Ah! Now I know why we can't tune in on the same wave lenght. The chargepump is as Jeff said, a watchdog signal. Unfortunately, if you do not control all the Parallel port signals, and at start up, the states of the pins is unknown, and with my breakout boards which has led indicators, I can see unvoluntary activety on the port pins on all PC setups I've seen. But having a signal at some khz is very unlikely to be coincidental uncontrolled port pin activety. I have worked with the Rofin Sinar lasers and Messer Griesheim CNC machines some years back when Messer Griesheim was still a good company to work for (sorry to folks at Messer) and believe me, a watchdog is necesary.
The best way and the only way I've seen industry specific implementation of the watchdog signal, was a constant frequency similar to Mach3 on a decoupled circuit. Mostly inductive decoupling. If you don't mind, how were you going to implement this signal?
Whacko
Addon:
I see you did answer that question, your post slipped by whilst I was typing mine.