Re: FBUS and PDU
The term PDU is most commonly used when talking about Networking. If you are familiar with the 7 layers of the OSI model, the term PDU is used to refer to the output of the different protocols in each layer. (Physical, Data Link, Network, etc). I believe this is explained in the Wikipedia site. So that means the term PDU is a "general" word to refer to data produce by a protocol after "encapsulating" the data coming from the lower layers. The FBUS is one type of such protocol where we can use the term "PDU" to refer to its data. Although I'm not sure up to what layer the FBUS protocol defines (I think up to Physical layer only, the lowest layer).
If you have taken a CCNA course, the PDU is very much explained. I hope this clears up some confusion. Please feel free to ask if you have more concerns and I will try to explain it further.