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Appendix B
Register-Level Programming
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National Instruments Corporation
B-25
DAQCard-DIO-24 User Manual
single interrupt output for port A. After enabling interrupts from
the 82C55A, set the appropriate enable bit or bits in Interrupt
Register 1; for example, if you selected both mode 2 interrupts from the
82C55A, you would set IRQ0 in order to interrupt the host computer.
External signals can be used to interrupt the DAQCard-DIO-24 when
port A or port B is in mode 0 and the low nibble of port C is configured
for input. If port A is in mode 0, use PC3 to generate an interrupt; if
port B is in mode 0, use PC0 to generate an interrupt. When you have
configured the 82C55A, you must set the corresponding interrupt
enable bit in Interrupt Register 1. If you are using PC3, set IRQ0; if you
are using PC0, set IRQ1. When the external signal becomes logic high,
an interrupt request occurs.
Although the host computer's interrupt-monitoring circuitry is triggered
by the positive-going edge of the interrupt signal, the signal must
remain high until the interrupt routine is entered and interrupts are
masked out. Make sure your external interrupt signal meets these
qualifications. To disable the external interrupt, clear the appropriate
IRQx bit or clear the INTEN bit.