Micro Channel Slots

Slot Pinouts

MCA Slot Types
Matched-Memory Extension (MME)
Auxiliary Video Extension (AVE)
Base Video Extension (BVE)

Drawings by Tomáš Slavotínek.


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.


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"

32-bit MCA slot with MME and BVE "Video 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
BVE - Base Video Extension
MME - Matched Memory 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. The Models 70 and 80 support MME - and therefore can be upgraded with a 32-bit memory expansion card, the Models 90 and 95 do not support MME and should be upgraded only with memory on the planar.

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. This adapter can utilize the planar video-system through this connections and the video-signal in the low-resolution modes come in fact from the onboard VGA / onboard XGA, but is passed through to make it available on the 8514/A monitor port. Therefore you need only one monitor.

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.

See the Auxiliary Video Extension page for more information.

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 (Bermuda-based), 85, and 95.

The machines' video card (XGA / XGA-2 or SVGA) plugs into this slot and is enabled to communicate with a card which sits in a probable AVE slot. All sync-signals, blanking and pixel-data is supplied over this BVE connector.

On the XGA adapter the BVE connector is the short one to the rear, the MME part is left out, therefore there is a very large gap between the BVE part and the rest of the MCA connector. If the XGA is set to boards with Base Video this BVE part plugs nowhere and is just hanging free, because systems with onboard video card don't have an BVE connector.

Content created and/or collected by:
Louis Ohland, Peter Wendt, David Beem, William Walsh, Tatsuo Sunagawa, Jim Shorney, Tim Clarke, Kevin Bowling, Tomáš Slavotínek, and many others.

Ardent Tool of Capitalism - MAD Edition! is maintained by Tomáš Slavotínek.
Last update: 25 Jun 2022 - Changes & Credits | Legal Info & Contact