Re: Let's pay a developer to make a VGA output card

["David Wilson" <mcs6502@gmail.com>]

mdj wrote:
> I thought perhaps even the IIc's and IIe's could be
> 'fixed' with a crystal change, but by the sounds of it, the IOU is
> timing dependant in terms of enabling the AUX bank for the video
> hardware reads... :-(

I think that the problem is to do with refresh. We know that the
standard 64KB Aux RAM bank is refreshed purely by the video scanner. To
refresh a 1MB card we need more bits in the refresh counter. These
refresh cycles cannot occur during phase 1 when the video is being
displayed nor in phase 2 as the CPU might need to access the RAM. My
theory is that the Meg80z (and other large Aux slot RAM cards) need to
work out when the vertical blanking interval is occuring and refresh
the rest of the card during this time. The fact that a 50Hz IOU has
more lines in this time than a 60Hz IOU causes the Meg80z to select the
wrong bank during the first character time.