CGA on a NEC LCD monitor the cheap way

[vwestlife]

My NEC LCD1550M monitor is known to support 15 kHz video. I tried it in the past with my Apple IIGS using a Mac 15-pin video to RGB adapter (since the IIGS uses the same pinout as older Macs) and it didn't work, because the NEC doesn't support composite sync, which the IIGS uses.

But it turns out the LCD1550M does support analog 15 kHz RGB video as long as you have separate horizontal and vertical sync. There are various converters you can build or buy to convert digital RGB (such as from CGA or EGA) to analog RGB, but I'm cheap and lazy, so I decided to see what would happen if I simply used a $9 adapter cable from eBay and tried to display digital RGB on it.

Attached below are the results. It works, but as expected, you only get 8 colors instead of 16. But still, 8 colors are better than none! It even displays the overscan area, too. And since the Tandy I was using it with also outputs MDA/Hercules video on the green pin, I also tried it with that. Unfortunately the NEC has no vertical size adjustment and the image was way too tall, cutting off the bottom of the image. (Or you can adjust the vertical position to display the bottom part, cutting off the top.) I didn't have an EGA card on hand to test it with, but if it can support 15 kHz and 18 kHz, I'm pretty sure it can support 21 kHz EGA, with the same 8-color limitation if you're not converting it to analog RGB.

At least this proves that the NEC LCD1550M does support 15 kHz video, and all you'll need to display CGA is an adapter cable and a digital to analog RGB converter. For analog RGB video, such as from an Amiga or Atari ST, you wouldn't even need that -- just wire up the necessary cable and you'd be good to go.

 

[SomeGuy]

Very nice. I wish there were more detailed information out there about what lower resolutions VGA LCD or CRT monitors really supported. (Perhaps we need to make a database?)

If this adapter does nothing with the Intensity/secondary green, then what system is this adapter designed for? (Edit: I see the cable is a simple passive cable meant for use with an RGB converter box that would make use of the extra signals)

 

[Chuck(G)]

You could certainly handle the I signal with a simple circuit, but brown would still display as yellow.

 

[njroadfan]

Try running the composite sync to the v-sync pin, sometimes the monitor will work with that. Otherwise sync-seperators are relatively easy circuits to build with off the shelf chips. Some Mac-to-VGA convertors already have them built in (to support early Mac video cards that only do composite sync or sync-on-green). I have a Belkin one here that does this.