Warning - Old Content
This post is quite old, and it might not apply anymore, or maybe there's a better way to do the same thing nowadays. Take with a big grain of salt.
A few months ago I installed Ubuntu on my old 2009 Mac Mini (and apparently neglected to write about it), which went well except that it would only boot up properly if there was a monitor attached. For something that I’m using as a headless server, that amounts to a giant pain in the butt. We had a power outage last night and the thing once again didn’t boot up, so I finally did something about it.
Some quick searching led me to believe that using reFind or some other boot magic could solve the problem, but I didn’t feel like reinstalling my OS again. Luckily I came across a post about the same problem, in which the author used a simple hardware hack to convince the mini that it has a monitor. Basically, create a null VGA cable out of parts on-hand.
I used 100Ω resistors because they were what I had on-hand, and it worked - reboots think they have a monitor, and the mac mini is useful as a headless server. Total project time was about 20 minutes, most of which was spent trying to remember where I put my electronics.
Here’s a pin-out diagram showing which pins I bridged with the resistors:
That’s it! One mac mini that happily thinks it’s connected to a monitor at all times.