[Coco] CoCo 3 to RBG...

[gene heskett]

On Wednesday, July 20, 2011 04:52:43 PM Steven Hirsch did opine:

> On Wed, 20 Jul 2011, gene heskett wrote:
> > On Wednesday, July 20, 2011 01:00:14 PM Steve Batson did opine:
> >> Steve,
> >> 
> >> Can you provide details on building your 2 chip circuit to get it
> >> working?
> >> 
> >> I received mine this week, built cables to Map Coco 3 RGB signals to
> >> the on board 8 pin connector as they describe. The board powered
> >> fine, I can access it's menu and functions, but no Coco output. Just
> >> a green screen, but I don't think it's Coco output though. Coco
> >> works fine with composite  out to TV.
> >> 
> >> Any help will be appreciated!  :)
> > 
> > This is going to be best troubleshot with a decent scope, only that
> > can give you a definitive answer.
> > 
> > Oh Wait, you said the 8 pin connector?  Humm, I may be half a bubble
> > off here, but ISTR reading in the excruciatingly fine print (I had to
> > drag out an old projector lens & use it for a magnifying glass to
> > read that teeny little booklet) that the 8 pin connector only handles
> > 31khz & up sources, so I had not considered doing anything but
> > stuffing the coco's signals into the adjacent DB15, which claims to
> > work down to 15 khz, and which means one must pay attention to the
> > pinout of the 10 conductor cable from the coco as to where it goes in
> > the DB15.
> 
> Where in the manual does it say that?  The one I'm looking at claims
> that 15Khz. video can be applied to P3 (The large pins on the left),
> P11 (8-pin SIL) or P12 (HD15).

Did you not get the xx20 version of that board?  Mine came with dual db15 
outs on the back edge.  And my booklet text says P3, P10, & P11 for the 15 
khz input.

> Since P3 appears to bring out only the
> composite sync input I never tried it.  I can tell you that H+V on P11
> never worked for me.  I made the (perhaps erroneous) assumption that
> the H and V pins on the HD15 were connected to the same circuit nodes
> as H+V on P11.  But you know what they say about "assume" :-).

Yes, its been applied to me on numerous occasions.

A digital meter in ohms mode can tell that story.
 
> I've never been able to get mine to work with H+V sync on a 15Khz.
> signal. I wrote to their tech support and recall being told that it
> wasn't intended to work with H+V at that horz. freq - only composite
> sync.

That's a genuine class A bummer if by composite, they also mean fully 
interlaced.  Taking the coco's H & V, and combining them does not a 
composite signal make.  In std Never Twice Same Color, the V Sync is not a 
solid 3 line long signal, it is 'serrated' by returning to black at 2x the 
h-rate, such that there is a 4.7 u-sec hole, which returns to the sync 
output such that each serration end, is co-incident with the falling edge 
of the H-sync for every other serration.  That is why, years ago, you could 
adjust the vertical hold to make the picture roll down, and the vertical 
sync bar you could see had a square tail on the left end of the bar from 
the middle to the right of the screen.  The one you couldn't see at the 
left end would have been identical.  Its actually slightly more complex 
than that because for 3 lines ahead of V-sync, the H sync pulse was reduced 
to half its normal time to reduce any dc offsets created by the apparent 
doubling of the repetition rate.  Ditto for 3 lines after the V pulse for 
exactly the same reason.  This is what forms the arrow's 'tail' you could 
see.

Making composite by Nanding the two signals substitutes a solid bar 3 H 
lines long, which contains no H sync info, and causes the hooking to the 
left or right at the top of the picture because the H sync is momentarily 
lost.  That might well be the only thing we can do in which case there is a 
bit of false advertising involved.

However I am still confused by the apparent differences in our booklets, 
and now wonder which is correct.  Or are you miss reading the PCB because 
the photos do not call the connectors out by their P3, P10 or P11 
designations?

> Steve


Cheers, gene
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