EGA out on compact macs

#1 Sun, 20 Nov 2011 - 04:58 68kMLA

Recently, I got a Mac Plus board from a member of these forums for a project that I am working on. I am not far enough in this project to where I need the board, but figured it would be nice to have on hand for when I am ready... and decided to play around with it in the meantime. Since I just have the logic board, I decided to hook it up to a CGA EGA monitor.

Anyways... here are the results:

Normal, low intensity green.

In Amber Monochrome mode

I also have a nice new wallpaper!

Here is a quick schematic, it uses a 74ls04 hex interter, 4 SPST switches to adjust the foreground color (BG is black), and two SPDT switches for both the H Sync select and Video select.

#2 Sun, 20 Nov 2011 - 07:59 68kMLA

Are you sure that's not an EGA monitor? CGA is 15 kHz and compact Macs put out 22.3 kHz, if I remember correctly. EGA is 21.8 kHz, which is close enough that it should work, but I can't picture CGA reaching that far without some tweaking.

#3 Sun, 20 Nov 2011 - 10:00 68kMLA

I think it should be EGA, too -- ot maybe that's an old multi-scan monitor? CGA has 200 lines only, while EGA has 350 -- very close to the Compact's 342, so it's very likely to work OK.

Anyway, it's a very interesting work! I see that you can choose between straight and inverted signals for H-sync and Video... which are the 'proper' settings? Maybe you'd like to use a couple of your unused inverters in series for the "straight" video connection -- just in order not to overload the Mac's output in case someone likes to get a bright white image (the four inputs in parallel!)

Thanks for sharing!

#4 Mon, 21 Nov 2011 - 04:30 68kMLA

WhoahMan, nice work indeed! Yes please do clarify for those asking in regards to CGA vs EGA

#5 Mon, 21 Nov 2011 - 05:17 68kMLA

That's pretty cool! What have you got planned next?

#6 Mon, 21 Nov 2011 - 05:28 68kMLA

I apologise, the monitor is actually an EGA monitor and is running in EGA mode due to the negative vsync. I do have to admit, I am curious as to what would happen if i inverted vsync and tried it in CGA mode, maybe I will have to try that out sometime tomorrow on both this monitor and another that is strictly CGA.

#7 Mon, 21 Nov 2011 - 05:33 68kMLA

zuiko21, most people would want to use set sw-hselect and sw-vselect to use the outputs from the 7404. The only reason i added the ability to switch between /hsync and hsync has to do with a few arcade "CGA" monitors that I have sitting around... just in case i decided to hook it up to a bigger screen

#8 Thu, 24 Nov 2011 - 00:27 68kMLA

Title fixed

Really wishing I hadn't ditched my 23" EGA CRT now :/