Solution:

All interrupt inputs on PCL-833 are controlled and managed by on-board 8259 interrupt controller. You can set the interrupt mast register to disable the index input interrupt. The demo program enable all the interrupt inputs, so it will resoponse to any interrupt input. When you write your program, you can set the interrupt mast register to disable any interrupt input you want. Please refer to manual for 8259 registers and controls. The 8259 mask register is at base+13 register. The relative bits for each interrupt input are as follows: bit 0 - counter 1 overflow bit 1 - counter 2 overflow bit 2 - counter 3 overflow bit 3 - counter 1 index in bit 4 - counter 2 index in bit 5 - counter 3 index in bit 6 - DI0 input bit 7 - DI1 input or Timer pulse (bit 3 of base+9 selects DI1 or timer)