The wiki for the 400 says it too uses a USB cable for programming.
However, let's back up a minute- you say you are having problems programming a 780 (which is relatively straightforward, when you compare it to a 996). Precisely what problems are you having?
You are aware that ProScan can work with a 780, and download data from the RadioReference database, since you are a premium subscriber? The link for the package is:
http://www.proscan.org/index.htm
More specifics - like what system(s) you are trying to program (look them up in the database, and supply the URL) and what your problems are would be helpful. If you are having USB issues; they seem to often come down to 2 basic things;
a. Not having up to date drivers from the manufacturer's website - or -
b. Trying to assign a COM port that really isn't available, since something else is using it.
??? 73 Mike