Attachment Unit Interface

Short description: Physical and logical interface defined in the original Ethernet standard

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)

The Attachment Unit Interface (AUI) is a physical and logical interface defined in the original IEEE 802.3 standard for 10BASE5 Ethernet and the previous DIX standard. The physical interface consists of a 15-pin D-subminiature connection that provides a path between an Ethernet node's physical signaling and the Medium Attachment Unit (MAU), 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 medium access controller MAC are directly attached to one another. On Ethernet implementations without separate MAU and MAC, the AUI is omitted.

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). Gigabit Ethernet and 10 Gigabit Ethernet have respectively the GMII and XGMII interfaces. A 10 Gigabit Ethernet interface called XAUI was developed to extend the operational distance of the XGMII and to reduce the number of interface signals.

A modified form using a smaller connector called the Apple Attachment Unit Interface (AAUI) was introduced on Apple Macintosh computers in 1991, and its use discontinued in 1998.

Connector and signals

AUI Connector with numbered pins on the DEC EtherWorks LC (DE100) Ethernet controller

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

Pin	Signal	Direction	Description
03	DO-A	AUI→MAU	Data Out Circuit A
10	DO-B	AUI→MAU	Data Out Circuit B
11	DO-S	AUI→MAU	Data Out Circuit Shield (not used)
05	DI-A	MAU→AUI	Data In Circuit A
12	DI-B	MAU→AUI	Data In Circuit B
04	DI-S	MAU→AUI	Data In Circuit Shield
07	CO-A	AUI→MAU	Control Out Circuit A (not used)
15	CO-B	AUI→MAU	Control Out Circuit B (not used)
08	CO-S	AUI→MAU	Control Out Circuit Shield (not used)
02	CI-A	MAU→AUI	Control In Circuit A
09	CI-B	MAU→AUI	Control In Circuit B
01	CI-S	MAU→AUI	Control In Circuit Shield
06	VC	AUI→MAU	Voltage Common (0 V)
13	VP	AUI→MAU	Voltage Plus (+12 V)
14	VS	AUI→MAU	Voltage Shield (not used)
Shell	PG	AUI→MAU	Protective Ground