There's 4 COM port headers on the motherboard, but afaik they didn't get used on any games. I could be wrong though. I have an X4 with the drivers installed for the IO, but it's really strange trying to extract them from the image. It's also running Windows 8, so it's possible the driver is only for windows 8, and that's why i couldn't make it run on Windows 10.
I'm sure I could make a TTX3 DMAC work in a X4 system.... easy JVS, or just use a RS485 usb dongle.
The motherboard is just an off the shelf part, with Taito's custom (poorly protected) bios.
TTX3? Now those are very available and not too expensive. They are a beast for emulation and running other systems. It's amazing how good the hardware on these runs arcade systems. They also come with JVS and FASTIO support in every machine. Upgrade the video card and it gets even better! I don't really see any reason why you would buy a TTX2 over a TTX3 these days, the TTX3 hardware is just so much more powerful. Topic for another thread i guess.