[Coco] CoCo 3 to RBG...

[Steve Batson]

Steve,

If I understand your message here, you are saying that using the 15 Pin 
connector may be the better way to go because it has all the proper signals 
on it where the on board connector does not. Is this correct?

Should I proceed with just building the appropriate Coco3 RGB to standard 
15 pin VGA connector cable mapping the connections to the correct pins 
between the two?

If this is the case, to keep the noise down, I'm thinking of shortening the 
Ribbon cable to around 6 inches and have the rest be shielded cable going 
to the 15 pin D connector. Think this will work?

----------------------------------------

From: "Steven Hirsch" <snhirsch at gmail.com>
Sent: Wednesday, July 20, 2011 2:38 PM
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Subject: Re: [Coco] CoCo 3 to RBG... 

On Wed, 20 Jul 2011, gene heskett wrote:

> 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.

I have an older revision. But, I think I misread your paragraph above. I 
thought you were claiming that ONLY the DB15 accepted 15.75Khz. video. 
Ain't the English language great? I agree that all three are intended to 
sync at that freq. But, the big pins on P3 do not have anything brought 
out for H+V sync.

>> 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'll check after dinner this evening.

>> 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.

No nothing that draconian. It simply needs a dumb two-chip sync combiner. 
I built one up on a proto-board and it worked just fine. I'll send you 
the GIF of the schematic in private e-mail.

> 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.

Wow. Now I know who to ask about video particulars!

> 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?

No, I think we were saying the same thing.

Steve

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