Is Plug-and-Play functionality affected by video extension?

Plug-and-play is a hardware/software feature that allows for automatic discovery, installation, and configuration of hardware components in a PC. With regard to PC video, the foundation of this process is a data structure referred to as Extended Display Information Data (EDID). EDID information is stored in non-volatile memory on a display device and is a readable by a graphics card via a serial bus called the I2C bus. Together, this functionality is referred to as the Display Data Channel, or DDC.

EDID information is used by the video driver and configuration utilities to allow supported modes of the display to be utilized and to disallow unsupported modes from being selected. The following is a subset of information provided by an EDID viewer on my own PC:

Power Management and Features:
Standby : Supported
Suspend : Not Supported
ActiveOff : Supported

Established Timings :
800 x 600 @ 60Hz (VESA)
640 x 480 @ 75Hz (VESA)
640 x 480 @ 67Hz (Apple, Mac II)
640 x 480 @ 60Hz (IBM, VGA)
1024 x 768 @ 60Hz (VESA)
832 x 624 @ 75Hz (Apple, Mac II)
800 x 600 @ 75Hz (VESA)

Preferred Detailed Timing:
Input Type : Digital Separate
Interlaced : False

Stereo Display: Stereo Display : Normal display (no stereo)

Digital and analog video extenders, rarely provide direct connectivity of the DDC channel from the source to display device. Some analog Cat5 transmitters, for example, provide ‘canned’ EDID information to the source device. Others offer no EDID information, in which case the OS assumes that a non plug-and-play device is attached. Yet other analog extenders provide EDID pass through from a display that is locally-attached to the transmitter. Some extenders are able to cache EDID information on the transmitter, thereby allowing the locally-attached display to be disconnected. Digital extension products may actually read EDID information from the display and pass it back to the transmitter. The transmitter may edit the information to remove modes that it does not support before passing the information to the source PC.

Implementations that rely upon canned or no EDID information may prevent selection of video modes that are supported by your display. Likewise, canned information may allow a user to select a mode which the display does not support. In the case of my sample EDID above, my laptop might not be able to go into Standby if the canned EDID did not indicate this support. Conversely, if allowed for in a canned EDID, a user might be able to select a 16:9 resolution such as 720p or select interlaced video even though these are not supported by the display. Many HDMI displays support embedded audio. However, if this was not enabled via a canned EDID, then the speakers would be rendered useless.

When selecting a video extension product, let your supplier know what aspects of your display device are important to you. For example, you may require: a certain resolution, digital audio, HDCP, interlaced video, power management, the flexiblity to change displays from time to time, the ability to restrict modes that a user can select, or a specific aspect ratio. For multi-point extension, you’ll want to ask how to select a particular display device as the EDID source, especially if video is being extended to a variety of monitors.

Don’t panic if a extension product results in a plug-and-play limitation. Most qualified vendors and integrators should be able to offer a software or hardware work around. Or, if you wish, feel free to reply to this post for suggestions.