This is a standalone 6502 computer consists of CRC65 and VGA6448 on a 5-slot RC2014 backplane. CRC65 is a SBC with RC6502 expansion bus running at 14.7MHz. VGA6448 has a monochrome 64 columns X 48 rows (512 x 480) text output and a PS2 keyboard interface. While this 6502 computer can operate standalone, it can also receive serial input from the laptop or PS2 keyboard and display the text output serially on the laptop as well as the VGA monitor. This is the mechanism to load software from laptop to the 6502 computer. Displayed on the screen is mandelbrot benchmark in EhBasic. Forum member okwatts wrote much of the vterm emulation for VGA6448 as well as modified the EhBasic to work with VGA6448.

The work is not done; the display has strange artifacts on the top line as well as bottom line which may be monitor-specific because they were not present on a different monitor. I need to improve the scan code for PS2 keyboard; I like to add a quad serial board to this computer and I want to port DOS/65 to it.
Bill

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Nice work!

What you are seeing on the monitor is the pixel clock attempting to stretch certain pixels in order to best fit the screen. Since you are pushing 512 pixels across either the 640 or 800 standard, the circuitry in the monitor will try to compensate.

You can completely get rid of this by either dividing the horizontal perfectly such as 800/2 for 400 pixels using a 20mHz clock or attempting to push 640 pixels (not easy) using the 25MHz or 25.175 MHz clock.

Another way to remove the banding is to tweak the pixel clock on your monitor settings. This is either called phase or clock on the menu. You first shove the visible image to the center as best you can by adjusting the h position, and then tweak the clock until the banding is gone. It takes some patience, and some monitors may readjust each time they power up, so the matching horizontal rate may be better.

Brad

BDD,
I have an existing quad serial board based on OX16C954. The part I used is 80-pin QFP but I recently found out OX16C954 is available in 68 pin PLCC and cheaper, too. So I've designed a board for the PLCC (the board on top) but still waiting for the part to show up.

The standalone 6502 computer does not use interrupt so far, but quad serial port will need to be interrupt driven and possibly a jiffy timer as well.
Bill

Brad,
Thanks for the insight to monitor adjustments. Indeed, I can adjust another monitor to completely eliminate the banding, but I have more problem with the particular monitor of the original post. I'll go back and try again with your suggestion.

I am pushing out pixels at 25.175MHz, but cut off output after 512 pixels for every horizontal trace. The fact I need to adjust monitor to accommodate the VGA output suggests my VGA output is not conforming to the expected standard. Perhaps I need to adjust front porch or back porch timings?
Bill

Ah, ok, that makes sense.

If you can put out a border during your active horizontal line, that would also stop the banding.
In your case, 64 pixels left and 64 pixels right of any color above black will automatically center your screen and remove all phasing.

Here is a copy of my VGA timing calc program. Compiled in Visual studio. This allows you to tweak timing based on an pixel clock.
Hope it helps for a project one day.



Cheers,
Brad