Substream through 8th style connect to the external DVD player through standard serial (DB9). 1st style, for some reason, uses the MINIDIN-8 port. It is a proprietary pinout that requires RTS/CTS connected in order to pass checks. This is a tested working diagram for the pinout to connect a DVD player to a Twinkle to play background movies on 1st Style. Colors are arbitrary to highlight the correct pins:
Pin 1 (red) - RXPin 2 (green) - RTSPin 3 (blue) - TXPin 4 (yellow) - CTSPin 5 - NCPin 6 - NCPin 7 - (brown) GroundPin 8 - (brown) Ground
Note that you can simply bridge pins 2 and 4 to get IIDX 1st Style to boot, but in technicality it should be passed forward to the serial cable.