What is the polarity of the sync signals ?
Sync signals applied to monitor BNC connectors are traditionally negative polarity. PC VGA card uses different sync polarities for telling the monitor about which resolution is in use, so the sync signal polarity changes from mode to mode. If you don’t have specs of your monitor then negative polarity in both HSYNC and VSYNC is the best configuration to start experimenting. If you get the sync polarities wrong then the the monitor might not display any picture or it does not sync to the video signal. Other possible problems might be that you see only part of the picture. Some monitors work with many sync signal polarities, but might have different monitor settings for every sync signal polarity combination (this applies to some older PC multisync monitors).
