NMEA 0183, NMEA 2000, RS-232, RS-422, RS-485 and CAN: What They Really Mean on Boats

One of the most common sources of mistakes in marine electronics is mixing up four different concepts: the data protocol, the electrical layer, the network topology and the connector.

A connector that looks familiar does not guarantee the signal behind it. A DB9 can carry RS-232, RS-422, RS-485, CAN or a proprietary interface. A twisted pair can carry NMEA 0183, Modbus RTU, CAN or something manufacturer-specific. And RS-485 is not Modbus: RS-485 defines how bits travel electrically; Modbus defines what those bits mean.

Practical rule: first identify the electrical layer, then the speed, then the protocol. Never connect by connector shape alone.

The Four Layers People Often Confuse

• Data protocol: NMEA 0183, Modbus RTU, NMEA 2000, J1939, CANopen, proprietary messages.
• Electrical layer: TTL-UART, RS-232, RS-422, RS-485, CAN.
• Topology: point-to-point, linear bus, backbone with drops, star, multidrop listener network.
• Connector: DB9, screw terminals, Micro-C, Deutsch, RJ45, M12, or a proprietary plug.

The same protocol can use different electrical layers, and the same electrical layer can carry many different protocols. This distinction matters because connecting the wrong interface can damage ports, create intermittent faults or make a network impossible to diagnose.

Quick Comparison Table

TTL-UART: Useful Inside Equipment, Dangerous Outside It

UART defines asynchronous serial characters: baud rate, data bits, parity and stop bits. It does not define voltage. When people say “TTL serial” they usually mean logic-level signals such as 0–3.3 V or 0–5 V.

Device A TX  ---------->  Device B RX
Device A RX  <----------  Device B TX
Device A GND -----------  Device B GND

• Three conductors are normally required: TX, RX and GND.
• TX and RX are crossed.
• It is normally point-to-point.
• It is not designed to run around a yacht.
• It is sensitive to ground differences, noise and overvoltage.

Important warning: a 3.3 V TTL port can be damaged by 5 V, by RS-232 voltages, or by differential RS-422/RS-485/CAN signals. A USB-TTL cable is not the same as a USB-RS-232 adapter, even if both appear as a serial port on the computer.

RS-232: Point-to-Point Serial with Positive and Negative Voltages

RS-232 is a single-ended electrical interface. Each signal is measured against a common ground. Typical drivers operate around ±5 V to ±12 V, and the logic is inverted: idle is normally negative.

TXD  ---------->  RXD
RXD  <----------  TXD
GND  -----------  GND

Three conductors are enough for basic communication without hardware flow control. Additional lines such as RTS/CTS, DTR/DSR, DCD and RI may be present.

DB9 Does Not Mean RS-232

A PC-style DB9 DTE port often uses pin 2 as RXD, pin 3 as TXD and pin 5 as GND. But the connector alone proves nothing. Manufacturers sometimes place RS-422, RS-485, CAN or proprietary signals on a DB9 shell.

With a multimeter, an RS-232 TX line measured to GND will often sit around -5 V to -12 V when idle. If you see positive and negative swings, it is probably RS-232 rather than TTL.

RS-422: Differential Serial, Usually One Talker

RS-422 uses a differential pair. The receiver reads the voltage difference between two conductors instead of reading one conductor against ground. This greatly improves noise immunity.

TX+  ----------------------  RX+
TX-  ----------------------  RX-

For full duplex communication, two pairs are normally used: one pair for transmit and one pair for receive.

Device A TX+  ---------->  Device B RX+
Device A TX-  ---------->  Device B RX-

Device A RX+  <----------  Device B TX+
Device A RX-  <----------  Device B TX-

• One driver can feed one or several receivers.
• There should normally be only one transmitter on each pair.
• Termination is placed at the far receiver end, usually around 100–120 Ω.
• Use twisted pair cable with appropriate impedance.

In marine electronics, RS-422-like interfaces are commonly associated with NMEA 0183, IEC 61162-1/2, gyrocompasses, satellite compasses, autopilots, repeaters and navigation systems.

RS-485: A Multidrop Differential Bus

RS-485 is electrically similar to RS-422, but it is designed so multiple drivers and receivers can share the same bus. This is why it is widely used for Modbus RTU, PLCs, chargers, inverters, monitoring devices and proprietary control systems.

2-Wire RS-485

120 Ω                                         120 Ω
  |                                              |
  +======+==========+==========+==========+======+
         |          |          |          |
       Node 1     Node 2     Node 3     Node 4

The same pair is used for transmit and receive. Only one node should transmit at a time, and the higher-level protocol decides who is allowed to talk. This is half duplex.

4-Wire RS-485

Four-wire RS-485 uses two pairs: one pair from master to slaves and one pair from slaves to master. Full duplex is electrically possible, although many master/slave protocols still operate sequentially.

Master TX+  =================  Slave RX+
Master TX-  =================  Slave RX-

Master RX+  =================  Slave TX+
Master RX-  =================  Slave TX-

RS-485 Is Not Modbus

RS-485 is an electrical standard. Modbus RTU is a message protocol. A twisted pair marked A/B may carry Modbus RTU, but it may also carry a proprietary protocol. Likewise, Modbus can be transported over different media depending on the implementation.

• Use a linear bus, not a passive star.
• Place approximately 120 Ω termination at the two physical ends only.
• Do not install a terminator at every device.
• Use failsafe biasing where required, normally at one point.
• Use galvanically isolated adapters in marine environments whenever possible.

Also be careful with A/B naming. Some manufacturers use A as positive, others use A as inverting or negative. Always check the manual for +, −, non-inverting or inverting indications.

CAN: Differential, But Not RS-485

CAN also uses a differential pair, but its bus logic is different. CAN has dominant and recessive states, non-destructive arbitration and built-in error handling. Multiple nodes can begin transmitting, and the identifier priority resolves access to the bus.

CAN-H  ==================================
CAN-L  ==================================
GND    ----------------------------------

In high-speed CAN, both lines are typically around 2.5 V in the recessive state. In the dominant state, CAN-H rises and CAN-L falls, creating a differential voltage of roughly 2 V.

• Use a linear bus with two 120 Ω terminators.
• Keep stubs/drop lines short.
• Do not build passive stars.
• A USB-CAN adapter is not a simple serial cable: it needs a CAN controller, CAN transceiver, bitrate configuration and usually a dedicated driver/API.
• Use listen-only mode when investigating an unknown CAN bus.

NMEA 0183: Marine Serial Messages over a Differential Interface

NMEA 0183 combines marine sentence formats with an RS-422-like electrical interface. It is usually unidirectional: one talker sends to one or more listeners.

Talker TX-A / TX+  ----------  Listener RX-A / RX+
Talker TX-B / TX-  ----------  Listener RX-B / RX-

Typical speeds are 4,800 bit/s for standard NMEA 0183 and 38,400 bit/s for high-speed use such as AIS. Messages are normally ASCII sentences starting with $ or !, such as HDT, GGA, RMC, MWV, RSA or VDM.

However, an RS-422 connection is not automatically NMEA 0183. It may carry binary data or a proprietary protocol from a manufacturer.

NMEA 2000: CAN-Based Marine Networking with Power

NMEA 2000 is based on CAN and runs at 250 kbit/s. It is multi-device, multi-transmitter and self-configuring, with no central network controller. It uses a backbone with drops rather than separate point-to-point links.

Terminator                                               Terminator
   |                                                          |
===T===========T==============T==============T=================
   |           |              |              |
 Device     Device         Power tee       Device
  drop       drop                            drop

• Backbone/trunk: the main line.
• T-connector: connection point to a device or power insertion.
• Drop/spur: the branch to a device.
• Terminators: one at each physical end of the backbone.
• Power insertion: feeds the network, normally around 12 V nominal.

With the network powered off, a healthy NMEA 2000 backbone with two correct terminators normally measures approximately 60 Ω between NET-H and NET-L. Around 120 Ω usually means one terminator is missing. Around 40 Ω may indicate three terminators. Do not measure resistance on a powered network.

Protocol vs Interface: Practical Examples

Modbus RTU
└── UART framing
    └── RS-485 electrical layer
        └── Twisted pair cable
            └── Screw terminals

NMEA 2000
└── PGN messages
    └── CAN frames
        └── CAN-H / CAN-L
            └── NMEA 2000 cable
                └── Micro-C connector

How to Identify an Unknown Marine Interface

Three wires: TX, RX and GND

• Could be TTL-UART or RS-232.
• Negative idle voltage relative to GND suggests RS-232.
• Idle around 3.3 V or 5 V suggests TTL-UART.

Four lines: TX+, TX−, RX+, RX−

• Could be RS-422, 4-wire RS-485, bidirectional NMEA 0183 or a proprietary differential interface.
• You cannot determine the protocol from the wiring alone.

Two lines marked A/B

• Often RS-485 2-wire, but not guaranteed.
• Check termination, polarity, baud rate and protocol framing.

CAN-H and CAN-L

• Likely CAN.
• With power off, approximately 60 Ω between H and L suggests two terminators.
• Both lines often sit around 2.5 V at rest when powered.

Five wires: red, black, white, blue and bare

• Very likely NMEA 2000 or a related CAN-based marine network.
• Red/black are network power, white/blue are CAN data, bare is shield/drain.

Rules for Working on Boats

1. The connector does not identify the protocol.
2. The cable does not fully identify the protocol.
3. Do not confuse GND with shield/drain.
4. On RS-422, RS-485 and CAN, think differentially.
5. Use galvanically isolated interfaces whenever possible.
6. Use listen-only mode on CAN when investigating.
7. Never feed NMEA 2000 network power into a generic USB-CAN adapter.
8. Do not add terminators before checking existing termination.
9. Identify the electrical layer first, the speed second and the protocol third.

Mental shortcut: TX/RX/GND with negative idle voltage suggests RS-232. TX/RX/GND at 3.3–5 V suggests TTL. TX± and RX± suggests RS-422 or 4-wire RS-485. Shared A/B suggests RS-485. CAN-H/CAN-L with about 60 Ω powered off suggests CAN. A five-pin Micro-C network with power is usually NMEA 2000.

Final Advice

In yacht electronics, guessing interfaces is expensive. A wrong adapter can damage a port; a wrong terminator can make a network intermittent; a wrong assumption about protocol can waste hours of diagnostics.

If you are integrating navigation electronics, NMEA 0183, NMEA 2000, Modbus devices, CAN gateways, tank monitors, battery systems or monitoring equipment onboard, identify the layers carefully before connecting anything.

Área Electrónica works with marine electronics, yacht connectivity and onboard technical integration in Mallorca. If you need help identifying, adapting or troubleshooting a vessel interface, request a technical inspection before modifying the wiring.

References

• Texas Instruments — The RS-485 Design Guide
• Analog Devices — RS-485/RS-422 Circuit Implementation Guide
• NMEA — NMEA 2000 Standards
• Maretron — NMEA 2000 Network Installation Guide
• Actisense — How to Test an NMEA 2000 Network
• FTDI — TTL-232R USB to TTL Serial Converter Cables
• Texas Instruments — Introduction to CAN
• Analog Devices — Isolation in RS-485 Applications