USB Connected Monitors
ddcutil supports monitors that implement the Monitor Control Command Set (MCCS) over USB, following the USB Device Class Definition for Human Interface Devices (HID) ) and the USB Monitor Control Class Specification.
Note that some monitors use USB to communicate, but do not follow the specification for MCCS over USB. For example, the HP Dreamcolor 2480zx uses a proprietary, undocumented protocol.
Also, bears emphasizing that when ddcutil refers to USB connected monitors, it means monitors that communicate MCCS over USB, NOT to monitors that communicate the video signal over USB.
Limitations:
- The MCCS over USB specification has no mechanism for querying a capabilities string. The ddcutil capabilities
command synthesizes a response based on the feature codes it detects.
- Table type features are completely unsupported.
- Apple Thunderbolt displays might work, but are not supported. Thunderbolt integrates video, USB, and ethernet into a single cable, and
appears to require a Thunderbolt capable computer, as I can find no adapters that split out all the signals.
Other than actual Macs, these are uncommon.
On the other hand, MCCS over USB has advantages:
- Unlike I2C, the protocol is inherently reliable. No retry logic is needed.
- Unlike I2C, USB communication does not require waits between system calls. It is therefore faster.
Thanks to Ojdrej Zary, whose usbmonctl was used as the basis for USB support in ddcutil.
If there are multiple monitors, a USB connected monitor can be selected in the usual manner using its display number (option --display), or by its bus number/device number pair (option --usb) or hiddev device number (option --hiddev).
There are also 2 ddcutil commands specific to USB connected monitors, chkusbmon, and usbenvironmentc
For details on USB device permissions, see USB Device Permissions.
Options for Display Selection
Option: --usb
USB connected monitors can be specified by their USB bus number and device number. The numbers are separated by either a period or colon.
For example:
ddcutil --usb 3.5 ...
selects the monitor at USB bus number 3, device number 5.
Option --hiddev
USB connected monitors can also be specfied by their hiddev device number, which specifies the /dev file by which they are accessed. That is, --hiddev 2 refers to /dev/usb/hiddev2.
Command ddcutil chkusbmon
Command ddcutil chkusbmon helps to detect USB HID compliant monitors. It is intended for use in udev rules.
It uses Read-Only access to check that a USB device is of class User Interface Device, but is not a keyboard or mouse.
ddcutil chkusbmon ***hiddev device name***
e.g.
ddcutil chkusbmon /dev/usb/hiddev3
Returns 0 if a device represents a USB attached monitor, non-zero if not. See Device Permissions
Command ddcutil usbenvironment
Explores the system's USB environment as an aid to diagnosing USB device detection and communication issues.
Option --verbose increases the amount of information.
The word "usbenvironment" can be abbreviated down to its first 6 letters, e.g.
$ ddcutil usbenv