An Attachment Unit Interface (AUI) is a 15 pin connection that provides a path between a node's Ethernet interface and the Medium Attachment Unit (MAU), sometimes known as a transceiver. It is the part of the IEEE Ethernet standard located between the Media Access Control (MAC), and the MAU. An AUI cable may be up to 50 metres long, although frequently the cable is omitted altogether and the MAU and MAC are directly attached to one another.
An AUI connector is a DA-15 (D-subminiature). It has a sliding clip in place of the thumbscrews normally found on a D-connector to hold two connectors together. This clip permits the MAU and MAC to be directly attached to one another even when their size and shape would preclude the use of thumbscrews. This clip is however frequently found to be awkward and/or unreliable.
AUI connectors became rarer beginning in the early 1990s. This was because it became more common for computers and hubs to incorporate the MAU, particularly as the 10BASE-T standard became more common and use of 10BASE-5 (thicknet) and 10BASE-2 (thinnet) declined. The electrical AUI connection was still present inside the equipment. By the mid-1990s AUI had all but disappeared as fast Ethernet became more common (fast Ethernet has an equivalent known as MII). Gigabit Ethernet and 10 gigabit Ethernet have respectively the GBIC and XAUI standards which are equivalent to AUI.
A modified form using a smaller connector called the AAUI was used on Apple Macintosh computers in the early 1990s.
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