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

[Luis Antoniosi (CoCoDemus)]

well 24 inches is a pretty long cable. I'm using 12 inches maximum.

Did you check all the groundings on the board ? check the coco3 pins 1&2
and also altera's pins GIPO0 12 & 30.

in my prototype the lm1881 is detachable, so it's never plugged when I use
the coco3.

Try assembling another board without the lm1881, 680K, and the 2 caps, or
remove them for your current board.


Luis Felipe Antoniosi



On Sat, Nov 29, 2014 at 11:30 AM, Richard Goedeken <
Richard at fascinationsoftware.com> wrote:

> 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
> >
> >
>
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