Barcode Scanner to Arduino/Interfacing
Clock and data are open-drain on the old AT keyboards. Put some pullup resistors (like 4K7 on each) and have another go at it. 

Okay, given that you actually had the resistors, how about that 5kSa/s on your scope waveform. I'd expect to see more like 1Gsps on a proper scope. 

Those angled lines are just the scope connecting the dots with really horrible horizontal resolution.