Micro Channel Architecture can be bit confusing on a first glance. There multiple variants of Micro Channel slot. We can divide them to two main groups by the data/address bus width - to 16-bit and 32-bit slots. Apart from this there are 3 special bus extensions - BVE, AVE and MME.
Let's check how does each slot variant looks and what is the purpose of bus extensions.
All drawings made by Tomas Slavotinek.
The "component side" (A) and "solder side" (B) refers to an installed adapter card.
The "Rear" arrow points towards the back side of the planar/system unit.
The physical key between pins 45 and 48 serves as a reference point for all connector types.
MCA Slot Types
16-bit standard MCA slot
16-bit MCA slot with AVE "8514/A Slot"
32-bit MCA slot
You will rarely if ever see the 32-bit MCA slot in its base form, typically it will have at least the MME extension.
32-bit MCA slot with AVE "8514/A Slot"
32-bit MCA slot with MME "Memory Card Slot"
All views are from the top looking down to the slot. The "REAR" arrow is pointing towards the back of the planar/system unit.
AVE - Auxiliary Video Extension
Matched Memory Extension (MME)
The MME part of the MCA connector consists out of 8 pins (2 x 4) at the rear end of a 32-bit MCA connector. There is no separation between the 32-bit connector and the MME-part.
The MME part is used from 32-bit Memory Expansion adapters, which support Matched Memory Cycles. This can improve the data transfer capabilities between the system master and channel-resident memory (means: between planar memory and that on expansion cards).
The use of MME is system-dependent and varies between the line of PS/2-machines.
Auxiliary Video Extension (AVE)
The AVE part of the MCA connector consists out of 20 pins (2 x 10) at the rear end of the MCA connector, separated by a key (notch in the cards' connector).
The AVE is intended for cards that have no own base-video system - like the IBM 8514/A Display Adapter /A.
Other additional video-cards that *do not* use the AVE cannot display the VGA text/graphic mode on the same screen along with their own native modes. They need a second monitor attached to the onboard VGA / XGA card.
In a way this AVE is the MCAs Video Feature Connector: cards can use the onboard video-system and -for example- synchronize their output to the onboard video.
Base Video Extension (BVE)
The BVE part of the MCA connector consists out of 20 pins (2 x 10) at the rear end of a 32-bit MCA connector and after the MME-part. There is a separating key between MME and BVE, which causes a notch in a cards' MCA connector with a size of 2 connecting pins.
This slot can only be found on machines, that do not supply an onboard Base-Video system, like the Models 76, 77, 90 and 95.