[Coco] CoCo 1 & 2 Compatibility Question...

[Darren A]

On Mon, Jun 8, 2015 at 11:36 PM, Barry Nelson  wrote:

> Actually, later versions of the CoCo 2 had true lower case. If I remember
> correctly, these machines also lacked some of the semi-graphics modes, but
> I am not sure.
>
> Very late in the CoCo 2 production run, an enhanced VDG was available.
> Called the 6847T1, it included a lower case character generator and the
> ability to display a green/orange or black border on the text screen. Its
> other changes were mainly to reduce parts count by incorporating an
> internal data latch. The lower case capability of this VDG is not enabled
> by default on this system and is not even mentioned in the manual. Only
> through some tinkering and research was this feature discovered by intrepid
> CoCo users.
> The 6847T1 may also carry the part number XC80652P; these may have been
> pre-release parts.
>


The 6847T1 used in the later CoCo 2 models eliminated the Semigraphics 6
mode. This mode was already crippled in the earlier models (and likewise in
the Dragon and MC-10) due to the fact that the D7 line of the data bus is
hard-wired to the A/S input, allowing use of only 2 of the 4 colors in each
color set.

In addition to incorporating an internal data latch, the 6847T1 also has
built-in support for the NTSC artifacting hack that Tandy employed in
earlier CoCo 2 models (but not in the CoCo 1). As Zippster recently pointed
out to me, CoCo 2 models with the regular 6847 also have a 555 timer chip
that is used as a one-shot triggered by the horizontal sync pulse (HS*) of
the VDG. This is used to apply a negative bias to the phase A color output
just long enough to cover the Color Burst period, ensuring red/blue
artifacting rather than the magenta/green you would sometimes get with a
CoCo 1.

The datasheet for the 6847T1 provides a very brief description of pin 25 as
the "Burst Pseudo Color" open-drain output which pulls low at burst time
when using a BW graphics mode. This was almost certainly provided to
eliminate the need for Tandy's 555 timer circuit in the CoCo 2.  It makes
me think that the newer VDG was, at least partially, created to fulfill
specific requests from Tandy. Maybe that explains the use of "T1" in the
part number.

- Darren