MCA wasn't just a replacement for ISA... ISA was still at 16 bits (or 8 bits if you were really old), whereas MCA brought in a 32 bit inteface with some form of bus mastering as well. Great.

Unfortunately, it used different connectors to ISA. And not only that, you couldn't even put MCA and ISA connectors on the board at the same time. And even worse, IBM charged a fortune to license it. So it didn't take off.

Shortly after, Compaq and some others designed EISA - Extended ISA. This was also a 32 bit slot, that at first sight looked like a 16 bit ISA slot. Indeed, a 16 bit ISA card could be inserted and worked fine. But, there was a second row of contacts underneath the ISA contacts that a 32 bit EISA card mated with. So same performance and functionality as MCA, with backwards compatibility with EISA.

However, neither of these really took off as (with the exception of servers) most people didn't need the power of a 32 bit bus mastering controller at that point.

Of course, by the time VL-bus and PCI came around, the power was needed. VL-bus was like ISA in as much as it was an extension to the 16 bit ISA bus, and the slots could take 16 bit ISA cards and 32 bit VL-bus cards. PCI was a different slot altogether, but it could co-exist with 16 bit ISA slots on the same board (in fact, the ISA slots were usually implemented as an on-board PCI card, which provided the ISA slots.

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