VGA blacks out whenever a character appears in the first column

ElectricAye

I'm trying to learn to work with VGA and I'm using a Mitsubishi Diamondtron Plus 72 that I rescued from the junk. The Diamondtron seems to work okay, for example it works okay with the VGA_text demo. But whenever a character is allowed to appear in the first column (leftmost edge) of the screen when using VGA_text, a large black rectangle appears over not only the character itself but also over everything that follows, taking up about 40 percent of the total screen all the way across. After the black rectangle, I get the standard blue background again. Does this sound like a familiar problem to anybody? or is something probably wrong with the Diamondtron? Or could I not be using VGA_text correctly?

I'm also getting some ghostly smearing of the characters even when things are working well, but I'm guessing that has to do with my long wires (6-8 inches) that attach my Prop to the VGA plug via resistors. But I consider this to be just a minor problem. It's the black rectangle that has me freaked out.

thanks,
Mark

Post Edited (ElectricAye) : 4/7/2009 10:22:00 PM GMT

kwinn

First, I would try a VGA cable to see if that helps. Then I would see what happens if you put a space in column 1, then 1 and 2, etc and characters after. It sounds like it may be sync related, but more info is needed.

ElectricAye

I just tested the VGA monitor using the VGA_Tile_Driver_Demo 2 and 3 and everything looks fine, including the characters appearing in the first column. So now I'm wondering what could I be doing wrong with VGA_text? Is there some kind of taboo for having characters in the first column?

ElectricAye

kwinn said...
... I would see what happens if you put a space in column 1, then 1 and 2, etc and characters after...

Thanks, kwinn, I did exactly that and the black rectangle disappears. But this phenomenon raises the question: what exactly am I doing wrong?

Peter Jakacki

Sounds like your monitor is not very tolerant of your VGA timing. It could be that it is expecting blanking on the "back porch", that is the period between the synch pulse and the video data. But the monitor might have been on the junk pile for a reason or you might have a problem with your circuit somehow. Don't try to second guess or assume, just check it methodically and you will find the problem.

Does your VGA work with other monitors? Does you monitor work with other VGA sources?

*Peter*

ElectricAye

Thanks, Peter,

I think you are probably right: my wig of a wiring job is probably creating some kind of synch problem.

Since I have the VGA_Tile_Driver_Demo 2 working, I've been using that code as a starting point to learn how I can use VGA. I'm hoping after I get a PCB put together for all of this, the ghosts and synching problems will disappear.



Mark