always out put redstone signal

so i have a piston under my computer and that i need to retract when a password has been entered (it opens the a ladder) all i dont know how to do is leave a redstone signal always on please help

You can turn on a signal from a computer with rs.setOutput("side",true) but it will go off when the computer is rebooted. the best way would be to invert the signal using a redstone torch. that way it will be power by default ant the computer will not affect it when it reboots.
[EXAMPLE]

Spoiler

If your using Redpower put a nor gate in the redstone circuit for the piston.

thanks