NMEA Standards Overview

Summary from Boat Lore

NMEA 0183

ASCII text sentences beginning with $ (eg, $GPRMC, $GPGGA, $IIVHW)
Two-letter talker prefix: GP = GPS, II = integrated instrument, VH = speed log, etc.
Fields are comma-delimited; each sentence ends with a checksum
One-way broadcast per wire pair — a single talker, multiple listeners

Connect A(+) —▶ A(+) and B(−) —▶ B(−); never cross the polarity
Do not share one talker across too many listeners without an active multiplexer or buffer
Ground loops can be an issue — a ground isolator sometimes helps
Some older gear uses single-ended RS-232 (one wire + ground) instead of differential pairs
Common mistakes: reversed TX/RX, missing common ground, baud rate mismatch

Modern standard based on the automotive CAN bus (Controller Area Network) protocol
True network: a single backbone with T-connector drop cables to each device
Bidirectional — any device can be both talker and listener simultaneously
Runs at 250 kbps; much higher data density and update rates than 0183
Devices draw 12V power directly from the backbone
Engine data (RPM, temperature, fuel) flows natively — a major advantage over 0183

Binary messages identified by PGNs (Parameter Group Numbers), not human-readable text
Multiple devices share the bus simultaneously without collision
Backbone cable has two twisted pairs: NET-H/NET-L (data) and NET-S/NET-C (power/ground), plus a shield — DeviceNet-style connectors

Run the backbone as a single straight line — no star topologies
120 Ω terminating resistors required at each end; missing one causes network instability
Short drop cables (typically ≤ 6 m) connect devices to the backbone via T-connectors
Max backbone length ≈ 100 m; use a repeater for longer runs or high device counts
One and only one 12V power source on the backbone; avoid the engine starting battery if possible
Common mistakes: missing terminators, power injected at multiple points, loose connectors, voltage drop on long runs

Many modern chartplotters act as gateways between the two standards automatically
Dedicated gateway devices (e.g. Actisense NGT-1) also bridge 0183 and N2K
Each standard requires its own separate bus — they cannot share wiring

AIS transponders use high-speed NMEA 0183 (38,400 baud) or N2K PGNs on newer installs
If AIS shows a stale or fixed position, suspect loss of GPS input — the unit transmits its last known valid position when GPS data is absent
Most AIS position errors trace back to wiring or network faults, not the GPS itself

A newer Ethernet-based standard (announced ~2021) for high-bandwidth data such as radar and camera feeds
Adoption on recreational boats is still in early stages

NMEA 0183: point-to-point, RS-422, 4,800 baud, ASCII sentences, one talker per wire pair, no power over cable
NMEA 2000: bus network, CAN, 250 kbps, binary PGNs, bidirectional, power over backbone
0183 is simple to wire but easy to misconfigure; N2K is robust and plug-and-play but topology rules must be followed exactly