Hi jujuyy,
What module is the 1300 connected to? I'm guessing that the module is actually filtering the signal you are inputting, which is basically smoothing out your square wave and making it look sinusoidal. Depending on the module you are using, this filter could be controlled by either hardware jumpers or by software settings.
As for the 1520 signal you are seeing, can you explain how the signal looks different from the oscilliscope (or possibly take a screen shot you can post)? Also, how are you looking at the signal? Are you just using a test panel in Measurement and Automation Explorer (MAX)? If so, you could try unchecking the "Autoscale" box and manually adjusting the limits of the graph, just to see if "zooming in" on the signal makes it look any better.
Thanks,
Justin M.
National Instruments