titus-larsen-titus-1981-apple-interfacing

If this type of a "polled" interrupt is used, with the computer polling
each of the devices that could have generated an interrupt, a priority
can be established in the software. Thus, if the computer checks
the devices in the order A, B, C, and so on, device A has the highest
priority, since it will be checked first when an interrupt is detected.
The interface slots also have two other interrupt lines that may be
of interest, depending upon your application. These signals are the
interrupt input (INT IN) at pin 28, and the interrupt output (INT
OUT) at pin 23. These signals are used to "daisy chain" interrupt
signals form one card to the next. The signals are only connected between
the interface connectors, as shown in Fig. 7-6. Thus, the INT
OUT signal on slot 1 is connected to the INT IN signal on slot 2, the
INT OUT on slot 2 is connected to the INT IN on slot 3, and so on.
The INT IN and IN OUT lines are only connected to the adjacent
interface slot, and they do not go any further.
REAR OF APPLE
0
23 28 23 23 28
INT OUT
INT IN
*NOTATIONS ARE INTERCHANGEABLE, DEPENDING UPON USE.
A simple daisy-chained interrupt scheme is shown in Fig. 7-7, The
lower-priority interrupting devices are further down the chain, further
from the INT connection to the 6502 microprocessor chip. In
this circuit, a higher priority device can pass its interrupt request up
the chain, blocking any interrupt requests from the lower priority
devices that are further down the chain. Once the higher-priority
device has been serviced and its interrupt flag has been cleared, it
will "open" its gate and allow the lower priority interrupt request to
pass on to the computeL
As you can see, the computer still needs some way of determining
which device is generating the interrupt, so that it can select the corresponding
service subroutine. This type of