I would like to turn off my dual monitors by running:
xset dpms force off but just the primary monitor is beeing turned off while the other just gets a "black screen" (like if it was turned off) but it is not off. The led is still gleaming green indicating the monitor is on.
What exactly are you trying to accomplish? For managing monitor usage you can/should use the randr extension where
If you actually want the monitor to turn off, your problem is that
In effect I'd guess you have to issue your
Check the output of
Clarification of the question: I'm guessing Rox wants to turn off his monitors when he goes away (but leave the computer running). This normally happens after the screensaver has shown pretty patterns for a while. Using the "xset dpms force off" command is a way to be able to test if things are working without having to wait half an hour for the DPMS screensaver to kick in. (you can also set the timeout to DPMS off really short, but that's just another way of speeding up the testing).
I just tested this on my system. It has: 01:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI RV620 LE [Radeon HD 3450] and when I type "xset dpms force off" both monitors show "no signal" for three seconds and then turn the led amber (i.e. sleep mode). "it works for me"!
So... To figure out what's wrong in your situation: Can you swap monitors to verify that your second monitor is DPMS capable? If that's the case, as I expect, this might be a bug. Probably quite specific to your chip/software. Are you running the latest software? (e.g. ubuntu 12.04).