Transport Output

[wanders]

I'm messing around with a Rasp. Pi transport and seemingly not getting any signal from the trans to the DAC. Probably a software issue. But, I'd like to be able to check the output without having having to plug it into my system. I could measure voltage with a multimeter or use the scope to see if there is any waveform. I've Googled around, but still have questions: What does a digital output waveform look like? Should there be voltage? (I read it was a square wave, but ???)

Thanks