Raster Scan Signals - Zenith Z-100 Reference Manual

Page 7.26
Video Graphics Programming

Raster Scan Signals

The raster scan outputs, RAO through RA4, provide the interface between the
6845 and the character generator logic. These outputs may represent scan line
counts of from 0 to 31, or up to 32 scan lines per character row. Register R9,
the scan lines/row register, determines the maximum count from the scan output
registers before reset occurs. Scan line counts are incremented at the HSYNC
rate but are also dependent on the interlace format selected.