Dr. Marc Zuckman reports that one can completely power off the Coolpix 990
completely (to keep the CCD cool, thereby minimizing thermal noise) without
having to disconnect and reconnect the USB cable, if you follow these
instructions on Linux 2.4.3mdk with firmware 1.1. However, I was not able
to reproduce this behaviour under Linux 2.4.4 (RedHat 6.2), so it seems
that the following procedure does not work with all hardware/software
configurations:

1. Turn the camera on and wait for the display to come on.

2. Plug in the USB (or serial) cable. 

3. Press xphoto OFF button (or execute "photopc -u mode Off").

4. Turn camera operations dial to off. The camera WILL be recognized with
the next use merely by turning it on again.

5. Turn camera operations wheel to A-rec from off (with all cables still
attached).

6. Press the xphoto ON/RESET button (or execute "photopc -u mode Record").

7. Take photos.

8. Go to step 3.