[Coco] CoCo 3 to RBG...

[Steven Hirsch]

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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