RS-232 works well for DNC (drip-feed) applications, but if you want to transfer programs back and forth much easier and faster, use the Ethernet and hard-drive features.
There is some helpful info here:
Connecting Your CNC Machining Center to a PC | Haas
Here is some info that supports using Ethernet:
Downloading Large Files: RS-232 vs. Ethernet | Haas
Documents for troubleshooting RS-232: