Design Ideas: October 26,, 1995
In "Software configures IBM PC interrupts," by Said Jackson (EDN, Nov 10, 1994, pg 90), the author addresses a problem and proposes a practical solution. The circuit in the author's design uses four ICs and a resistor package and requires an external address decoder. A more practical solution is the GAL20V8, or equivalent, PLD (Fig 1), which integrates most of the vital features of Jackson's circuit in one chip.
You can control virtually all settings that use headers and jumpers through software; indeed, newer PC add-on boards usually use such jumperless control. The interrupt-select circuit here is a demultiplexer with tristate outputs and routes board-generated request signals to one of the available interrupt-request (IRQ) bus lines. Its operation closely resembles setting a jumper: The selected (closed-switch) output follows the input state, and all other outputs assume tristate, open-switch status.
You can also disable interrupts by setting all outputs to the off state; this action corresponds to removing jumpers. The all-off condition resumes upon system reset as well. In addition to demultiplexing, the circuit integrates an address decoder and an asynchronous latch. Because the PLD uses only address lines, interrupt selection requires "OUTing" (a CUPL term) to predefined locations. The decoder senses 10 address bits and an address-enable signal that distinguishes CPU and DMA-bus transactions.
The PLD's SEL output flags CPU accesses to the address area that contains locations reserved for IRQ selection, as well as to several free locations. This output drives a simple decoder in which the SEL signal, IOW and the least significant address lines generate controls for onboard logic. You can download the CUPL code from EDN BBS /DI_SIG #1772. The definitions in the PLD description allot locations 3E016 to 3E416 for IRQ control. SEL signals accesses to 3E016 to 3E716. You can, thus, map onboard ports to locations 3E516, 3E616, and 3E716. The base address and range are definable, so you can easily move and resize the window. (DI #1772)