Expansion Buses in PCS

Expansion Bus :
The expansion bus allows the CPU to communicate with peripheral devices. Expansion bus and associated slots are required because basic PC system cannot satisfy all needs of users. Expansion bus slots or I/O bus designs in general, provide the opportunity to add more devices to PC. We can plug sound cards, video cards, SCSI controller cards, network interface cards, etc. Into expansion bus slots provided on PC motherboard.

Block diagram is shown below :

Functions of Expansion Bus :

1. Expansion bus slots are long thin connectors provided on motherboard near backside of computer and these are used to connect all add-on cards in a computer system.
2. They provide direct electrical connection with PC’s main logical circuitry. Therefore, its purpose is to enable user to add expansion boards to computer system that plays a Crucial role in enhancing power and capabilities of a PC.
3. Expansion bus determines what exactly can be added to system – how many add-on cards, how much memory on expansion cards, what other system components and how easy it will be to setup system.
4. It provide a common pathway for data, address and control signals that links various components of PC.
5. Bus design also has to assure that data actually goes to place where it is meant to go, i.e. through bus circuits computer sends definite patterns of digital bits in a definite sequence from one point to another.
6. Expansion bus provides special signals to synchronize working of add-on cards with the rest of the computer.

In modern PCs, a number of basic peripheral devices are built into motherboard. As a result, it may happen that expansion slots may remain unused, but that doesn’t mean that expansion buses remain unused in that system. All built-in controllers and ports still use I/O bus to communicate with CPU, i.e. They operate in such a way as it they are add-on cards plugged into the system’s expansion bus slots.

Since introduction of first PC till, a number of expansion bus types have appeared on PC platform with basic objective of providing faster I/O speeds for better system performance. The need for better performance arises mainly due to improvements in three main areas :

• Faster processors
• Increasing application software requirements
• Faster multimedia requirements in audio and video
• Faster peripherals.