What's the best way to connect Focusrite Trakmaster to Terratec EWX24/96?

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Trakmaster doesn't have XLR outs so I have to use a plug. My soundcard has stereo RCA I/O and an optical digital I/O. I don't think I will be using the optical one because I don't have the A/D converter for the trakmaster? So what kind of cable do I need? I'll be recording in mono so can I just use one RCA channel?