Different Types of RAM Buses:-
• 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
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. Improvements done over the years eventually made
the AT bus ran at a clock speed of 8MHz.
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.
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.
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.
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 color.
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:-
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 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.
