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MICROCHANNEL BUS OVERVIEW

A Good Idea Which Never Caught On

MicroChannel was introduced in 1987 by IBM as a solution to the inadequacy of the ISA bus. However, because MicroChannel (MCA) was prohibitively expensive, and since it was not backward compatible with older systems, the bus never caught on. It merits brief discussion here because design features first implemented in the MicroChannel architecture are at the heart of all subsequent bus designs. Quatech is a registered MCA developer, and at one time carried a large line of MicroChannel adapters. There is not much call for them today, and all Quatech MCA boards have been discontinued. However, support files for the MCA product line remain on our website.

Improvements Over ISA

The MCA bus itself, operating at 10MHz, was not enormously faster than its ISA predecessor, but its implementation provided for dramatically increased system performance. With MCA, IBM took bus control away from the processor and set up a system of hardware-mediated bus sharing, whereby individual devices could temporarily take control of the system. This significantly lightened system overhead and allowed for much faster processing. In some systems the MCA bus could reach speeds of 40 Mbytes/sec, a significant improvement over ISA.

MCA improved over ISA in other ways as well. It used full 32-bit addressing which allowed 4-byte data transfers. To minimize interference, a ground or a power supply conductor was located within 3 pins of every signal. With the bus mastering feature, the MCA bus allowed multiple devices to compete for system resources at once. To avoid potential conflicts this could create, a burst mode feature was designed, which would exclusively allocate system resources to a single device for 12ms periods.

The First Plug & Play Boards

Another large improvement in the MCA architecture was the introduction of Plug & Play boards. Gone was the necessity to set jumpers and cables, MCA cards are automatically configured using a utility program which reads a unique identity number coded into a board's firmware. An MCA system uses CMOS memory to remember its system configuration. At setup it compares its file to the hardware installed, and makes necessary adjustments. The identity numbers on each board correspond to instructions indicating how the board should function within the system hierarchy. All MCA boards use the same setup procedure which is completely handled by the system, making the process appear seamless.

MCA Specs


Bus Clock Signal   10 MHz

Bus Width   32-bit

Theoretical Max. Transfer Rate   40 Mbytes/sec (240 Mbits/sec)

Advantages   higher speed than ISA

Disadvantages   obsolete

Click here to see how MCA compares with other busses.

 


 
 
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