Page 6-7
Manual 00650-526-1
Sharing Interrupts on the ISA Bus
As noted on page 1-1, DIO48S/AT-P can facilitate an external interrupt via bit C3 at each 24-bit
port. (The external interrupt is OR'ed with the change-of-state interrupt.) On occasion, however, a
system application will require more interrupt levels than are available on the ISA bus. While this
is not recommended, IRQ sharing is possible. Each card that is going to share an IRQ must
strictly adhere to a special standard for accessing the IRQ lines as follows:
1.
The interrupt must be held in a high impedance state until asserting an interrupt.
2.
The interrupt must be asserted in the form of a low signal lasting at least 500 nanoseconds
followed by a rising edge and then immediately returning to a high impedance condition.
3.
The card must contain a status register or flag of some kind to indicate that it generated the
interrupt. There is an exception to this rule. This is the case where only one card (of those
sharing the interrupt level) does not provide a status bit to indicate that it asserted the
interrupt but is otherwise capable of sharing the IRQ. In this case, it may share the
interrupt level with other cards if (a) it is the only card on that IRQ level that does not
have a status bit and (b) it is installed onto the IRQ vector first. (This makes it the last
card to be called in the vector chain.) This scheme will work because it can be assumed
that if every other card in the vector chain did not cause the interrupt, then the last card
must be the one that did.
Note
Note that if two cards assert the IRQ line within 500 nanoseconds of each other, the second card in
the ISR chain will not be serviced. It is possible to alleviate this problem by writing a single ISR
that can detect the bit flag on every card and therefore detect the fact that two cards (or more)
report generating an interrupt even though only one interrupt was processed by the CPU.
Summary of Contents for DIO48S/AT-P Series
Page 1: ...DIO48S AT P Series Product Manual MANUAL NUMBER 00650 526 1B...
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Page 13: ...DIO48S AT P Manual Page 1 4 Manual 00650 526 1 Figure 1 1 DIO48S AT P Block Diagram...
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