VGA Dongles for Mac Mini Headless Booting

At work we’re placing Mac Minis as network appliances into backbone wiring closets, in part to decentralize DHCP and DNS services so they survive partial failures (or scheduled outages) of the network any time the local backbone drop also survives, in part to run local proxy service for WCCP because our backbone doesn’t support GRE.

We reinstall the Minis with OpenBSD; and whether this would be true with OS X on them or not, at least with OBSD, they don’t like to boot without a monitor connected. A few weeks ago I made a batch of VGA “dongles” to trick the Minis into thinking a monitor was present.

VGA dongles, sloped pyramid

Comment 14 by TasDevil on this blog post summarizes the problem and the solution — put a resistor in the range of 75Ω across the analog green signal pins (analog green signal to coax shield / ground). Some computers apparently want all the analog lines terminated and some even the sync lines, but the Mini is happy with just green.

VGA dongle, first test

I didn’t have any 75Ω resistors on hand, but a number of sources suggested that the Mini wasn’t particular, so I made up a test dongle with a 100Ω resistor. The plugs for these came from cables I cut off a batch of dead VGA monitors; the remainder of the cable makes a nice supply of stranded wire and lightweight coax for.

This particular cable had red, grey, and blue coaxes in it. Since they’re supposed to be for the RGB signals, the grey coax had to be green. Rather than continuity-test it against the pins, I lazily slopped the first one together and took it to Andy in the Unix group to try out.

VGA dongle, resistor view

He said it worked fine, so I cleaned up the resistor placement,

VGA dongle, heatshrunk

heatshrinked it, and gave it back to him to confirm before making more.

VGA dongles, end view

After hearing that the first one still worked after the rebuild, I made up the other four, which all also tested good.

That’ll do, pig.

2 Responses to “VGA Dongles for Mac Mini Headless Booting”

  1. JamesS says:

    I recently co-located a newer mini with a DVI port running OSX and it has no problem booting up without any monitor attached. It must be peculiar to the VGA output models or the BSD install. Hasn’t it been a really long time since they made one without a DVI port? What a great way to recycle some old Apple gear!

  2. Keith Neufeld says:

    JamesS, these Minis have DVI-I, but they’re a little older — one of the last PowerPC models. We actually bought them for this purpose and have been fiddling with them altogether too lazily, that we’re just now getting them put into service.

    Still way more CPU power than we need for the minimal work they have to do, and the form factor is fantastic for the job.

Leave a Reply