When the link I/F you use is the CPU Direct type, setting of External Device is not necessary. When you use a function extension board and a communication adapter, make the settings as shown below.
Uncheck the checkbox for [Operate communication setting] in [PLC system (2)] of Mitsubishi's GX-Developer.
Store data "0" in D8120
and between D8173 and D8180. Then, set M8070 and M8071 to OFF.
When using Channel 2 for FX3U, FX3UC, FX3G or FX3GC, store data "0"
in D8420 instead of D8120. When using Channel 1, store data "0"
in D8120.