NTSC Colour board. :)

discussion of games, software, hardware & emulators relating to the Acorn Atom
waltermixxx
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NTSC Colour board. :)

Postby waltermixxx » Fri Dec 01, 2017 4:56 pm

Hi there,

If I understand correctly, the colour board for the Acorn Atom provides 2 functions:

Changes the frequency of the video out for 50Hz operation (from 60, as the 6847 in an ntsc chip), and combines the various other outputs of the MC6847 to produce a colour signal namely (I think):

CLK
0B
0A
HS
CB
Y
FS
INV

because I am in North America, and don't need to change the 60Hz portion, would a colour board be much simpler? Could I instead use something like this instead with little to no modification, (just a connector to PL4 and a couple of flying leads for the Y (composite video) signal? would I also need to change the 3.58 mhz crystal to a proper ntsc colour burst 3.579 crystal...? :)

this is the MC1372 used in the coco 2. :)

1372.png

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hoglet
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Re: NTSC Colour board. :)

Postby hoglet » Fri Dec 01, 2017 5:03 pm

Yes, much simpler colour boards are possible.

This is the simplest I could come up with, if you have a TV/Monitor with component-video inputs:
viewtopic.php?p=59728#p59728

And the MC1372 has been used on the Atom before:
viewtopic.php?p=142454#p142454

Dave

waltermixxx
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Re: NTSC Colour board. :)

Postby waltermixxx » Fri Dec 01, 2017 5:38 pm

thank you, I will look into those. :)


Cheers. :)

waltermixxx
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Re: NTSC Colour board. :)

Postby waltermixxx » Tue Dec 05, 2017 6:45 pm

Zippster, a guy in the U.S. builds lots of very cool projects for the Tandy Colour Computer.
I'm looking to purchase one of these from him, it's designed for the CoCo 2.

I just have to double check with him if the 1372 is included. :)

20171203_092654.jpeg


20171203_092724.jpeg
Last edited by waltermixxx on Tue Dec 05, 2017 7:55 pm, edited 1 time in total.

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hoglet
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Re: NTSC Colour board. :)

Postby hoglet » Tue Dec 05, 2017 6:54 pm

Is it this one?
https://sites.google.com/site/thezippst ... -the-coco2

I'll be interested to hear how well it works.

waltermixxx
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Re: NTSC Colour board. :)

Postby waltermixxx » Tue Dec 05, 2017 8:41 pm

its a newer version of it. it looks like...
:) I will let folks know how it works out.

I sent a note to Ed, (Zippster) and he confirmed the board does come with the MC6847 and MC1372.

I purchased the board and will update the thread and post pictures of the Atom when it arrives, with the colourboard and AtoMMC installed.

I will just have to reroute the ribbon cable from the video ram board to the Atom.

IMG_0832.JPG

waltermixxx
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Re: NTSC Colour board. :)

Postby waltermixxx » Fri Dec 08, 2017 8:54 pm

A question about "snow or noise on the screen" when playing games, on a real Acorn Atom, with 6k of Video ram: will this happen in black and white as well as colour games? If I understand correctly the snow is from a timing issue caused when the cpu and the MC6847 are accessing video ram at the same time? The Acorn Atom i will be receiving has a single 6264 8k Ram chip for video ram, as shown above, will this configuration also produce noise on the graphics screens on the ATOM?

:)

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hoglet
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Re: NTSC Colour board. :)

Postby hoglet » Fri Dec 08, 2017 8:58 pm

waltermixxx wrote:The Acorn Atom i will be receiving has a single 6264 8k Ram chip for video ram, as shown above, will this configuration also produce noise on the graphics screens on the ATOM?

In a word, yes. There are a couple of ways to avoid screen noise:
- run the CPU off the VDC clock (1.79MHz)
- add a noise killer / screen noise eliminator, see here

Dave

waltermixxx
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Re: NTSC Colour board. :)

Postby waltermixxx » Fri Dec 08, 2017 11:58 pm

thank you... :)

lots of good info in that document. :)

Cheers. :) the 74LS573 looks like a nice fit, along with the 74LS123. :)

i have lots of stuff to do to my atom:

32k RAM upgrade (deadbug on my 6502) 0000-7FFF and remove existing ram chips
(unless I get lucky enough to get on the list of folks who will receive a RAM/ROM board. :)

Install the ATOMMC and the eprom in #A000

Zippster Colour composite output (designed for COCO2)

Noise eliminator.

it's going to be a very good winter... :)

thanks everyone for your help thus far.

Cheers.

waltermixxx
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Re: NTSC Colour board. :)

Postby waltermixxx » Tue Dec 12, 2017 3:17 pm

Me again :)

I have a question, as you can see above, (scroll up a few posts to the picture of my Acorn Atom Motherboard.) i have 8k of ram connected in the "video ram" section of the Atom board. :)
I know that normally the MC6847 can use up to 6K of ram #8000-97FF. because there is 8k i suspect it goes from
#8000-9FFF. I suspect the ram from 9800 to 9FFF can be used for other things? Are there any programs that take advantage of the ram here? :)

just wondering really :)

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oss003
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Re: NTSC Colour board. :)

Postby oss003 » Sun Feb 04, 2018 7:59 pm

waltermixxx wrote:Me again :)

I have a question, as you can see above, (scroll up a few posts to the picture of my Acorn Atom Motherboard.) i have 8k of ram connected in the "video ram" section of the Atom board. :)
I know that normally the MC6847 can use up to 6K of ram #8000-97FF. because there is 8k i suspect it goes from
#8000-9FFF. I suspect the ram from 9800 to 9FFF can be used for other things? Are there any programs that take advantage of the ram here? :)

just wondering really :)

No commercial programs are using this area but you can use it for your own programs.
The only problem is that screen noise will be generated is this area is accessed.

Greetings
Kees


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