Bus: In engineering, a bus is a defined set of signals. Television's two wire and the household telephone four wire (actually paired 2 wire) buses are common examples of buses. Computers contain a number of buses -- some, like the CPU internal data bus, are hidden from the user, others are brought out to connectors. In most cases several devices can be attached to a bus simultaneously. Buses often include a clock signal used to coordinate activities of devices on the bus. A clocked bus is said to have a speed -- usually measured in thousands or millions of clock cycles per second (KHz or MHz).

The following buses are frequently encountered in PCs:

XT Bus: A 4.77 MHz 8 data bit wide (nominal -- sometimes "turbo"ed to higher speed) that carries virtually all signals in 8088 based PCs. Many 486 motherboards include one or two XT Bus slots. AT/ISA bus cards will often operate in 8 bit mode if placed in an XT slot.

AT Bus: An 8.333 MHz 16 data bit wide(nominal -- sometimes "turbo"ed to higher speed) superset of the XT bus that carries virtually all signals in 80286 based PCs.

ISA (Industry Standard Architecture) Bus: A version of the AT bus used in 386 and later computers. Typically the ISA bus is used for slower, older peripherals. The CPU, memory and fast peripherals may be on faster buses.

MCA (MicroChannel Architecture) Bus: A high speed bus introduced by IBM in the late 1980s. MCA uses a different connector than the ISA bus. Although a technical success, MCA was an economic failure and is not widely used.

Extended ISA Bus (EISA): A high speed bus introduced by a consortium of companies in competition to IBM's MCA Bus. EISA uses a different connector than ISA (or MCA) and is rarely encountered. EISA usually requires specialized drivers and/or BIOS support. Some features of the EISA BIOS appear to live on in the mid-1990s Plug aNd Play interface.

VESA Bus (VLB): A 32 bit bus. A 64 bit version is defined but rarely, if ever, seen. VLB is a 33 MHz (sometimes accelerated to 40 or 50 MHz in later versions of the standard) extension to the ISA bus that allows 2 or 3 high speed cards to by used in a PC. It is widely used in 486 PCs. Most VLB Bus implementations will support one 40MHz card. Speeds supported may vary with the number of cards installed. Only one slot allows Bus Mastering. Video cards may not run properly in the "Master" slot.

PCI Bus: A 32 bit 33 MHz (sometimes run at 25 or 30MHz) non-ISA compatible bus used in some 486 and almost all Pentium PCs. IDE bus masters may require a driver such as Intel's BMIDE.SYS to operate as bus masters.

Local Bus: A term for the direct memory and address bus seen at the CPU (or sometimes chipset interface) In many designs, the local bus is an XT, AT, ISA or VLB bus, but in others it is separate.

CPU Bus(es) the signals seen within the CPU. CPU buses are synchronized with the external Local Bus. In 486 and later designs, they may operate at a multiple (commonly 2x, 2.5x or 3x) of the external Local Bus.

Return To Index Copyright 1994-2002 by Donald Kenney.