You use the INTRDEF function enable the interrupt input channel, the function number of the interrupt service routine tie to this channel, and the interrupt edge (rising, falling or both edges).
The interrupt service has the priority above the ladder logic scan. Also if the CPU is executing a custom function then the ISR will run after the current statement has finished execution. i.e the interrupt service routine does not need to wait for the current function to return before it is called.
By default the ISR is not called when the PLC is performing its I/O scan cycle (update of its physical inputs and outputs status). This can be changed by the SETSYSTEM 256, 1.
The priority of the user-defined interrupt is in numerical order. i.e. Interrupt 1 has higher priority than 2, and 2 has higher priority than 3, etc.
