RGB2VGA

OVERVIEW

Luis Antonioso has created a great 15KHz to VGA video converter based on the
Teresic DE-0 Nano FPGA development board.  A number of 15KHz video sources
from various computers of the 80's and early 90's are supported, including...

The Tandy Color Computer 3
The Apple ][ series
RGBI systems such as the C128 and others 

The RGB2VGA project consists of an I/O PCB that mounts on the DE-0 Nano via 2 40-pin
connectors.  This PCB provides the connectors and resistor networks needed to interface
the Nano to the video sources and a VGA monitor.  The signal processing is done by the
FPGA, according to several options selectable by dip switch and jumpers.

An LM1881 sync separator IC is included on-board to allow for separating composite sync
signals imbedded in some video formats (such as Apple ][ composite).

Luis has made the project available as a shared project on OSH Park for those who would
like to build their own boards.  The Quartus project for the FPGA code is available on github.
Links for these are provided at the end of this page.

Some minor improvements have been made to Luis's design in the form of improved resistor
values for the DAC, making for a better range of colors when converting analog RGB.
These were worked out by Roger Taylor of the Color Computer Facebook page.

I myself made some improvements to the PCB which include rerouting the board, revising
the silkscreen, some minor adjustment of parts placement, mitering corners, etc.
Basically just polishing the design.  These are the RGB2VGA SE boards, made for a run
of converters for members of the CoCo mailing list and Facebook groups.  Fifty PCBs were made.

Original Layout

RGB2VGA SE Layout

Functionally, they are the same.  Other than the resistor values, PCB layout, and one small pin-assignment correction to the code (for a dip switch position),
the SE boards are the same as Luis's original project.  Here are some pictures of the two types side-by-side, assembled with the same components.

Both versions, assembled with same components


OPERATION

In operation, the VGA_OUT is connected to a VGA monitor of course.  The board is mated to the DE-0 Nano, and the appropriate
connection must be made to one of the video inputs, depending on the system to be converted.  The DE-0 Nano is powered either by
the USB connection or the alternate power connector when the converter is in use.

Different dip switch settings can be made to affect how the video is processed, they will be explained a bit more in detail under setup
for the various systems below.


SETUP FOR COCO 3

The video put out by the CoCo3 RGB port is 15KHz analog RGB with separate HSYNC and VSYNC signals.

To set the converter up for use wit the CoCo3, you'll need a straight-through ribbon cable connection between the RGB port on
the bottom of the CoCo3 to the CoCo3 RGB connector on the RGB2VGA.  You can see how it should be attached in this picture.

CoCo3 cable connection

Once the cable is connected as shown in the picture, connect a VGA monitor to VGA_OUT, and connect a USB cable for power.

For the CoCo3, none of the jumpers should be bridged.  Start with all dip switches in the OFF position.
The SHRINK dip switch will shrink the picture somewhat.  This is needed for some CoCo3s as otherwise a small portion of the
left-hand side of the screen can be cut off.  I think this is is the case for machines with '87 GIME chips, if I recall correctly, Luis
mentioned that he developed the RGB2VGA using an '86 GIME machine.  The '86 GIME does not require the shrink switch to
be on.

The MSX, DENTERLACE, and APPLE ][ switches are not used when converting CoCo3 video.  Leave these in the OFF position.

While running software that uses composite video artifacts, such as hi res CoCo 1 and 2 games, the ARTIFACT switch is placed
in the ON position to enable artifacts.  The ART. INV. switch is used to change the artifact color set red/blue vs blue/red.

The SYNC switch should be left in the OFF position for use with the CoCo3.

The SCAN LINE switch causes simulated scan lines to be added to the video processing for an interesting "retro" look.

While in operation, the potentiometer at the bottom of the board is used to fine-tune the picture.  This may need to be adjusted
slightly to get the best picture for the different video modes, so it should be accessible during use.


SETUP FOR APPLE ][ 

(add picture here)

The video put out by the Apple is a 15KHz NTSC Monochrome Composite signal with embedded composite sync.

To use with an Apple ][ computer, connect the RCA video output jack on the Apple to the RGB2VGA RCA jack.
Bridge all three jumpers on the board, as we'll be using the LM1881 to separate the composite sync embedded in
the video signal into HSYNC and VSYNC.  Start with all dip switches in the OFF position.

The functions of the dip switches as used with the Apple ][ are as follows...

The SHRINK switch will shrink the picture displayed on VGA, not needed for Apple ][ mode.

The MSX and DEINTERLACE switches are not used with Apple ][ video, leave these in the OFF position.

The APPLE ][ switch when in the OFF position displays a green monochromatic screen, when in the ON position, white.

ART. INV. is not used in Apple ][ mode.

Use the ARTIFACT switch in the ON position, in conjunction with the APPLE ][ switch to display simulated composite artifacts (colors).

SYNC must be in the ON position when converting Apple ][ video.

The SCAN LINE switch causes simulated scan lines to be added to the video processing for an interesting "retro" look.


SETUP FOR C128

(to be added)

The video put out by the C128 RGB port is 15KHz TTL (digital) RGB, with an intensity signal, also known as RGBI.
The C128 RGB port also outputs separate HSYNC and VSYNC signal.

To use the RGB2VGA with an RGBI system, we'll need to construct a special cable.

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