ATI Graphics Solution REV 3 - DIP / Jumpersettings

The monitor is connected to the Hercules card through a DE9F connector using 5V TTL electrical signaling; the same as for an MDA card.[5][6] Nominally, the Hercules card provides a horizontal scanning frequency of 18.425 ±0.500 kHz, and 50 Hz vertical.[7]


However, the Hercules actually ran at two slightly different frequencies depending on whether it was running in Text or Graphics mode. This is a side effect of the CRT controller using 9-pixel, 14-line character cells when impersonating the MDA, but having to use 8-by-4 pixel blocks to turn memory bytes into 1-bit pixels in graphics mode, with neither the total pixel count in a horizontal period nor total number of scanlines matching up completely. As the card's onboard clock crystal was also slightly different, at 16.000 MHz vs the MDA's (and later, EGA's) actual 16.257 MHz, neither mode was an exact match for the original sync structure, with the exact figures being 18.141 kHz and 49.84 Hz in text mode, plus 18.519 kHz and 50.32 Hz in graphics. However both are close enough to MDA's own 18.432 kHz and 49.82 Hz timings to fit within the sync tolerance of and work reliably on a typical IBM monochrome monitor or compatible. This tolerance was taken advantage of by the various Hercules clone cards, which themselves often exhibited less than accurate timings versus the original.[8]