[Coco] MC6847 VGA (FPGA) Project underway...

[L. Curtis Boyle]

I wasn’t referring to SG6 - any of higher SG modes will have the characters offset if you strictly emulate the characters from the T1 VDG vs. the older VDG’s, as they do not line up the same. That is what I was referring to; sorry, should have made it clear that I was not referring to the broken SG6.

L. Curtis Boyle
curtisboyle at sasktel.net



> On May 26, 2016, at 1:39 PM, Arthur Flexser <flexser at fiu.edu> wrote:
> 
> What Curtis said about mixed in characters seems to be in disagreement with
> what Darren said and with my recollections.  I do not believe there are ANY
> mixed-in text characters in SG6 mode--Darren says they would have to come
> from an external character ROM, which doesn't exist.  I think Curtis may be
> confusing SG6 with another of the semigraphics modes.
> 
> Art
> 
> On Thu, May 26, 2016 at 3:04 PM, Brendan Donahe <brendan at polylith.com>
> wrote:
> 
>> Art,
>> 
>> That's good input.  Although I had ADOS, "back in the day" I didn't have a
>> machine with a 6847T1 and wasn't aware of the re-purposing of SG6 for
>> lowercase.
>> 
>> Curtis,
>> 
>> I noticed that the characters shifted up and left by a single pixel in the
>> newer font set.  I hope we will be able to select between them to avoid
>> this problem - Steve implemented both of them when we started this project
>> and I can switch between them on the prototype board.  To be sure, it
>> sounds like I'll have to try out Protectors II.
>> 
>> Thanks,
>> Brendan
>> 
>> On Thu, May 26, 2016 at 1:55 PM, L. Curtis Boyle <curtisboyle at sasktel.net>
>> wrote:
>> 
>>> That is the safest way, although it will (partially) break anything that
>>> used the text characters intermixed between scanlines, as the characters
>>> have shifted within the character cell somewhat, and a few characters
>> have
>>> changed (the main one affect by this would be Synapse Software’s
>> Protectors
>>> II, and the mixed text characters they used to do the waving people. You
>>> will also notice some games that did scores, etc. with regular text to
>> the
>>> exact height of the text character (not including extra, unused space on
>>> the top and bottom) will also be cut off somewhat, if you exclusively use
>>> the Coco 2B/Coco 3 style lowercase VDG mode.
>>> 
>>> L. Curtis Boyle
>>> curtisboyle at sasktel.net
>>> 
>>> TRS-80 Color Computer Games website
>>> http://www.lcurtisboyle.com/nitros9/coco_game_list.html
>>> 
>>> 
>>> 
>>>> On May 26, 2016, at 12:48 PM, Arthur Flexser <flexser at fiu.edu> wrote:
>>>> 
>>>> What Darren says corresponds with my recollection as well, although I
>> did
>>>> not have an explanation for it.  You get vertical colored stripes but
>>> there
>>>> are black gaps between them.  Also, no ability to mix in letters, a
>>> useful
>>>> feature of other semigraphics modes.  When the CoCo 2B with the real
>>>> lowercase came out, with SG6 eliminated, I heard of zero complaints of
>>>> software that wouldn't work.  (I was selling ADOS at the time, which
>>> could
>>>> be configured to default to the real lowercase, so I was in a position
>> to
>>>> hear such complaints.)  SG6 is a completely useless mode.  If you are
>>> going
>>>> to put in lowercase characters, I think you should substitute this mode
>>> for
>>>> SG6 for compatibility with CoCo 2B and CoCo 3 software that uses the
>> real
>>>> lowercase.  That is, have the real lowercase mode turned on by what
>> would
>>>> previously have turned on SG6, as is the case with the CoCo 2B and the
>>> CoCo
>>>> 3.
>>>> 
>>>> Art
>>>> 
>>>> On Thu, May 26, 2016 at 12:22 PM, Brendan Donahe <brendan at polylith.com
>>> 
>>>> wrote:
>>>> 
>>>>> Thanks, Darren - that explains some of the things I have seen in SG6
>>> mode.
>>>>> 
>>>>> The Mini-MPI looks great, Ed!
>>>>> 
>>>>> Brendan
>>>>> 
>>>>> 
>>>>> 
>>>>> On Thu, May 26, 2016 at 11:18 AM, Darren A <mechacoco at gmail.com>
>> wrote:
>>>>> 
>>>>>> On Thu, May 26, 2016 at 7:41 AM, Brendan Donahe wrote:
>>>>>> 
>>>>>>> 
>>>>>>> I had the understanding that SG6 didn't work right on the CoCo, too,
>>>>>>> however, it looks to me like it can on some systems, and I don't
>> want
>>>>> to
>>>>>>> exclude the Dragon or MC-10.  (Also, I could swear that I saw 6
>> pixels
>>>>>> per
>>>>>>> character block in one of Steve Spiller's tests of SG6 on my CoCo 2.
>>>>> Can
>>>>>>> someone describe clearly what is broken?  Is it, perhaps, a limited
>>>>>> number
>>>>>>> of colors, caused by what pins are tied together on the board?  I
>>>>>> obviously
>>>>>>> have not spent much time investigating this yet...)
>>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> The problem with SG6 is that all 256 byte values are used to produce
>>>>>> graphics characters in that mode. But because the CoCo, Dragon and
>>> MC-10
>>>>>> all have the A/S pin hard-wired to data line D7, you can only use
>>> values
>>>>>> 128-255, eliminating half of the available colors in each color set.
>>> Byte
>>>>>> values 0-127 will give you the External Character ROM mode. Since
>> there
>>>>> is
>>>>>> no external character ROM you end up with vertically striped
>> characters
>>>>>> corresponding to the byte's bit pattern.
>>>>>> 
>>>>>> - Darren
>>>>>> 
>>>>>> --
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>>>>>> 
>>>>> 
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