JimDrew wrote:Ok, so the VGA output is strictly 15Khz?
You read it wrong. I've said that output frequency depends on core. Usually core can output both 15KHz and 31KHz depending on scandoubler option. 31KHz is not the only thing required for VGA compatibility. VGA also requires the vertical frequency to be at least 60Hz while many cores provide 50Hz or even 48Hz - so most VGA monitors won't work. That's why i've started MiSTer project. I couldn't agree that FPGA board must be used with special VGA monitor supporting non-standard frequencies and couldn't be used with random modern TV.
VGA connector is just connector - doesn't mean VGA compatible resolutions/frequencies. Usually core outputs video format which original retro system provided. Output format is fully controlled by FPGA core.
JimDrew wrote:Also, how close if your R2R ladder for the VGA proper colors? It's not perfectly linear based on the resistors you used, but I wonder if it is worth the added expense to make it perfectly linear - or if you adjust the colors in the cores to compensate for this?
Color resistors are not strictly correct for high precision, but enough for hobbyist project. VGA strictly speaking is for debugging purpose while main video output is HDMI. VGA is 18bit while HDMI is 24bit. So, full color systems like Minimig will have some banding on VGA regardless how R2R is correct. R2R should provide linear voltage on the output and cores don't compensate it. So, if you will provide better linearity then it will be better. You also need to compensate FPGA output resistivity if you want good linearity.
JimDrew wrote:Please take a look at SW4 in your schematic for v5.5 I/O board (probably other versions are the same as well). You have the connections 4 and 6 swapped compared to the PCB and the data sheet for the switch.
These switches are not standardized as ICs, so different manufacturers may have different pinout while physically they look and work the same. So, just follow the PCB footprint and schematics - it works correctly.