pulse generator

I need a circuit that sends a stream of 0's to a parallel port by default when there is no voltage input, and then send 1's while voltage is applied, and returns to sending 0's when the voltage is removed. The 0's and 1's have to be sent at a specific rate somewhere around 1KHz. Can anyone point me in the right direction as to what kind of circuit I should be looking for?

-James

A stream of 0's is just a continuous low level, and a string of 1's is a continuous high level. There's no need to send any kind of pulses.