Learning objectives

• Understand the physical layer
• Understand the various interconnection methods of devices from different NMEA 0183 versions.
• Understands the Importance of the prevention of RF interference, and how this is achieved.

Physical Layer

 This is the rawest most basic layer, where the logistics and hard work of physically sending the data signals takes place. The electrical and mechanical aspects of the physical layer forms an electrically compatible link between devices specifying the voltages to be used and how signals are sent. This lesson covers the NMEA 0183 physical layers, and highlights the differences that exist between versions.

Key points

• NMEA 0183 V1 is single ended, and can only power up to 1 listener. Very rare as it was replaced in 1992
• NMEA 0183 V2 is widely still in use on board small craft from 1992 and 2005, but still very much in use in commercial shipping. V2 can power 3 or 4 listeners, the NMEA out wires A and B are both connected to the NMEA in wires A and B but the RF Ground wire is ONLY connected at the talker end.
• NMEA 0183 V2(SE) fairly common on small devices, and can be connected to both V1 and V2 listeners, but not approved under NMEA as a standard, and only short run capability, can only power 1 device, and is subject to interference.
• NMEA HS Designed primarily only for AIS systems is not directly compatible with any other version of NMEA without a dedicated converter connected to a suitable listener able to listen for, and comprehend non-AIS data.