Two ICOM 6021, two OPC 1939, and Allstar Node Single URI Interface:

URI DB25 pin to OPC 15 pin Transmitter

1 to 14
13 to 1
22 to 13

Same URI DB25 pin to OPC 15 pin Receiver

8 to 4
13 to 1
21 to 2

Set Transmitter TX and RX frequency and PL the same. Do the same for the Receiver TX, RX, and PL. To have PL work correctly, plug a blank plug into the speaker jack on the back of 6021. Turn the volume up 1/4 turn.