Priya,
When doing pulse train generation, a pulse is generated by toggling the output of the counter after some number of clocks have been counted. In this case, a clock will be routed to the source of the counter and the counter will toggle it's output after 'lowTicks' periods of this clock, and again after 'highTicks' periods of this clock. You have specified an empty string for the counter's source, so the counter's default source will be used. For M Series (of which PXI-6229 is one) the default clock is a 80 MHz clock. Therefore, lowTicks and highTicks specify how long the counter's output will be low and high in terms of periods of this 80 MHz clock.
Hope that helps,
Dan
