Garmin GDL 39 User Manual page 24

NOTE: Secure unused wires to prevent unintentional connections.
The GDL 39 and GDL 39R have a pass-through serial connection so a serial
device can be connected to a display device that has a single serial connection.
Some display devices do not support serial pass-through (such as older products
like the GPSMap 396/496).
NOTE: If the external serial device is not used, TxB should be left
unconnected and RxB should be connected to ground.
3-6
Garmin GDL 39 Portable ADS-B and GPS Receiver User's Guide
Figure 3-6 Serial Pass-Through Connection
190-11110-00 Rev. G