I realized that I was not very clear in my earlier response. Are you trying to read pin 20 of same module?
0x07 frames are used to read/set values of remote module using local module.
If its local module, then you need to send 0x08 frame.
If you want to read pin 20 of module2 from module1 then set address field in 0x07 frame with address of module2. Address of module2 is its SH + SL parameters. It will look something like 000000409D5D7375.