The daisy-chaining method of creating priority includes a serial connection of all devices that request an interrupt. The device with the highest priority is located in the first position, followed by lower-priority devices up to the device with the lowest priority, which is situated last in the chain. This technique of connection between three devices and the CPU.
The interrupt request line is average to all devices and design a wired logic connection. If some device has its interrupt signal in the low-level state, the interrupt line goes to the low-level state and enables the interrupt input in the CPU. When no interrupts are pending, the interrupt line continues in the high-level state and no interrupts are identified by the CPU. This is similar to a negative logic OR operation.
The CPU responds to an interrupt request by enabling the interrupt to acknowledge the line. This signal is acknowledged by device 1 at its PI (priority in) input. The acknowledge signal passes on to the next device through the PO (priority out) output only if device 1 is not requesting an interrupt.
If device 1 has a pending interrupt, it blocks the acknowledge signal from the next device by locating a 0 in the PO output. It then proceeds to insert its interrupt vector address (VAD) into the data bus for the CPU to use during the interrupt cycle.
A device with a 0 in its PI input generates a 0 in its PO output to inform the next-lower-priority device that the acknowledged signal has been blocked. A device that is requesting an interrupt and has a 1 in its PI input will intercept the acknowledge signal by placing a 0 in its PO output.
If the device does not have pending interrupts, it transmits the acknowledge signal to the next device by placing a 1 in its PO output. Thus the device with PI = 1 and PO = 0 is the one with the highest priority that is requesting an interrupt, and this device places its VAD on the data bus.
The daisy chain arrangement provides the highest priority to the device that receives the interrupt acknowledge signal from the CPU. The farther the device is from the first position, the lower is its priority.
It displays the internal logic that should be included within each device when linked in the daisy-chaining scheme. The device sets its RF flip-flop when it needs to interrupt the CPU. The output of the RF flip-flop goes through an open-collector inverter, a circuit that supports the wired logic for the common interrupt line.