The RS232 port on the denon amp is used for integration with home automation systems such as Crestron and AMX. It provides a two way protocol to control and receive feedback from the amp.
The RS232 port on the Sky box is designed for engineers only, which is a real shame as I would love to use this to control sky. According to all the people at Sky I have talked with, this port does not provide a protocol to control the box.