GB of memory.
Like the MCA, EISA offered a disk-based setup for the cards, but it still ran at 8MHz in order for it to be compatible with ISA.
The EISA expansion slots are twice as deep as an ISA slot. If an ISA card is placed in an EISA slot it will use only the top row of connectors, however, a full EISA card uses both rows. It offered bus mastering.
EISA cards were relatively expensive and were normally found on high-end workstations and network servers.
It was also known as the Local bus or the VESA-Local bus. VESA (Video Electronics Standards Association) was invented to help standardize PCs video specifications, thus solving the problem of proprietary technology where different manufacturers were attempting to develop their own buses.
The VL Bus provided 32-bit data path and ran at 25 or 33 MHZ. It ran at the same clock frequency as the host CPU. But this became a problem as processor speeds increased because, the faster the peripherals are required to run, the more expensive they are to manufacture.
It was difficult to implement the VL-Bus on newer chips such as the 486s and the new Pentiums and so eventually the VL-Bus was superseded by PCI.
VESA slots had an extra set of connectors and thus the cards were larger. The VESA design was backward compatible with the older ISA cards.
Features of the VESA local bus card:-
- 32-bit interface
- 62/36-pin connector
- 90+20 pin VESA local bus extension
Peripheral Component Interconnect
Peripheral Component Interconnect (PCI) is one of the latest developments in bus architecture and is the current standard for PC expansion cards. Intel developed and launched it as the expansion bus for the Pentium processor in 1993. It is a local bus like VESA, that is, it connects the CPU, memory, and peripherals to a wider, faster data pathway.
PCI supports both 32-bit and 64-bit data width; it is compatible with 486s and Pentiums. The bus data width is equal to the processor, such as a 32-bit processor would have a 32 bit PCI bus, and operates at 33MHz.
PCI was used in developing Plug and Play (PnP) and all PCI cards support PnP. This means a user can plug a new card into the computer, power it on and it will “self-identify” and “self-specify” and start working without manual configuration using jumpers.
Unlike VESA, PCI supports bus mastering that is, the bus has some processing capability and thus the CPU spends less time processing data. Most PCI cards are designed for 5v, but there are also 3v and dual-voltage cards. Keying slots used help to differentiate 3v and 5v cards and also to make sure that a 3v card is not slotted into a 5v socket and vice versa.
The PCI Slots
Accelerated Graphics Port
The need for high quality and very fast performance of video on computers led to the development of the Accelerated Graphics Port (AGP). The AGP Port connects to the CPU and operates at the speed of the processor bus. This means that video information is sent more quickly to the card for processing.
The AGP uses the main PC memory to hold 3D images. In effect, this gives the AGP video card an unlimited amount of video memory. To speed up the data transfer, Intel designed the port as a direct path to the PC’s main memory.
Data transfer rate ranges from 264 Mbps to 528mbps, 800 Mbps up to 1.5 Gbps. AGP connector is identified by its brown colour.
Personal Computer Memory Card Industry Association (PC Card)
The Personal Computer Memory Card Industry Association was founded to give a standard bus for laptop computers. So it is basically used in the small computers.
Small Computer System Interface
Short for Small Computer System Interface, a parallel interface standard used by Apple Macintosh computers, PCs and Unix systems for attaching peripheral devices to a computer.
The SCSI Port
Universal Serial Bus (USB)
This is an external bus standard that supports data transfer rates of 12 Mbps. A single USB port connects up to 127 peripheral devices, such as mice, modems, and keyboards. The USB also supports hot plugging or insertion (ability to connect a device without turning the PC off) and plug and play (You connect a device and start using it without configuration).
We have two versions of USB:-
First released in 1996, the original USB 1.0 standard offered data rates of 1.5 Mbps. The USB 1.1 standard followed with two data rates: 12 Mbps for devices such as disk drives that need high-speed throughput and 1.5 Mbps for devices such as joysticks that need much less bandwidth.
In 2002 a newer specification USB 2.0, also called Hi-Speed USB 2.0, was introduced. It increased the data transfer rate for PC to a USB device to 480 Mbps, which is 40 times faster than the USB 1.1 specification. With the increased bandwidth, high throughput peripherals such as digital cameras, CD burners, and video equipment could now be connected with USB.
The IEEE 1394 is a very fast external serial bus interface standard that supports data transfer rates of up to 400Mbps (in 1394a) and 800Mbps (in 1394b). This makes it ideal for devices that need to transfer high levels of data in real-time, such as video devices. It was developed by Apple with the name firewire.
A single 1394 port can connect up 63 external devices.
- It supports Plug and play
- Supports hot plugging, and
- Provides power to peripheral devices.
The IEEE 1394 Expansion Card
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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2011 Patrick Kamau