Abstract: An interface which connects input/output (I/O) controllers to a data channel in a data processing system. A bidirectional priority bus is provided interconnecting the channel with the controllers. Each controller is assigned a priority level. When a controller requires service, it signals the channel over a common request line and the channel responds with a channel select signal. Each requesting controller gates a binary number corresponding to its priority level onto the common priority bus. Contending controllers resolve priority among themselves by monitoring the priority bus. If a controller detects a higher priority level than its own level on the bus it removes its priority number from the bus. The highest priority controller then activates an acknowledge signal and places its device address on a bidirectional data bus in response to a ready signal from the channel.