(M-C-A)

A bus architecture developed by IBM to replace the original PC bus. Because of the lack of backward compatibility and high license fees, MCA never caught on very well in the market.

The first thing you notice about the MCA bus is its connector. The PC bus uses 62-pin connectors (0.1-inch spacing). MCA uses 90-pin, 22-pin, and 84-pin connectors (0.05-inch spacing). There are also auxiliary connectors designed for special purposes.

Card size is also some 40% smaller in the MCA specification. This, along with the connector specifications, made the cards difficult to construct; limiting the market.

But, like beta was technically superior to VHS but still lost in the market, MCA was technically superior to the PC bus. The bus supported multiple processors using a system of priorities. Also, eight DMA channels were supported. The PC bus used edge-triggered interrupts whereas MCA used level-triggered interrupts. The level-triggered interrupts made it easy for multiple devices to share interrupt lines. Additional memory address lines were also included in the MCA specification. MCA also supported a 16-bit I/O; increasing the nunber of devices that could be addressed.

MCA also introduced an effective plug-and-play system with its programmable option select feature. Jumpers and switches on cards were replaced with a programmable register that was configured automatically when the computer started. This was accomplished through a manufacturer-supplied Adapter Description File provided on disk with the adapter card and keyed to the card by a unique identification number.

There were other benefits to MCA but market factors made it only a limited success.

More Information

• Technical Summary
• PCGuide Article

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