[Coco] Last video I promise... :)

[Richard Goedeken]

I probably need to change the front porch delay, because the left-most column
of characters is cut off in 80-column mode.

I think the noise is from a ground loop, but I don't know where.  The Coco is
plugged in with a 2-prong plug.  I was powering the DE0-Nano board from a USB
port on a laptop which was plugged in with a 3-prong.  But I changed that to
use a 2-prong USB power adapter instead.   The noise didn't go away.  The
monitor has a power brick which uses a cable with a 3-prong plug, so I guess
my next test will be to block the ground connection on the monitor and see if
that helps.

I also accidentally discovered that the sync resistors weren't right.  When I
was building up the board, at first I couldn't find the 82-ohm resistors in my
pile of parts from digikey.  I thought that I had forgotten to order them, so
I looked at the schematic to see what they were being used for.  I saw that
they were for the H/VSYNC going to the VGA, so I searched online to find the
input impedance of these on a VGA monitor.  I found something which said that
most VGA cables have a characteristic impedance of 60 ohms in the lines used
to carry the sync, so drivers are supposed to use 60 ohm resistors.  I looked
around and found some 68 ohm resistors and used those.  Anyway, the VGA output
is perfect, because the test pattern looks super sharp and clean.  The noise
is definitely coming in on the input ADC side.

I actually bought all the parts before you put the digi-key parts list on your
blog.  I was looking at a BOM that you had posted here on the mailing list,
and selected all of the parts on the digi-key website.  For some reason I
ended up with two 4-position DIP switches instead of one 8-pos, and the
particular 4-position DIP switches that I got are too wide to fit next to each
other.  That's why I don't have any switches mounted yet.

I do have other Cocos that I could test with.  The RGB cable that I'm using is
a plain flat ribbon cable, so not shielded.  It's not very long, maybe 20 or
24 inches.

Richard

On 11/28/2014 08:37 PM, Luis Antoniosi (CoCoDemus) wrote:
> The customization is about the front porch, the code now is pretty stable
> to be generic. But yours seem to work. Steven Hirsch had to change on him.
> He didn't report about noise. So I can't tell if is a particularly from
> yours or if this board has indeed noise. Until my mules decide to cross the
> border I can't help you on this.
> 
> BTW, Steven has found out theat the correct values for the sync resistors
> are 47 ohms instead of 82. I doubt this could be the source of problem.
> 
> You don't need the LM1881, the 680K and the 2 caps if you're not planning
> to connect any other computers on it.
> 
> The scalines help improve the colors. You may turn them on, use a dip
> switch. I have changed the dip switches, the scanline is now the label
> shrink.
> 
> You don't have another coco to test right ? Also, how long is your cable
> from the coco3 ? Is it shielded ? Do you have anything else that could be
> potentially noisy ? Tried connecting the altera's usb cable in a different
> power source, like iphone or samsung usb chargers ?
> 
> Luis Felipe Antoniosi
> 
>