I am designing a dev tool (called Ninja* for some random reason) that will interface with the PC VIA the controller port (for ease of use). Here is my question:

Are the controller ports half-duplex (read or write, but not both) or monodirectional, as far as the D0 D3 D4 lines go?

Eg: If I set the expansion port to write mode, I can set those data lines hi or low, but if i set it to read, I can retrieve a hi or lo made by the NES.