Using high res on my Atari STe with the CosmosEx and a wire

This image shows a CosmosEx screencast of an Atari Mega STe in high resolution.

Throughout my Atari ST-owning life, I have only ever owned Atari’s color monitor, the SC1224. It can display two of the ST’s video modes: low resolution (320×200, 16 colors) and medium resolution (640×200, 4 colors).

The Atari also had a 640×400 high resolution mode, but it required a different monochrome monitor.

I never had one, but recently I came across some high-res software I wanted to test. What to do?

I immediately thought of the CosmosEx and its “screencast” function, which lets me see and control my Atari Mega STe remotely via a web browser. I thought: what if I turn off my monitor, so the ST doesn’t know what sort of monitor is attached, and then run it remotely via the CosmosEx screencast function?

I tried this idea, but alas, it didn’t work. The “ST High” option remained grayed out among the “Set screen resolution” choices.

So, I asked on atari-forum, and AdamK pointed out that the ST detects the presence of a monochrome monitor when the MONO DETECT signal is connected to the GROUND.

I looked up a fantastic wiring diagram of the Atari Video Connector, and found that pin 4 is MONO DETECT, and pin 13 is GROUND.

This photo shows a wire connecting pins 4 and 13 of the Atari Video Connector, tricking the Atari into thinking a monochrome monitor is attached.

I decided to try connect these two pins with a wire. But first, I copied CE_CAST.PRG into the AUTO folder on my boot drive to make sure the screencast function would be available since my monitor would be disconnected. (I normally load CE_CAST on demand, rather than boot with it, since it can conflict with programs.)

Make sure that CE_CAST.PRG is copied into your AUTO folder first.

After that, I disconnected the monitor cable, inserted the wire, rebooted the Atari, then launched the screencast webpage in a browser on my Mac.

Voila! It booted into monochrome!

This is awesome. The screencast can be a bit stuttery, and the mouse is hard to control, but it works! Best of all, I didn’t have to buy a second Atari monitor.

1 thought on “Using high res on my Atari STe with the CosmosEx and a wire

  1. denpashogai

    Looks great! If you can get a hold of a working SM125 monitor though, there’s something really charming about it, and they’re not huge (for CRTs, that is).


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