Computer Bus Architecture Types

Computers comprises of many internal components and in order for these components to communicate with each other a ‘bus’ is used.

A bus is a common pathway through which information is connected from one component to another. This pathway is used for communication and can be established between two or more computer components. We are going to review computer bus architectures that are used in computers.

The functions of buses can be summarized as below:

FUNCTIONS OF BUSES

1. Data sharing - the expansion bus must be able to transfer data between the computer and the peripherals connected to it.

The data is transferred in parallel, which allows 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 or even 32-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 that are 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 the easy connection of additional components and devices on a computer for example the addition of a TV card or sound card.

Bus Terminologies

Computers can be viewed to be having just two types of buses:
1. System bus:- The bus that connects the CPU to 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, (Acronym for input/output), connecting various peripheral devices to the CPU -these are connected 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
• PCMCIA - Personal Computer Memory Card Industry Association (Also called PC bus)
• AGP - Accelerated Graphics Port
• SCSI - Small Computer Systems Interface.

ISA Bus

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. It was improved over the years and eventually the AT bus ran at a clock speed of 8MHz.

MCA (Micro Channel Architecture)

This bus was developed by IBM as a replacement for ISA when they designed the PS/2 PC which was launched in 1987.

The bus offered a number of technical improvements over the ISA bus. For instance, the MCA runs at a faster speed of 10MHz and can support either 16-bit or 32-bit data. It also supports 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 perform other tasks.

One advantage of MCA was that the plug-in cards were software configurable i.e. 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 an obsolete technology.

EISA (Extended Industry Standard Architecture)

It was developed by a group of manufactures as an alternative to MCA. It 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, whereas 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.

VESA Bus

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 provides 32-bit data path and can run at 25 or 33MHZ. 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 have extra set of connectors and therefore the cards are 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

PCI

Peripheral Component Interconnect (PCI) is one of the latest developments in bus architecture and is the current standard for PC expansion cards. It was developed by Intel and launched as the expansion bus for the Pentium processor in 1993. It is a local bus like VESA i.e. it connects the CPU, memory and peripherals to wider, faster data pathway.

PCI supports both 32-bit and 64-bit data width; therefore it is compatible with 486s and Pentiums. The bus data width is equal to the processor, for example, 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 i.e. the 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 i.e. the bus has some processing capability and therefore 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 are used to differentiate 3v and 5v cards and slots to ensure that a 3v card is not slotted into a 5v socket and vice versa.

AGP

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 is connected to the CPU and operates at the speed of the processor bus. This means that video information can be 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 upto 1.5 Gbps. AGP connector is identified by its brown colour.

PCMCIA (PC Card)

The Personal Computer Memory Card Industry Association was founded to provide a standard bus for laptop computers.

SCSI

Short for Small Computer System Interface, a parallel interface standard used by Apple Macintosh computers,PC's and Unix systems for attaching peripheral devices to a computer.

USB - Universal Serial Bus

This is an external bus standard that supports data transfer rates of 12 Mbps. A single USB port can be used to connect up to 127 peripheral devices, such as mice, modems, and keyboards. The USB also supports hot plugging/insertion (ability to connect a device without turning the PC of) and plug and play (You connect a device and start using it without configuration).

We have two versions of USB:-

USB 1x

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.

USB 2x

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 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.

IEEE 1394

The IEEE 1394 is a very fast external bus 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.

A single 1394 port can be used to connect up 63 external devices.

• It supports Plug and play
• Supports hot plugging, and
• Provides power to peripheral devices.