[Coco] Re: [Color Computer] CoCo 2 questions.

[Phill Harvey-Smith]

Mike Pepe wrote:
> 
> Phill Harvey-Smith wrote:
> 
> 
>> Humm I get a forbidden error, though I can get to the other two 
>> files in http://www.doki-doki.net/~lamune/temp/
>> 
>> file permissions on coco2b.tif perhaps ?
>> 
> Yeah, oops... try that again please.

Yep downloading now....Thanks  :)

>> Right, though I meant 8x4164 :)
> 
> Actually you can indeed put 8 41256 chips in a CoCo 1/2 with a 
> 74LS785. A 41256 ram also has 256 refresh cycles. There was a circuit
>  provided in Rainbow ages ago that let you have 4 banks of 64k by 
> installing 41256 DRAMs and a LS785...
> 
> ...Would like to find that article again...

Yes that would indeed be good, I would think that the 256 chips have an
extra address line or two that would be controled by a latch or something.

Actually tried adding 41256's to one of my Dragons, and it seemed to
work, though the refresh would be miles out of spec.

>> Ahh, so effectivley it and the SAM become a chipset that can't 
>> operate independent of each other (well you could use the sam on 
>> it's own but not the 6847TI), also I suppose iliminating the latch 
>> means 1 less chip so drives down production costs when you are 
>> making thousands of them.
> 
> The SAM/VDG connection isn't really relevant in the CoCo/Dragon world
>  because of course both chips are always present. The internal latch 
> is absolutely a cheap-factor enhancement.

No but IIRC the MC-10, has a 6847, but without the SAM, as does the
Acorn Electron (the non UK people here my not have heard of that one tho
   :)  ).

>> Heheheh, well I suppose if they mentioned it, people without it 
>> would also want lower case etc, and perhaps they didn't want to 
>> offer an upgrade path :)
> 
> Well the T1 chip is not drop-in compatible with the regular 6847- 
> presumably though you can bypass the TTL latch and make a minor mod 
> or two to the older board to drop in a T1. It's on my list of things 
> to try.

Yeah that should be easy enough actually, asuming that otherwise they
are pinout compatible. I went back and checked, the chip appears not to
be a -TI, and the latch is there.

>> Though I do seem to remember that someone mentioned a mod that you 
>> could do to allow you to use an external character rom to get lower
>>  case, shame that's not on the web somewhere.
> 
> Yes, that's also on my list. Check the 6847 data sheet if you can 
> find one, it shows you how to add the external CG ROM. It only takes 
> one counter TTL chip and a ROM.

I have a copy of "Inside the Dragon" which has the datasheets for the
6809, 74LS783, 6847 and 6821, which is really usefull for Dragon/CoCo
hacking :)

>> Hehehe, well I don't think either of my CoCos, or any of my Dragons
>>  for that matter has not been modded in some way :), even if it's 
>> just to upgrade them to the latest version of basic, and max 64K 
>> ram :)
> 
> That's about all you can ask for!
> 
> Anyway, hope that helps

Thanks that was helpfull :)

Phill.



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