I see someone is still relying on outdated information.
There is NO READ/WRITE TOGGLE on the NES Expansion Port! There never has been, and there never will be.
This is how it REALLY works:
1. Write a value to the bottom 3 bits of $4016. These values will appear on appropriate pins on the expansion port, and D0 will also appear on the controller ports (the 'strobe' signal).
2. Read $4016 or $4017. The bottom 5 bits will be loaded from appropriate pins on the expansion port and/or from the appropriate controller port (D0/D3/D4 only).
The D0/D3/D4 lines on the standard controller ports are read-only. The 'strobe' line on the controller ports is write-only. If you want more signals, you'll have to connect to the expansion port (in which case you simply get 3 write signals and 10 read signals instead of 1 write signal and 6 read signals).
P.S. If you don't get this note, let me know and I'll write you another.