To set the DL850 in Roll Mode, turn the Time/Div knob until it is set to 100msec/Div or longer.
If the Time/Div is set to 1Hr/Div or longer, waveform scrolling can be difficult to observe in real-time. This is due to the fact the waveform appears compressed on the time axis.
To observe the waveform in real-time, turn the Zoom window ON, and move the Zoom Position all the way to the right. Set the Main Ratio to 50% and adjust the MAG knob accordingly.