PortObject question

Is there any documentation or simple example code which explains how to create a new PortType (which are the important methods to be implemented).

I had a look accross some code but I did not get the essentials.

Can anybody help? It's not part of the developer manual, isn't it?

So far, I did fill in the registration via the extension point and created the classes xxxPortObject and xxxPortObjectSpec.

Since PortObject and PortObjectSpec are interface you must implement all methods. The documentation of both these interfaces is quite extensive. If this still isn't enough, simply have a look at some existing PortObject implementations. There are quite a few in KNIME. If you have any specific questions, feel free to ask them.