The Attachment Unit Interface (AUI) is the IEEE 802.3 Clause 7 interface between an Ethernet station’s MAC layer and its Medium Attachment Unit (MAU), defined since IEEE 802.3-1985. It uses a 15-pin DA-15 D-subminiature connector with a sliding-clip latch and carries four differential-pair circuits — Data Out (DO), Data In (DI), Control In (CI, collision detection), and Control Out (CO) — together with a +12 V power supply and per-circuit shields. This null-transceiver cable connects two network interface cards directly by crossing the DO and DI pairs, removing the need for external transceivers. A jumper at each end ties VC (pin 6) to Collision Detect B (pin 9) to enable collision detection signalling in the loopback arrangement.
15 pin D-SUB male connector to the network interface card 1
15 pin D-SUB male connector to the network interface card 2
| Name | AUI1 | AUI2 | Name |
|---|---|---|---|
| TxD A | 3 | 5 | RxD A |
| RxD A | 5 | 3 | TxD A |
| TxD B | 10 | 12 | RxD B |
| RxD B | 12 | 10 | TxD B |
And make the following jumper at each end:
| Name | Pin |
|---|---|
| Vc | 6 |
| Collision Detect B | 9 |
Notes
The AUI cable has a characteristic impedance of 78 Ω; the maximum drop cable length is 50 m for 10BASE-5 and 1.33 m for 10BASE-2. The DA-15 connector uses a sliding-clip latch rather than thumbscrews. On the DTE side the DA-15 is female; on the MAU side it is male. The CO circuit (AUI pins 7, 8, and 15) is not used in this wiring and may be left unconnected.
References
- IEEE 802.3-1985: CSMA/CD Access Method and Physical Layer Specifications — IEEE Standards Association
- Apple Tech Info Library KB009980: AAUI, Pinout Equivalents to AUI — Apple Computer, Inc. (archived by Steve Savage)