F&C Marine Technical Guide · Engine Data · CAN Bus · NMEA 2000
J1939 to NMEA 2000 Explained – PGNs, SPNs, CAN Bus Data & Engine Integration
J1939 to NMEA 2000 conversion starts with a deceptively simple problem: both networks use CAN signalling, but the information carried on them is not automatically interchangeable.
This guide follows an engine message from the ECU, through CAN-H and CAN-L, into a J1939 PGN and SPN, through the LXNAV CAN2CAN conversion table, onto an NMEA 2000 network and finally to a compatible marine display.
The story starts with two wires
Two CAN networks can look alike and still speak different languages
A service engineer connects an oscilloscope to an engine network. CAN-H moves upwards. CAN-L moves downwards. Frames are present. The installation is alive.
A few metres away sits an NMEA 2000 backbone carrying engine, navigation and sensor information around the vessel. It also uses CAN-based communication.
At first glance it is tempting to think that the job is simply a matter of joining the two CAN networks together.
That single point explains why a proper J1939 to NMEA 2000 gateway exists. The gateway has to understand supported information on one side, interpret it correctly and recreate the corresponding information in the format expected on the other.
1 Physical layer
J1939 to NMEA 2000 starts with the same fundamental CAN idea
At the physical level, high-speed CAN communicates using a differential pair: CAN-H and CAN-L. Rather than relying on one wire referenced only to ground, the receiver responds to the voltage difference between the pair.
Recessive state
In a typical high-speed CAN implementation, CAN-H and CAN-L sit close together in voltage, producing a small differential voltage.
CAN bit 1 RecessiveDominant state
During a dominant state, CAN-H rises and CAN-L falls, increasing the differential voltage seen by the transceiver.
CAN bit 0 DominantThis also creates one of CAN’s most useful behaviours: if one node sends a dominant bit while another sends recessive, the bus resolves to dominant. That property is fundamental to CAN arbitration.
2 J1939 application data
What J1939 actually carries: PGNs, SPNs and engineering meaning
Once the electrical waveform is healthy, the story moves up a layer. The CAN controller is carrying frames, but the engineer still needs to know what those frames mean.
SAE J1939 provides the application-layer conventions used to describe parameter groups and individual parameters within those messages. SAE J1939/71 describes the parameter and message concepts, encoding conventions, parameter placement and transmission-rate conventions.
3 Marine application layer
What NMEA 2000 does differently
The vessel side of the story also sits on a CAN foundation, but NMEA 2000 is designed specifically for marine electronic equipment.
The National Marine Electronics Association describes NMEA 2000 as a CAN-based, bidirectional, multi-transmitter/multi-receiver vessel network. It is multi-master and self-configuring, with compatible equipment sharing data over the network.
Same foundation
NMEA 2000 is CAN-based, so the presence of CAN-H, CAN-L and extended identifiers is not surprising.
Different definitions
The meaning assigned to marine PGNs and their data fields is defined by the NMEA 2000 standard rather than by J1939.
Vessel integration
Engine, navigation, GNSS, battery, fluid, environmental and other compatible marine data can coexist on the same network.
For a wider comparison of vessel data formats, see NMEA 0183 vs NMEA 2000 Explained .
4 The gateway
What LXNAV CAN2CAN actually translates
We have now reached the point where simply joining the wires is no longer enough.
The LXNAV CAN2CAN J1939 to NMEA 2000 Gateway is designed to receive supported J1939 engine information and translate corresponding information into NMEA 2000 messages.
If the engine has J1939
CAN2CAN is the relevant LXNAV route when supported engine information is already available on the J1939 engine network.
Depending on the engine connection, an appropriate interface lead may also be needed. F&C Marine lists the LXNAV CAN2CAN M12 to Deutsch Adapter Cable 0.5m .
If the engine uses analogue senders
That is a different integration problem. The LXNAV SmartEMU Analogue Engine & Sensor to NMEA 2000 Interface is the relevant LXNAV retrofit route for supported analogue and digital sender inputs rather than a J1939 engine bus.
The distinction is important when planning a retrofit. Our wider LXNAV NMEA 2000 Retrofit & Integration article looks at both routes in the context of an existing vessel.
5 Real conversion examples
From J1939 PGN and SPN to an NMEA 2000 PGN
This is where the theory becomes much easier to visualise. Instead of saying that CAN2CAN “converts engine data”, we can follow several individual parameters through the conversion table.
| Engine parameter | J1939 PGN | J1939 SPN | NMEA 2000 PGN |
|---|---|---|---|
| Engine speed | 61444 | 190 | 127488 |
| Engine oil pressure | 65263 | 100 | 127489 |
| Engine coolant temperature | 65262 | 110 | 127489 |
| Fuel rate | 65266 | 183 | 127489 |
| Total engine hours | 65253 | 247 | 127489 |
Look at engine speed first. On the J1939 network it is identified as SPN 190 within PGN 61444. CAN2CAN’s conversion table maps that supported information to NMEA 2000 PGN 127488.
Oil pressure, coolant temperature, fuel rate and total engine hours are different J1939 parameters carried in different J1939 Parameter Groups, yet the corresponding values shown above are delivered through NMEA 2000 PGN 127489.
6 Signal analysis
A good waveform is the start of diagnosis, not the end
Now imagine that the expected engine value does not appear on the marine display. The first temptation is to blame the gateway.
A better approach is to prove the data path one layer at a time.
Level 1
Oscilloscope
Use the waveform to assess electrical activity: CAN-H/CAN-L levels, timing, signal quality, ringing, reflections and some wiring or termination faults.
Level 2
CAN analyser
Capture frames and inspect 29-bit identifiers, DLC, payload bytes, traffic timing and error behaviour.
Level 3
Protocol interpretation
Decode the relevant J1939 PGNs and SPNs to establish what engineering information the ECU is actually transmitting.
What the scope trace cannot tell you by itself
A waveform alone does not tell you:
PGN = ?
SPN = ?
Engineering value = ?
Protocol meaning = ?
Those require frame and higher-layer protocol analysis.
This layered approach also applies to other vessel electronics faults. See our wider Marine Electronics Diagnostics guide for additional diagnostic examples.
7 Identifier anatomy
Inside the J1939 29-bit CAN identifier
When the analyser starts returning hexadecimal identifiers, the traffic can initially look like little more than a list of numbers. The extended CAN identifier is structured, however, and those fields help establish the J1939 message identity.
Priority
Three priority bits participate in CAN arbitration. A lower numerical priority value represents a higher arbitration priority.
Source Address
The source-address field identifies the network node transmitting the message, allowing similar Parameter Groups to originate from different devices.
PDU Format
The PDU Format field participates in deriving the Parameter Group and determines how the PDU Specific field is interpreted.
PDU Specific
For PDU1 format messages it acts as a destination address. For PDU2 format messages it acts as the group-extension field and becomes part of the PGN.
8 Engine reality
A J1939 connector does not guarantee every engine value you want
This is one of the most important practical points in the entire job.
An engine may have a J1939 interface and still not transmit every parameter that an installer or vessel operator expects to see.
This is why F&C Marine needs more than the phrase “the engine has J1939” before promising a particular display result.
If available, a capture of the actual J1939 traffic is particularly useful because it changes the discussion from what the engine might support to what the installation is actually transmitting.
9 Prove the path
Diagnosing J1939 to NMEA 2000 one layer at a time
We can now return to the original vessel. The display is missing engine information. Instead of replacing parts, we prove each stage.
1–4: Prove the J1939 network
Check CAN-H and CAN-L electrical behaviour with appropriate equipment, establish the actual network bitrate, confirm the termination required by the physical implementation and capture valid CAN frame traffic.
5–6: Prove the data exists
Decode the 29-bit identifiers, identify the relevant J1939 PGNs and confirm that the required SPNs are present with usable data.
7: Prove the mapping
Compare the observed J1939 PGNs/SPNs with the current CAN2CAN conversion documentation and verify that the required parameter is supported.
8: Prove the NMEA 2000 result
Finally, inspect the NMEA 2000 network and establish whether the expected converted PGNs are present and available to the receiving equipment.
After translation
Once converted, the engine information becomes part of the vessel network
A successful conversion does not end at CAN2CAN. The translated engine information is transmitted onto the NMEA 2000 network, where compatible equipment can use the relevant PGNs it supports.
LXNAV’s current E-series documentation includes reception of engine-related NMEA 2000 PGNs such as rapid engine parameters, dynamic engine parameters and transmission parameters.
Relevant LXNAV display options at F&C Marine
Putting the layers back together
The F&C Marine integration path
After working from voltage levels to frames, PGNs, SPNs and conversion tables, the complete system becomes much easier to understand.
The visible installation may therefore look simple. The important work is making sure that every layer underneath it is correct: the source data exists, the J1939 traffic is healthy, the required parameter is supported, the gateway is correctly connected and the NMEA 2000 network and receiving equipment are functioning as intended.
Common mistakes
Five assumptions that cause unnecessary J1939 integration problems
“Both are CAN, so connect them together.”
CAN signalling is only part of the system. The application-layer definitions still have to be translated.
“The engine has J1939, so everything is available.”
The actual set of broadcast engine parameters varies between engines and ECU configurations.
“The waveform looks good, so the data must be right.”
A scope can establish physical-layer behaviour without identifying the engineering meaning of the payload.
“Any NMEA 2000 display will show every value.”
Receiving equipment must support and present the relevant NMEA 2000 PGNs and parameters.
“One known mapping proves the whole engine.”
Each required parameter should be confirmed against the actual engine data and current gateway conversion documentation.
“No display value means the gateway is faulty.”
The fault can exist before the gateway, inside the conversion path, on the NMEA 2000 network or at the receiving display.
Planning before parts
What F&C Marine needs before specifying a J1939 to NMEA 2000 installation
The quickest route to a reliable installation is to start with the required information rather than with a connector.
Where the information is uncertain, the installation may need to move from specification into diagnosis: capture the J1939 traffic first, prove what is present, then decide what can be translated and displayed.
If the vessel does not yet have an NMEA 2000 backbone, use our NMEA 2000 network building guide before planning the device drops and gateway connection.
Technical FAQ
J1939 to NMEA 2000 questions we are likely to be asked
Can J1939 be connected directly to NMEA 2000?
They should not be treated as directly interchangeable simply because both use CAN-based communication. Their higher-layer data definitions differ. Supported J1939 engine information requires an appropriate gateway or converter when it is to be represented as NMEA 2000 data.
What is a PGN in J1939?
PGN means Parameter Group Number. It identifies a defined J1939 Parameter Group or message type at the application layer.
What is an SPN?
SPN means Suspect Parameter Number. It identifies an individual J1939 parameter such as a defined engine measurement or status parameter.
Does J1939 use a 29-bit CAN identifier?
J1939 uses the CAN extended frame format with a 29-bit identifier. Fields within that identifier are used in deriving the Parameter Group and identifying the transmitting source.
Can an oscilloscope identify a J1939 PGN?
A scope is extremely useful for physical-layer diagnosis, but a waveform by itself does not provide the engineering interpretation of a J1939 PGN or SPN. CAN frame capture and appropriate protocol decoding are required.
Will CAN2CAN display every engine parameter?
No blanket assumption should be made. The engine must transmit the required J1939 data and CAN2CAN must support the corresponding conversion. The receiving NMEA 2000 equipment must also support the resulting data.
What happens if the engine does not transmit a required parameter?
A gateway cannot create an engineering measurement that is not available from the source system. Another source, interface or sensor strategy may be required depending on the installation.
What if my engine uses analogue gauges rather than J1939?
That is a different retrofit path. For supported analogue and digital sender inputs, see the LXNAV SmartEMU rather than assuming CAN2CAN is appropriate.
Build the whole picture
Related F&C Marine technical guides
CAN2CAN, SmartEMU and the wider LXNAV retrofit ecosystem. How to Build an NMEA 2000 Network
Backbone, power, T-connectors, termination and devices. NMEA 0183 vs NMEA 2000 Explained
Understand the difference between serial NMEA 0183 and CAN-based NMEA 2000. NMEA 2000 Backbone Problems
Troubleshooting power, topology, termination and network faults. Marine Electronics Diagnostics
Practical fault-finding across integrated vessel electronics. Marine Electronics Knowledge Base
Browse further F&C Marine technical guidance.
Engine integration support
Before promising engine data, prove what the vessel actually has
F&C Marine can help with J1939 and NMEA 2000 integration planning, equipment supply, network checks, diagnostics, pre-dispatch preparation, installation and commissioning.
For an engine-data enquiry, send the engine make and model, ECU details, photographs of the existing interfaces, the parameters you want to display and details of the vessel’s existing NMEA 2000 equipment.
View LXNAV CAN2CAN Ask F&C MarinePrimary technical references
Standards and manufacturer documentation
- National Marine Electronics Association – NMEA 2000 — official overview of the CAN-based marine electronics standard.
- SAE J1939/71 – Vehicle Application Layer — SAE reference covering J1939 parameter and message definitions and conventions.
- LXNAV Marine – CAN2CAN — manufacturer’s CAN2CAN product information.
- LXNAV Marine – Manuals & Technical Downloads — current manuals for CAN2CAN and related LXNAV equipment.
Technical standards and manufacturer documentation can be revised. For installation or compatibility decisions, check the current applicable documentation for the exact engine, gateway, network and receiving equipment.
Products for J1939 to NMEA 2000 engine integration
The main LXNAV product covered by this article is CAN2CAN. Use it only where the required J1939 parameters are actually present and supported; a J1939 connector alone does not guarantee every engine parameter will be available.
- LXNAV CAN2CAN J1939 to NMEA 2000 Gateway.
- CAN2CAN M12 to Deutsch Adapter Cable — where this cable is appropriate to the confirmed engine/interface arrangement.
- LXNAV SmartEMU — the alternative route for analogue engine senders rather than a J1939 data bus.
- E360, E500, E700 and E900 — display options once compatible information is on the NMEA 2000 network.
Related technical guides: How to Build an NMEA 2000 Network · LXNAV Retrofit & Integration · E-series Display Comparison