Attachment Unit Interface

The Attachment Unit Interface (AUI) is a physical and logical interface defined in the original IEEE 802.3 standard for 10BASE5 Ethernet.[1] The (optional) physical interface consists of a 15-pin connection that provides a path between an Ethernet node's Physical Signaling and the Medium Attachment Unit (MAU),[2] sometimes also known as a transceiver. An AUI cable may be up to 50 metres (160 feet) long, although frequently the cable is omitted altogether and the MAU and MAC are directly attached to one another.

AUI Connectors. The male connector (left) is on the MAU and the female connector (right) is on the MAC device (normally either a computer or an Ethernet hub)
AUI Connector with numbered pins on the DEC EtherWorks LC (DE100) ethernet controller

AUI connectors became rare beginning in the early 1990s when computers and hubs began to incorporate the MAU, particularly as the 10BASE-T standard became more common and use of 10BASE5 (thicknet) and 10BASE2 (thinnet) declined. The electrical AUI connection was still present inside the equipment. With the introduction of Fast Ethernet the AUI became obsolete, and was replaced by the Media Independent Interface (MII).[3] Gigabit Ethernet and 10 Gigabit Ethernet have respectively the GMII and XGMII interfaces.

A modified form using a smaller connector called the AAUI was introduced on Apple Macintosh computers in 1991, and its use discontinued in 1998.[4]

Connector and signals

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 pinout
PinSignalDescription
3DO-AData Out Circuit A
10DO-BData Out Circuit B
11DO-SData Out Circuit Shield (not used)
5DI-AData In Circuit A
12DI-BData In Circuit B
4DI-SData In Circuit Shield
7CO-AControl Out Circuit A (not used)
15CO-BControl Out Circuit B (not used)
8CO-SControl Out Circuit Shield (not used)
2CI-AControl In Circuit A
9CI-BControl In Circuit B
1CI-SControl In Circuit Shield
6VCVoltage Common (0 V)
13VPVoltage Plus (+12 V)
14VSVoltage Shield (not used)
ShellPGProtective Ground

See also

References
  1. "IEEE Standard for Ethernet, Section 1, Clause 7". IEEE Computer Society. Retrieved 14 February 2017.
  2. IEEE 802.3 7. Physical Signaling (PLS) and Attachment Unit Interface (AUI) specifications
  3. Overview of the XAUI, XLAUI and CAUI: Part1, "...the existing AUI was not suitable and it was replaced by the MII interface..."
  4. Dan, Knight. "Apple's AAUI Ethernet Connector". Apple's AAUI Ethernet Connector. Retrieved 13 February 2012.
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