How multisync monitors detect different screen resolutions

There are tree different methods used for detecting the screen mode used. The oldest method is using sync polarity information. VGA card originally used this method for telling monitor when to be in 350, 400 or 480 line mode. Many monitors used the remaining signal polarity combination to detect the high resolution modes.

Horizonal Dots         640     640     640
Vertical Scan Lines    350     400     480
Horiz. Sync Polarity   POS     NEG     NEG
Vert. Sync Polarity    NEG     POS     NEG

Some ICL monitors have detected the screen mode from the length of the vertical sync pulse. The monitor electronics count how many scanline times the vertical sync pulse was active (the settings in use are selected from info if the verical sync pulse lasts more than 7 scanlines or shorter time).

Nowadays monitors have controlling microprocessor and it is not hard to make a program which extracts frame rate, horizonal frequency, interlace and other necessary info from the sync signals. If you cna detect the frequences and other timign issues, then it is qauite easy to tell what resolution is used (or at least detect when some resolution is same or different which is stored to monitor memory).

Thanks for Joonas Kekoni ( for sending me information about ICL monitor mode detection.
Tomi Engdahl <>