1. Using Surgery to Teach an Old Dog New Tricks
Steve Spiller

2. Backstory Friends since 6th grade … that’s over 30 years now
Met through interest in the CoCo
Sometimes annual summer vacation projects
MAME cabinets
Nintendo DS game
ATARI 400/800/5200 game
Using real hardware and tools

3. The TRS-80 Color Computer
Originally released in 1980 by Tandy and sold at Radio Shack
My first computer!
16k RAM
Cassette deck cable
Microsoft BASIC
Motorola reference design
MC6809E CPU <1Mhz
MC6883 Synchronous Address Multiplexer (SAM)
MC6847 Video Display Generator (VDG)

4. MC6847 Video Display Generator (VDG)
Generates NTSC signal fed to MC1372 which forms a local transmitter for channel 3 or 4
Lots of “clones” simply built the block design from the VDG datasheet
High resolution graphics was 2-bit but NTSC has magic: Artifacting!
Bit patterns 0101 and 1010 produced color
Which color was random

5. Genesis of a Project Looking for a project for the CoCo
Hardware based
FPGA in a cartridge
Hardware tile engine ala Gameboy, NES
VGA output
Too ambitious
F18A Project FPGA TMS9918A
TI-99/4a, ColecoVision, MSX
Full emulation
CoCo VGA is Born
Time to gather data

6. CoCoVGA Started out with full replacement
Timing interactions with the M6883 were tricky and not documented
Switched to a “piggyback” solution
We plug in line with the VDG and snoop the signals

7. The Real Thing Approached by Ed Snider to move CoCoVGA into reality
Refined our prototypes
Designed PCB layout
Other plans:
Map registers into the address space
Software control of custom hardware
Wavetable/FM synthesis
Try to take over the world

8. DONE!