Expansion Bus and Expansion Slots

Definition: BUS - a set of electronic signal pathways that allows information and signals to travel between components inside or outside of a computer.

Expansion Slot (connector)
Remember that the expansion bus, or external bus, is made up of the electronic pathways that connect the different external devices to the rest of your computer. These external devices (monitor, telephone line, printer, etc.) connect to ports on the back of the computer. Those ports are actually part of a small circuit board or 'card' that fits into a connector on your motherboard inside the case. The connector is called an expansion slot.

Note: Communication ports (com ports), printer ports, hard drive and floppy connectors, etc., are all devices which used to be installed via adapter cards. These connectors are now integrated onto the motherboard, but they are still accessed via the expansion (external) bus and are allocated the same type of resources as required by expansion cards. As a matter of fact (and unfortunately, in my opinion), other devices like modems, video technology, network and sound cards are now being integrated, or embedded, right onto the motherboard.

Expansion slots are easy to recognize on the motherboard. They make up a row of long plastic connectors at the back of your computer with tiny copper 'finger slots' in a narrow channel that grab the metal fingers or connectors on the expansion cards. In other words, the expansion cards plug into them. The slots attach to tiny copper pathways on the motherboard (the expansion bus), which allows the device to communicate with the rest of the computer. Each pathway has a specific function. Some may provide voltages needed by the new device (+5, +12 and ground), and some will transmit data. Other pathways allow the device to be addressed through a set of I/O (input/output) addresses, so that the rest of the computer knows where to send and retrieve information. Still more pathways are needed to provide clock signals for synchronization and other functions like interrupt requests, DMA channels and bus mastering capability.

As with any other part of the computer, technology has evolved in an effort to increase the speed, capability and performance of expansion slots. Now you'll hear about more busses - PCI bus, ISA bus, VESA bus, etc. Not to worry! These are all just types of expansion (external) busses. They just describe the type of connector and the particular technology or architecture being used. Thus, the adapter card being installed must match the architecture or type of slot that it's going into. An ISA card fits into an ISA slot, a PCI adapter card must be installed into a PCI expansion slot, etc. More on this later.

Continued...