| Schematic | KiCad Project & all artifacts |
This card is a color video digitizer made for the Apple IIgs. It is basically just a simple NTSC-to-RGB converter with an analog-to-digital converter that the CPU polls to read the video data. A full color scan is done one column at a time and takes 6 seconds to complete. Chapter 6 of the manual contains a high level description of how a capture is done.
I did some work to disassemble the original software using 6502bench to confirm my hunches on how a scan was performed (disassembly project included) and reimplemented a simple capture program so I can try this card out using my Apple II+.
The card itself has none of the components labeled so I took the liberty of labeling them
so the components can be linked to the schematic. There is an IC (labeled U7) that has the identifier sanded off
on all pictures of this card that I could find however I found that this IC appears to be a TDA3330 “TV Color
Processor”.
Front Image
Back Image
Device Registers
The device has some I/O registers located at address $c080 + $10*<slot>.
Register 0 (read) - Data Read
This register returns the RGB data and some control bits.
| bit(s) | Description |
|---|---|
| 7 | Will be 0 when the video signal is in a blanking period |
| 6 | SENSE. Will be the value that was stored at this register in bit 4. |
| 5:0 | The 6-bit sampled value for the selected color |
Register 0 (write) - Scan Control
This register is used to control the video scanner hardware.
| bit(s) | Description |
|---|---|
| 7 | Enable sampling of the blue color channel. Do not enable more than one color channel at a time. |
| 6 | Enable sampling of the green color channel. Do not enable more than one color channel at a time. |
| 5 | Enable sampling of the red color channel. Do not enable more than one color channel at a time. |
| 4 | SENSE. Will be readable as bit 6. |
| 3 | Disables color to capture a grayscale image |
| 2 | Use “fast” horizontal scans (6 seconds) |
| 1 | Switch the capturing input to the monitor output for preview |
| 0 | Reset the horizontal scanner delay |
Register 1 - Trigger ADC
write-only
This register is only used to trigger the analog-to-digital to perform a conversion. The value written to the register doesn’t matter.