PracticalExperience

Pilot Plug Design Considerations

From Ross Norsworth, L3-Communications

The idea of the "Pilot Plug" was from those of us who thought that pilots who would board ships already fitted with AIS would want to carry their own laptop PCs loaded with electronic charts, Tides & Currents programs, navigation software, and other applications that could interface with the AIS. We thought that they might bring a pair of wireless modems and hook one up to the AIS Pilot Port and another one up to their laptop (or palm-pilot) PC. Nowadays, many laptop PCs have imbedded wireless modems with the antenna integrated somewhere like into the display screen. We thought that the boarding pilot would appreciate a standard plug with the pins already defined. So, we decided to implement this. However, the AIS was already well down the road to standardization, so we were stuck with the rigors of IEC 61162-2 for the definition of interfaces (like RS-422, but just enough different to be a pain). We decided to keep it simple and just specify the essentials (RxA, RxB, TxA, TxB, and Shield (for a shielded drain wire that someone may use)). This would insure that there would be no conflict with grounds and signal returns, since the interface would be floating and purely differential, and it would permit the use of standard off-the-shelf RS-422-to-USB or RS-422-to-RS-232 converters. The portable laptop or palmtop unit would be happy to be connected thusly. In our system (the AIS produced by L-3), we also provide 8 Volts DC and a DC return so that the pilot can power the companion wireless modem that works with his carryaboard computer. We also have built-in diagnostics and a 5V logic interface. This is possible because the Pilot Plug has 9 pins, and 5 were designated. However, there is an action item to make further changers to this interface to add RxC and TxC leads to the definition because the latest version of IEC 61162-2 now demands this, but there is not yet any published revision to include it.