View source for Attachment Unit Interface

{{Short description|Ethernet standard interface}}
[[File:AUI Connectors.jpg|thumb|AUI Connectors. The male connector (left) is on the MAU and the female connector (right) is on the DTE device (typically either a [[computer]] or an [[Ethernet hub]]). Note the sliding clip.]]
[[File:AUI connector on DEC EtherWorks LC (DE100).jpg|thumb|AUI Connector with numbered pins on the [[Digital Equipment Corporation|DEC]] 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 [[Federal_Information_Processing_Standards|FIPS]] PUB 107) for [[10BASE5]] Ethernet<ref name="spec">{{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}}</ref> and the earlier [[Ethernet#History|DIX standard]]. The physical interface consists of a 15-pin [[D-subminiature]] connector that links an [[Ethernet]] node's [[Physical layer#Physical signaling sublayer|physical signaling]] to the [[Medium Attachment Unit]] (MAU),<ref>IEEE 802.3 ''7. Physical Signaling (PLS) and Attachment Unit Interface (AUI) specifications''</ref> 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 [[Digital Equipment Corporation|DEC]] [[Digital Ethernet Local Network Interconnect]] (DELNI) provided hub-like functionality using AUI-compatible connectors.<ref>{{cite web | title=DELNI - zxnet | url=https://www.zx.net.nz/computers/dec/networks/DELNI/ }}</ref><ref>https://vt100.net/mirror/hcps/delnitm1.pdf {{Bare URL PDF|date=July 2025}}</ref> Additionally, under certain conditions, it was possible to directly connect two AUI devices without the need for transceivers using a [[crossover cable]].<ref>{{cite web | title=Is it possible to cross-connect two AUI ports without transceivers? | url=https://serverfault.com/questions/375782/is-it-possible-to-cross-connect-two-aui-ports-without-transceivers }}</ref>

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.<ref>{{Cite web |url=https://www.mattmillman.com/projects/usbaui/ |title=USB Ethernet Adapter with AUI interface – Matt&#039;s Tech Pages |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 }}</ref><ref>{{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 }}</ref> 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).<ref>[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..."''</ref> 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.<ref>{{cite web|last=Dan|first=Knight|title=Apple's AAUI Ethernet Connector|date=4 September 2007 |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}}</ref>

== Modes ==
The AUI can operate in both ''normal mode'' and ''monitor mode''. In normal mode, it functions as a direct connection between the DTE and the network medium. Monitor mode, an optional feature, isolates the MAU's transmitter from the medium while allowing the DTE to observe network activity. This mode is useful for diagnostic and monitoring purposes without impacting the physical medium.<ref name="spec"></ref>

== Signaling and control ==
The AUI uses [[Manchester encoding]] for data transmission, which ensures clock synchronization without requiring a separate timing signal. The data and control circuits operate independently and are self-clocked. Control signals coordinate communication between the DTE and MAU, enabling error signaling, MAU isolation, and medium access requests.<ref name="spec"></ref>

== Connector and signals ==
An AUI connector is a '''DA-15''' ([[D-subminiature]]) type, where the DTE side has a female connector and the MAU side has a male connector.<ref name="spec"></ref>

The connector often uses a sliding clip instead of the typical thumbscrews found on D-connectors, allowing the DTE and MAU to be directly attached, even when their size or shape would not accommodate thumbscrews. However, the clip mechanism is sometimes considered awkward or unreliable.<ref>{{Cite web |url=https://www.mattmillman.com/projects/10base5/#comment-35680 |title=Building a 10BASE5 "Thick Ethernet" network – Matt&#039;s Tech Pages |access-date=2024-10-02 |archive-date=2024-08-29 |archive-url=https://web.archive.org/web/20240829114711/https://www.mattmillman.com/projects/10base5/#comment-35680 |url-status=live }}</ref>

In the case of incompatible fittings, the jackposts or sliding clip can be unscrewed and replaced, or adapter dongles and cables can be used.

Electrically, the AUI's differential signals are designed for use with a 78 Ω cable and can transmit data between DTE and MAU at 10 Mbps over the standard's specified 50-meter length.<ref name="spec"></ref>

AUI drivers and receivers are required to tolerate wiring faults without permanent impairment of the pair. Signal jitter is controlled to within ''1.5 nanoseconds'' across the interface.<ref name="spec"></ref>

== Pinout table ==
The DA-15 [[pinout]] is specified by the IEEE 802.3 standard and describes four differential pairs:

{| class="wikitable sortable " style="text-align: center;" | 
|-
|+!colspan="3" align="center"|Attachment Unit Interface [[Image:DA-15 DSubminiatures female.svg|300px]]
|-
!  Pin || Signal || Direction || Description
|-
| {{0}}3 || DO-A || AUI→MAU || style="text-align: left;" | Data Out Circuit A
|-
| 10 || DO-B || AUI→MAU || style="text-align: left;" | Data Out Circuit B
|-
| 11 || DO-S || AUI→MAU || style="text-align: left;" | Data Out Circuit Shield {{efn-lg|name=shield|Grounded in AUI and capacitively coupled to V<sub>C</sub> in MAU <ref name="spec"></ref>}}
|-
| {{0}}5 || DI-A || MAU→AUI || style="text-align: left;" | Data In Circuit A
|-
| 12 || DI-B || MAU→AUI || style="text-align: left;" | Data In Circuit B
|-
| {{0}}4 || DI-S || MAU→AUI || style="text-align: left;" | Data In Circuit Shield {{efn-lg|name=shield}}
|-
| {{0}}7 || CO-A || AUI→MAU || style="text-align: left;" | Control Out Circuit A
|-
| 15 || CO-B || AUI→MAU || style="text-align: left;" | Control Out Circuit B
|-
| {{0}}8 || CO-S || AUI→MAU || style="text-align: left;" | Control Out Circuit Shield {{efn-lg|name=shield}}
|-
| {{0}}2 || CI-A || MAU→AUI || style="text-align: left;" | Control In Circuit A
|-
| {{0}}9 || CI-B || MAU→AUI || style="text-align: left;" | Control In Circuit B
|-
| {{0}}1 || CI-S || MAU→AUI || style="text-align: left;" | Control In Circuit Shield {{efn-lg|name=shield}}
|-
| {{0}}6 ||   V<sub>C</sub> || – || style="text-align: left;" | Voltage Common (0 V) {{efn-lg|name=voltage|These two pins (V<sub>C</sub>/VP) use a single twisted pair between AUI and MAU <ref name="spec"></ref>}}
|-
| 13 ||   VP || – || style="text-align: left;" | Voltage Plus (+12 V) {{efn-lg|name=voltage}} {{efn-lg|VP is optional, but must provide a stable voltage for all current less than 500 mA <ref name="spec"></ref>}}
|-
| 14 ||   VS || – || style="text-align: left;" | Voltage Shield {{efn-lg|name=shield}}
|-
| Shell ||   PG || – || style="text-align: left;" | Protective Ground
|}

{{notelist-lg}}

== References ==
{{Reflist}}

== External links ==
* {{Commons category inline|Attachment Unit Interface}}

{{Ethernet}}

[[Category:Computer connectors]]
[[Category:Ethernet]]