XPS 8300 S/PDIF Toslink audio output needs enabling

XPS 8300 on WIndows 7 Home Premium. The Realtek High Definition Audio manager lists 3.5mm jack sockets for analogue audio in and out . It also shows the S/PDIF output on the rear for digital audio out.

While the analogue audio output works, the S/PDIF Toslink output has a continuous red signal light with no digital modulation. Linking it to digital input on a DAT audio machine shows no signal.

I've tried examining all device parameter selection options, but none that I have found brings up a Digital Output selection where I could enable S/PDIF. Anyone know what I am missing??

Thanks