Today I landed my mandatory corporate Windows laptop at a desk supporting a nice 24″ monitor. Willing to take advantage of the available extra display real estate, I plug the DE-15F cable into the laptop’s D-subminiature video port and proceed to set the extra monitor’s resolution in the “Settings” tab of the “Display Properties” dialog. Alas 1280×800 pixels is the most I can set – it is the laptop’s main display’s resolution and it is far below the 1920×1200 pixels the secondary display is capable of. Shutting off and on the monitor, disconnecting and reconnecting the cable on the laptop’s port, putting the laptop to sleep and even rebooting… Nothing worked : it seemed that the system was not detecting the monitor properly and chose to handle it with some sort of default resolution. I even uninstalled the operating system’s monitor drivers – with no visible result.

Suspecting a hardware problem I decided to check all the connections. A quarter of a century of experience has taught me that connections are the most frequent cause of incidents. I reseated and properly screwed the cable to the monitor… And I was mildly surprised to see the display properties settings tab let me choose the monitor’s nominal resolution at last. My instincts had been vindicated.

What happened was a loose VGA cable. All the pins necessary for display were making contact, but some of the ones necessary for plug’n’pray detection were not. Mere visual inspection could not have found that – only reseating the connector makes the problem evident by solving it. I’m not sure I would be able to misconnect the cable just right to reproduce this situation if I wanted to…

And that’s how I learned about Display Data Channel, a collection of digital communication protocols between a computer display and a graphics adapter that enables the display to communicate its supported display modes to the adapter. I’m sure you won’t resist following this link to learn about DDC1, DDC2, DDC/CI and E-DDC to understand some basic technology working in the shadows, taken for granted until it stops functioning…