There is alot of information on the internet about the Pelco-D protocol, if you have a Pelco D controller it should be fairly obvious what all the messages mean. Here is a short document that has the basics of the Pelco D protocol:
http://www.bitman.ca/d_protocol.pdf

There is more to the protocol but I don't know if I have the rest of the documents anymore. The above document contains all the major information but there are more commands it doesn't talk about (mostly query commands for magnification, pan position, etc.).