I`ve decided to abandon the current VGA board design. After assembling it and adding some corrections,and after long time of tests and improvements I couldn`t get rid of artifacts seen on the picture below:
It is obviously a hardware problem – the artifacts are smaller than pixels accessible to software. The problem lies in the video circuit design. Unfortunatelly after assembly, there is no way of major changes on the board. This fact, and also the ugly-looking interlaced image the board renders, I`ve decided to rework the display adapter from scratch using different approach.
Just for the record I`m posting photos of ready made adapter, the prototype and some test screens generated by AVR chip connected to the video board:
Also schematics and PCB layout in EAGLE can be examined here:
As now I have no display board for my computer, I`m starting all over again. This time I`ll use AVR chip (I didn`t want to do this, but it seems the only reasonable choice as opposed to use of a handfull of standard logic ICs and RAM chips…) , XC9536XL, 74LS165 shift register and some buffers for the system bus. The project assumes 320×200 resolution, this time without interlace, one color (!) – green most likely (for sentimental reasons of course 🙂 ). The use of just one color and a shift register should give me fairly high text resolution of 53×25 characters using 6×8 pixels font.