| You need to know what differentiates one expansion bus architecture from another and which are the most commonly used types. One bit of terminology adjustment is in order here: An expansion bus architecture is the same as an expansion slot type. A variety of expansion architectures have been used in PCs over the years, including 8-bit, ISA, EISA, MCA, VLB, and PCI. When you open a PC's case and look at the motherboard's expansion slots you likely see are ISA, VLB, and PCI. A motherboard can often support several types of expansion slots. Here is a brief description of each of the expansion slot architectures that have been used in PCs: |
| 8-bit bus: An 8-bit bus is characterized by a single slot that supports eight interrupts and four DMA channels, with all of them pre-assigned. Almost all the architectures that followed are backward compatible with 8-bit cards. |
Industry Standard Architecture (ISA): Pronounced "ice-ah," it was introduced with the IBM AT and also the AT bus; it provided a 16-bit data bus. The ISA bus is characterized by adding an additional short slot to a slot on the 8-bit bus to create the 16-bit connector. ISA added eight additional IRQs and doubled the number of DMA channels. ISA expansion cards were designated to the appropriate IRQ or DMA numbers through jumpers and DIP switches. The ISA architechture also separated the bus clock from the CPU clock to allow the slower data bus to operate at its own speeds. ISA slots are found on 286, 386, 486 and some Pentium PCs. |
Micro-Channel Architechure (MCA): Introduced with the IBM PS/2, MCA was the first 32-bit option, and featured bus mastering and a 10 MHz bus clock for expansion cards. The MCA expansion slot is about the same size as the ISA slot, but has about twice as many channels. MCA cards are also configured to their IRQ and DMA assignments by software, an improvement over the jumpers and DIPs of the ISA architechture. |
Extended Industry Standard Architecture (EISA): Pronounced "ee-sah," this architechture was developed by a group of companies to overcome the limitations of ISA and to compete with MCA, in effect, EISA takes the best parts of MCA and builds on them. It has a 32-bit data bus, uses software setup, has more I/O addresses available, and ignores IRQs and DMA channels. EISA uses only an 8 MHz bus clock to be backward compatible to ISA boards. |
Intelligent Drive Electronics (IDE): This architechture is used almost exclusively for disk drives. An IDE adapter card connects up to two different devices on each expansion slot, providing a low-cost way to add addtitional drives without needing another expansion card. The Extended IDE (EIDE) architechture allows up to four drives to be connected into a single slot. |
Vesa Local Bus (VLB or VL-bus): VLB was used first on 486 systems and grew out of the need for the data bus to run at the same clock speed as the CPU. VLB was developed by the Video Electronics Standards Association (VESA) to place a port more or less directly on the system bus with what was called a bus slot or a processor direct slot. This 32-bit architechture is called Big ISA because beyond the local bus slot, it's basically an ISA architechture - jumpers, DIP switches, and all. VBL slots are mostly proprietary and support expansion cards only from the PC's manufacturer. |
Peripheral Component Interconnect (PCI) bus: Introduced with the Pentium PC, PCI is a local bus architechture that supports either 32-or 64-bit bus, which allows it to be used with both 486 and Pentium computers. The PCI bus is also processor independent because of a special bridging circuit contained on PCI boards. Its bus speed is 33 MHz, giving it much higher throughput than earlier cards. The PCI architechture and expansion slot also supports ISA and EISA cards. PCI cards are also Plug-and-Play, which means they automatically configure themselves to the appropriate IRQ, DMA and I/O port adresses. |
Universal Serial Bus (USB): A newly developed architechture that allows for the connection of up to 127 external serial devices. The USB technology is supported only on Pentium motherboards, where it's a built-in feature on most. It can also be added to motherboards that don't include it through a PCI slot. USB provides support for both low-volume serial I/O devices, such as a mouse or keyboard, and some higher volume devices, such as a modem, CD-ROM, or scanner. All USB devices are Plug-and-Play, and you can hot-plug or hot-swap them; that is remove or insert them with the system power on. |
| Bus | Bus Width (bits) | Bus Speed (MHz) | How Configured |
|---|---|---|---|
| 8-bit | 8 | 8 | Jumpers and DIP switches |
| ISA | 16 | 8 | Jumpers and DIP switches |
| MCA | 32 | 10 | Software |
| EISA | 32 | 8 | Software |
| VL-Bus | 32 | Processor Speed | Jumpers and DIP switches |
| PCI | 32/64 | Processor Speed | Plug-and-Play |