Monitor hotplugging and DDC

Markus Kuhn Markus.Kuhn at
Wed Sep 8 03:12:29 PDT 2004

With regard to the recent discussion on what should happen if a X server
starts up while the DDC monitor is still switched off, I had a look at
the VESA DDC v3 specification:

Section 10.2.7 says that an ACCESS.bus host should be automatically
notified when a device is plugged in, though monitors that are only DDC1/
2B compliant do not have this capability. Therefore ACCESS.bus host
systems should periodically test for the presence of DDC1/2B monitors.

The next section 10.2.8 talks about video switches and notes: "Video
device drivers should be designed to assume the video display may be
connected and disconnected at any time. The device driver should ignore
monitor disconnect. It should not change video capabilities until it
receives a monitor connect messages. This minimizes the time it takes to
recognize and resynchronize when the monitor is reconnected.

Sounds like the current DDC implementation is somewhat simpler than what
the authors of the spec had in mind.

The DDC Spec also recommends that monitors supply the microcontroller
that answers DDC from pin 9 of the VGA connector (+5V, > 50 mA) in
section, to ensure that the monitor data can be queried even
when the monitor is switched off. This is particularly useful for
software theft-alarm systems, that test the presence of the (even
switched off) monitor by querying its serial number via DDC every few
seconds. Very useful for public workstations. I don't know though, how
widely it is implemented.


- VESA Display Data Channel Standard, Version 3, December 15, 1997.


Markus Kuhn, Computer Lab, Univ of Cambridge, GB | __oo_O..O_oo__

More information about the xorg mailing list