What it is
NMEA 0183 is a text-based serial protocol where one talker device sends ASCII sentences to one or more listeners. It is standardised as IEC 61162-1 and dates back decades.
How it works
Data runs at a low serial speed over RS-422/RS-232 as comma-separated sentences, each identifying its data type. AIS messages, for example, are carried inside dedicated NMEA 0183 sentences.
Limits and legacy
It is slow and effectively one-directional, and has largely been superseded by NMEA 2000. Yet it remains everywhere in older and specialised equipment, so it still has to be supported.