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The Emerald serial expansion board(s), serving serial ports ttyS5 to tty12 (and ttyS13-ttyS20 on m21) are configured to use ISA interrupt 3. The PC104 interrupts (in this case just IRQ 3) get multiplexed by a CPLD on the CPU board into a GPIO interrupt. On vipers, the PC104/GPIO interrupt is number 25, GPIO line 01. On titans, the PC104/GPIO interrupt is number 129, on GPIO 17.
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