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

[Arthur Flexser]

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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