How to Determine if LCD Monitor is Turned on From

Question

How do you tell if a computer's monitor(s) are turned on/off from the command line in Linux? I've traditionally thought of monitors as output-only devices, but I've noticed the Gnome Monitor Preferences dialog has a "detect monitor" function. Can this be generalized to determine if a monitor is physically turned off?

Answer 1

The VESA DDC connection is an I2C connection that can be used to query the presence of the monitor.

Linux exposes the I2C device and userland programs can communicate directly with the monitor with code such as that at http://jaffar.cs.msu.su/oleg/ddcci/

Notice this below: Control 0xe1: +/1/1 [SAM: Power control (0 - off/1 - on)]

# ddcci-tool /dev/i2c-2 -e -c -d


ddcci-tool version 0.03

Reading EDID : 0x50@/dev/i2c-2
    Plug and Play ID: SAM00BA
    Input type: Analog

Using ddc/ci : 0x37@/dev/i2c-2

Capabilities:
(type(LCD)vcp(04 05 10 12 60(1 3) B0(1 2) B6 C6 C8 C9 D6(1 4) DC(1 2 3 4) DF))

Controls (valid/current/max):
Control 0x04: +/0/1 [Reset Factory Defaults]
Control 0x05: +/0/1 [SAM: Reset Brightness and Contrast]
Control 0x06: +/0/1 [Reset Factory Geometry]
Control 0x08: +/0/1 [Reset Factory Default Color]
Control 0x0e: +/60/120 [SAM: Image Lock Coarse]
Control 0x10: +/0/100 [Brightness]
Control 0x12: +/50/100 [Contrast]
Control 0x16: +/8/16 [Red Video Gain]
Control 0x18: +/8/16 [Green Video Gain]
Control 0x1a: +/8/16 [Blue Video Gain]
Control 0x1e: +/0/2 [SAM: Auto Size Center]
Control 0x20: +/50/100 [Horizontal Position]
Control 0x30: +/25/54 [Vertical Position]
Control 0x3e: +/39/50 [SAM: Image Lock Fine]
Control 0x60: +/1/3 [Input Source Select]
Control 0x62: +/0/100 [Audio Speaker Volume Adjust]
Control 0x6c: +/140/255 [Red Video Black Level]
Control 0x6e: +/127/255 [Green Video Black Level]
Control 0x70: +/121/255 [Blue Video Black Level]
Control 0xb0: +/0/2 [Settings]
Control 0xb6: +/3/8 [???]
Control 0xc6: +/1/1 [???]
Control 0xc8: +/5/16 [???]
Control 0xc9: +/1/0 [???]
Control 0xca: +/2/2 [On Screen Display]
Control 0xcc: +/2/11 [SAM: On Screen Display Language]
Control 0xd6: +/1/4 [SAM: DPMS control (1 - on/4 - stby)]
Control 0xdc: +/4/4 [SAM: MagicBright (1 - text/2 - internet/3 - entertain/4 - custom)]
Control 0xdf: +/512/0 [VCP Version]
Control 0xe0: +/0/2 [SAM: Color preset (0 - normal/1 - warm/2 - cool)]
Control 0xe1: +/1/1 [SAM: Power control (0 - off/1 - on)]
Control 0xe2: +/0/1 [???]
Control 0xed: +/108/255 [SAM: Red Video Black Level]
Control 0xee: +/101/255 [SAM: Green Video Black Level]
Control 0xef: +/103/255 [SAM: Blue Video Black Level]

An interesting question is whether or not your monitor returns that piece of data, and if not, whether it responds at all if it's currently turned off.