CABLE FROM RCA TO AT&T FIBER OPTIC ???????????

Sorry, not a USB expert. Good luck!

Dweller:    I'm aware of that, it seems the Audio Research DAC has four inputs....one digital RCA, one BNC, one Tos-link and one AT&T................I have other stuff plugged so the only thing left are the two optics......OK here's another question ??   How long can I run a USB cable from my computer to my DAC  ??      Will

Toslink is not the same as AT&T (aka "ST") optical. Theta makes something called the "Lineque" converter that sometimes has an "ST" connector -sometimes not. See the following...
https://www.audiogon.com/listings/lis974e9-theta-timebase-linque-conditioner-signal-processor-13876-...

imhififan:   I ordered one of those devises, I hope that will do the job.......And if that doesn't work, we will keep trying......Will

Digital Coaxial (RCA) Audio to S/PDIF (Toslink) Digital Optical ?

https://www.amazon.com/Digital-Coaxial-Toslink-Optical-Converter/dp/B001622FL8