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 a ll
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