The Serial Loopback Adapter configures Radio2 to function as a repeater by looping data back into the module for retransmission
 
The Null Modem swaps The RX and The TX
 
The 3 wire Pigtail is utilized for RS232 (1000 Series COMM)
 
The 2 wire pigtail is utilized for RS485 (100 Series COMM)
 
If you are connecting your Digi Extends to the 100 series controller you would need to use the 2 wire pigtail, because the iGrow 800 uses RS 485 Technology.