Maybe some info on what game or games you’re trying to connect would also be helpful. Is it something, like a NAOMI, that natively outputs VGA?
Or, are you building a custom cable for the interface? Also, are you trying to split the signal, for (for instance) capturing through the OSSC, while playing on a cab?
nmalinoski – isn’t game PCB sync output typically TTL? So, it really would be a simple direct connection for RGBS to a HD-15 the appropriate pins on AV3 (csync on pin13)?