MCA Bus. The MicroChannel Architecture Bus was introduced by IBM in it's PersonalSystem/2 computers. It is a 32 bit bus that was capable of 10MHz at the time of its introduction. The bus was expanded to 64 Bits in 1991 and speed up to 20MHz in 1994. Laboratory measurements indicated that it could have been pushed to the 80-100MHz range without significant difficulties.

While technically superior to the AT Bus, the MCA bus suffered from several defects that doomed it to obscurity. MCA it is not pin compatible with the ISA bus. Since IBM did not design ISA bus slots into MCA bus computers, existing peripheral cards could not be used. IBM charged rather high licensing fees for the MCA bus. MCA cards were expensive. Installation and configuration software for the PS/2 was difficult to use -- which caused some technicians to recommend against IBM MCA bus machines. And finally, the PS/2 computers reflected a number of poorly thought out innovations apparently implemented more for product differentiation purposes than for user benefits. Customers quickly learned to avoid the PS/2 and other vendors, by and large, did not build MCA bus computers. As of 1997, the MCA bus is long dead.

The MCA bus used pins on .05 inch centers. The first 92 pins (46 on each side) supported 8 bit operations. The next 24 pins (12 on each side) extended the bus to 16 bits. A notch 2 pins wide separated an optional 58 pin (29 per side) 32-bit extension. Optional further extensions were defined for video (22pins) or Memory (8pins)

Maximum theoretical throughput for a 10MHz MCA bus is about 20 MBytes per second.

Return To Index Copyright 1994-2002 by Donald Kenney.