The NASA AIS Engine 3 unit has a DB9 female connector from which we can get the signal to drive a multiplexer. There is much confusion about which signals we should use and to clarify this I went to a drawer where I keep old electronic instruments and picked up the NASA AIS which I used in my boat before mounting the excellent Matsutec unit that I have now on the boat. I also had an antenna for the VHF band mounted near the roof of my house which has a coaxial cable that enters through a window in a corner of the basement room where I run my small electronic lab. Unfortunately the end of the cable had no connector and the NASA AIS needs a BNC antenna connector. I was luck enough to find one in my lab. Also from about a dozen of 12V power adapters I found one (lucky again) whose connector suits the NASA AIS DC in. So I was able to make the test. From my house I see the Atlantic Ocean. Here is the marine traffic when I made the experiment:
Now the connection and the DB9 (I am not considering DB25) connectors. Cables connect DTEs to DCEs. DTEs use male DB9 sockets and DCEs use female DB9. A DTE is a Data Terminal Equipment (like Computers, Routers, Terminals, etc). A DCE is a Data Communication Equipment like a modem. The NASA AIS is a DTE. If we look to this diagram
we note that the labels TX or RX can confuse us. For example we would think that the AIS transmits data to the multiplexer and so we would look for wire labelled as TX to connect to the multiplexer input port. That is wrong. We should use RX. We should think as the data RECEIVED (and so RX) by the modem from the telephone network (or by the AIS from the VHF radio waves) should go to the terminal equipment (or to the multiplexer in the case of AIS). So looking to the AIS photo shown here:
we should connect pin 2 to A1 and pin 5 to B1 when we want to receive data on port P1 of the multiplexer. I leave a video that shows my experiment. I was not able to get a picture with the scope but I was able to get some AIS sentences on the debug window of a NMEA3WIFI multiplexer (one of the demos that I have).
Regards, Luis