MicroChannel Architecture

MCA

Micro Channel architecture
32-bit Graphics Card IBM XGA-2
Year created	1987; 30 years ago (1987)
Created by	IBM
Supersedes	ISA
Superseded by	PCI (1993)
Width in bits	16 or 32
Speed	10 MHz
Style	Parallel
Hotplugging interface	no
External interface	no

Micro Channel architecture, or the Micro Channel bus, was a proprietary 16- or 32-bit parallel computer bus introduced by IBM in 1987 which was used on PS/2 and other computers until the mid-1990s. Its name is commonly abbreviated as "MCA", although not by IBM. In IBM products, it superseded the ISA bus and was itself subsequently superseded by the PCI bus architecture.

The development of Micro Channel was driven by both technical and business pressures.

The IBM AT bus, which later became known as the Industry Standard Architecture (ISA) bus had a number of technical design limitations, including:

In addition, it suffered from other problems:

These limitations became more serious as the range of tasks and peripherals, and the number of manufacturers for IBM PC-compatibles, grew. IBM was already investigating the use of RISC processors in desktop machines, and could, in theory, save considerable money if a single well-documented bus could be used across their entire computer lineup.

It was thought that by creating a new standard, IBM would regain control of standards via the required licensing. As patents can take three years or more to be granted, however, only those relating to ISA could be licensed when Micro Channel was announced. Patents on important Micro Channel features, such as Plug and Play automatic configuration, were not granted to IBM until after PCI had replaced Micro Channel in the marketplace.

The Micro Channel architecture was designed by engineer Chet Heath. A lot of the Micro Channel cards that were developed used the CHIPS P82C612 MCA interface controller; allowing MCA implementations to become a lot easier.

The Micro Channel was primarily a 32-bit bus, but the system also supported a 16-bit mode designed to lower the cost of connectors and logic in Intel-based machines like the IBM PS/2.

The situation was never that simple, however, as both the 32-bit and 16-bit versions initially had a number of additional optional connectors for memory cards which resulted in a huge number of physically incompatible cards for bus attached memory. In time, memory moved to the CPU's local bus, thereby eliminating the problem. On the upside, signal quality was greatly improved as Micro Channel added ground and power pins and arranged the pins to minimize interference; a ground or a supply was thereby located within 3 pins of every signal.