The five interface management lines (ATN, EOI, IFC, REN, SRQ) manage the flow of control and data bytes across the interface.
The ATN (Attention) signal is asserted by the controller to indicate that it is placing an address or control byte on the data bus. ATN is released to allow the assigned talker to place status or data on the data bus. The controller regains control by reasserting ATN; this is normally done synchronously with the handshake to avoid confusion between control and data bytes.
The EOI (End or Identify) signal has two uses. A talker may assert EOI simultaneously with the last byte of data to indicate end-of-data. The controller may assert EOI along with ATN to initiate a parallel poll. Although many devices do not use parallel poll, all devices should use EOI to end transfers (many currently available devices do not).
The IFC (Interface Clear) signal is asserted only by the system controller in order to initialize all device interfaces to a known state. After releasing IFC, the system controller becomes the active controller.
The REN (Remote Enable) signal is asserted only by the system controller. Its assertion does not place devices into remote control mode; REN only enables a device to go into remote mode when addressed to listen. When in remote mode, a device should ignore its local front panel controls.
The SRQ (Service Request) line is like an interrupt: it may be asserted by any device to request the controller to take some action. The controller must determine which device is asserting SRQ by conducting a serial poll. The requesting device releases SRQ when it is polled.
