From that, I take that you want the CPU to evaluate the signals programmatically.
At 2000 Hz, you would have to scan the signal every 0.2 millisecond - or faster ! (*). That is impossible in the PLC5 program, even if it was dedicated to this application alone.
Even Selectable Timed Interrupts cannot be faster than 10 milliseconds. And if you programmed Input Interrupts, I dont think it would be possible, but if it is possible it would make the CPU do nothing else but react to the interrupts.
Dedicated counter cards in the PLC, or standalone counter to analog converter, or changing to something else than 2kHz pulse signals is the solution to your problem.
*: 0.2 milliseconds scan assumes that the pulses have evenly distributed pause and pulse periods. If the pause and pulse is not evenly distributed, the scantime would have to be proportionally shorter !