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Monitor Sense wiring for a fake Multiple Scan 21"?


Active member
I'm trying to get the best out of my Quadra 950 built-in video. I have a SuperMac NuBus card driving a 1024x768 15" TFT at 60Hz I use as its main monitor.

The builtin video along with the Houdini II card however is something I'd like to configure to Multiple Scan 21" codes to my CRT for, but I really can't figure out what this means:
Table S-2 Extended Sense Line Code Assignments Sense 2 Low Sense 1 Low Sense 0 Low Monitor Type 1 & 0 2 & 0 2 & 1 Multiple Scan 21" * 10 00 11
Note: In this table, the column under "Sense 2 Low 1 & 0" indicates the values the
software is reading from monitor sense lines 1 and 0 while driving sense line 2
So, does anyone have a graps of it? I have it currently wired as (S)VGA, since that was a nobrainer, but 60Hz 640x480 and 56Hz 800x600 is flickery and although they were fine back in the day when I had a 14" VGA monitor when this computer was new, it's now heavily underutilizing the potential of my "modern" 19" LG Flatron 915FT Plus CRT.


Daring Pioneer of the Future
Staff member
What that mode will probably produce is 1152x870, which was Apple's favorite resolution for 20-21" monitors until the 2000s.

I don't think the 700/900 have quite as much support for multiple scan displays as newer machines, but they do support the 21-inch macintosh color display, whose sense codes will produce 1152x870@75Hz which a modern monitor will "probably" do. With 2 megs of VRAM that'll get you 256 colors. (the 21MCD is still a fixed sync display.) (EDIT: I misread your post and didn't realize for a moment that was a CRT. A good CRT should have no trouble at all syncing up to 1152x870@75.)

If you don't have this document yet: https://web.archive.org/web/2005022...ac/hardware/dale-adams/video-quad700-900.html may have some more info.

Are you building your own cabling/adapter or using an existing adapter? I think we have a thread hanging around somewhere documenting switch configuration patterns for a few different common Mac -> VGA adapters


Well-known member
1024x768 is the highest VGA resolution you can get from Apple’s on-board video on 040 machines. I have a 1280x1024 LCD that runs full-res on my 7500, but on my Duo 280 it won’t go higher than 1024x768 because, according to Apple’s tech note, if it senses a VGA signal, you only get 3 resolutions (640x480, 800x600, 1024x768). Also remember Apple used odd resolutions with their displays, like Cory5412 said. I had a Mac display that was 832x624, I guess to accommodate the menu bar and disk icons?