TX 750 was Yamaha's effort to build a more economical 750 class bike. They were notorious for mechanical issues, and went away fairly quickly. They were replaced with the XS750 triple. Shaft drive, DOHC, available as either standard, or a "special". I had a '79 standard that I restored before moving on to Honda machinery. It was a nice bike.
The TX/XS 500's were twins. Due to the poor reputation of the TX750, Yamaha renamed the TX500 to XS500, but I don't think they were much different mechanically - just styling, and details like mags vs. spokes.
1978 GL1000 - JUNE 2017 BOTM: Sold
2006 Kawasaki Concours: Sold
1995 Kawasaki Concours: Crashed/totalled.
1976 GL1000: LAST RESTORATION, sold
1981 Honda CB900F Supersport: 3rd restoration, sold.
1979 Yamaha XS-750F: 2nd restoration, sold.
1982 Honda FT-500 Ascot: First (only) new bike. Family forces sale.
1973 CB500 Four: First restoration, long gone.
1972 Suzuki TS-100: First bike, sold.
Only dead fish go with the flow . . .