Attachment Unit Interface

File:AUI Connectors.jpg or an Ethernet hub). Note the sliding clip.]]

File:AUI connector on DEC EtherWorks LC (DE100).jpg EtherWorks LC (DE100) Ethernet controller. Note the jackposts (as opposed to a sliding clip).]]

The Attachment Unit Interface (AUI) is a physical and logical interface defined in the IEEE 802.3 standard (additionally published in FIPS PUB 107) for 10BASE5 Ethernet{{cite web|title=Federal Information Processing Standards Publication: local area networks: baseband carrier sense multiple access with collision detection access method and physical layer specifications and link layer protocol|url=https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub107-Mar1985.pdf|publisher=American National Standard (doi:10.6028/NBS.FIPS.107)|access-date=3 October 2024}} and the earlier DIX standard. The physical interface consists of a 15-pin D-subminiature connector that links an Ethernet node's physical signaling to the Medium Attachment Unit (MAU),IEEE 802.3 7. Physical Signaling (PLS) and Attachment Unit Interface (AUI) specifications sometimes referred to as a transceiver. An AUI cable can extend up to {{convert|50|m|ft|abbr=off}}, though often the MAU and data terminal equipment's (DTE) medium access controller (MAC) are directly connected, bypassing the need for a cable. In Ethernet implementations where the DTE and MAU are combined, the AUI is typically omitted.

The IEEE 802.3 specification officially defines the AUI as an interconnect between a DTE and the MAU. However, devices like the DEC Digital Ethernet Local Network Interconnect (DELNI) provided hub-like functionality using AUI-compatible connectors.https://www.zx.net.nz/computers/dec/networks/DELNI/https://vt100.net/mirror/hcps/delnitm1.pdf Additionally, under certain conditions, it was possible to directly connect two AUI devices without the need for transceivers using a crossover cable.https://serverfault.com/questions/375782/is-it-possible-to-cross-connect-two-aui-ports-without-transceivers

AUI connectors became increasingly rare in the early 1990s as computers and hubs directly integrated the MAU, especially with the rising adoption of the 10BASE-T standard. This shift led to the decline of 10BASE5 (thicknet) and 10BASE2 (thinnet) which made use of the interface.{{Cite web |url=https://www.mattmillman.com/projects/usbaui/ |title=Archived copy |access-date=2024-10-02 |archive-date=2024-08-28 |archive-url=https://web.archive.org/web/20240828194459/https://www.mattmillman.com/projects/usbaui/ |url-status=live }}{{cite web | url=https://servercloud.com/the-evolution-of-network-port-sizes-from-10base-t-to-100-gigabit-ethernet/ | title=The Evolution of Network Port Sizes: From 10BASE-T to 100 Gigabit Ethernet | date=June 2022 | access-date=2024-10-02 | archive-date=2024-06-14 | archive-url=https://web.archive.org/web/20240614184417/https://servercloud.com/the-evolution-of-network-port-sizes-from-10base-t-to-100-gigabit-ethernet/ | url-status=live }} The electrical AUI connection remained internally within equipment for some time.

With the introduction of Fast Ethernet, the AUI interface became obsolete and was replaced by the Media Independent Interface (MII).[https://40gethernet.wordpress.com/2009/05/08/overview-of-the-xaui-xlaui-and-caui-part1/ Overview of the XAUI, XLAUI and CAUI: Part1] {{Webarchive|url=https://web.archive.org/web/20190517201905/https://40gethernet.wordpress.com/2009/05/08/overview-of-the-xaui-xlaui-and-caui-part1/ |date=2019-05-17 }}, "...the existing AUI was not suitable and it was replaced by the MII interface..." Subsequent Ethernet standards, such as Gigabit Ethernet and 10 Gigabit Ethernet, introduced the GMII and XGMII interfaces, respectively. A 10 Gigabit Ethernet interface, known as XAUI, was developed to extend the operational distance of XGMII and reduce the number of interface signals.

A smaller variant called the Apple Attachment Unit Interface (AAUI) was introduced on Apple Macintosh computers in 1991, and was phased out by 1998.{{cite web|last=Dan|first=Knight|title=Apple's AAUI Ethernet Connector|url=http://lowendmac.com/tech/aaui.html|access-date=13 February 2012|archive-date=26 March 2009|archive-url=https://web.archive.org/web/20090326062546/http://lowendmac.com/tech/aaui.html|url-status=live}}