Relationship between NMEA 2000 and J1939

[F. N.]

Are they the same?
Maybe not.
NMEA 2000 claims to be based on the SAE J1939 high-level protocols, but defines its own messages. The NMEA 2000 devices and J1939 devices can be made to co-exist in the same physical network. Even though they shall not disturb each other, they don't understand each other either. So, they are different.

Here are a few example:

Even though both are defined running at 250K baud rate, they are different at both hardware and software (protocol) levels:
Major hardware differences:
0: rule of thumb, everything related to NMEA 2000 also means, expensive:(From devices to cable, from backbone to terminal resistors, etc. That's a point what SmartCraft is laughing at NMEA 2000.
1. NMEA 2000 requires opto-isolation, which is not required at J1939.
2. NMEA 2000 recommends Molex mini-c or micro-c five-pin connectors and cables (same or similar with DeviceNet cables), while J1939 recommends Deutch connectors (2 pin, 4 pin, 3 pin, and 9 pin connectors. 18 and 16 AWG wires recommended.).
3. NMEA 2000 max power is limited to 4A and 8A, J1939 has no such power requirements/limits.
4. NMEA 2000 needs dedicated network power supply for opto-isolation circuits, which is not required by J1939
5. NMEA 2000 opto-isolation might bring electric protection, but in the mean time also brings potential issue for CAN bit time synchronization. Network and software must be well laid out strictly with the NMEA 2000 standard. J1939 is robust enough to survive in a Truck and automotive environment and seems much friendly on network layout.


Major software differences:
1. NMEA 2000 needs expensive certification, which, however, doesn't guarantee working with other certified products. This is an expensive but useless feature. NMEA 2000 paid more attention on market and commercial rather than technology and innovation. J1939 protocol balanced the relationship between marketing and innovation much better.
2. NMEA 2000 Level A and Level B certification confuse end-users. Each product will have a different name, A-, A+, B, B+. This software mass doesn't exist at J1939.
3. NMEA 2000 uses dynamic name arbitration, J1939 uses pre-defined name (source addresses) for network nodes.
4. Software implementation in the NMEA 2000 network management is not so easy while J1939 network management is relatively easy.
5. NMEA 2000 was defined and claimed to be matured around year 2003. J1939 was released in the middle of 1980s.

Other differences:
1. NMEA 2000 protocols and documents are not so matured as J1939. For example, NMEA 2000 1.x and 1.y might not be compatible on some PGNs and SPNs. This has never happened on J1939 protocols.
2. NMEA 2000 protocol documents cost x times more than the J1939 protocols, which brings an heavy over-head to NMEA 2000 developers/users. This is one of the main reasons that the NMEA 2000 isn't so popular in the marine industry. The situation might change if NMEA 2000 defines itself as the "Linux" in the marine world.
3. NMEA 2000 is designed for Marine industry, but this industry is dominated by another CAN protocol: SmartCraft. J1939 is designed for on-highway industry and it is the de facto for USA diesel and truck industry.

Benefit from NMEA 2000:
1. After paying so much to NMEA organization, You will get a NMEA 2000 company name ID which will be a future J1939 ID too. (Free, kind of, if your business covers both NMEA 2000 and J1939)

Typical J1939 Network:

Typical Micro cable network:

Typical Mini cable network:

[F. N.]

NMEA 2000 has been designed being so complicated to implement, then why do we still need it?
1. NMEA 2000 was originally designed to upgrade the aged NMEA 0183 for GPS and navigation application. It does introduce a Controller Area Network (CAN) for those marine radar, cruise, and navigation applications. So, those devices finally get a unique protocol to communicate with each other.
2. NMEA 2000 uses dynamic source address. This introduces some prons and cons to the CAN network. But it does provide some flexibility to remap the limited source address (00-255) for large network which might have as many nodes as 200. So, the limit on source address has been pushed to the max. capability of which a normal CAN network can handle.
3. opto-isolated network may be required for some extremely harsh environments.

[Mikeduan]

1. NMEA 2000 requires opto-isolation, which is not required at J1939.

NMEA 2000 recommends opto-isolation for devices with multiple electronic interfaces or multiple power supplies.
For devices with only one electronic interface and only one power supply, opto-isolation is not necessary.

J1939 network doesn't have official recommendation on opto-isolation. For most truck and school bus application, there may be only one main power supply, so it does not bother. Technically you can still add opto-isolation to J1939 network if multiple power supplies are used.

[Mikeduan]

NMEA 2000 device can be powered from a single bus power supply if no other electronic interface is used and single power supply is used. The node must consume less than 1A current.
(The max. bus power is 4A (light cable, e.g. Molex Micro cables) and 8A (heavy cable, e.g. Molex Mini cables), depending on different cables used.)

[Mikeduan]

2. NMEA 2000 protocol documents cost x times more than the J1939 protocols, which bring an heavy over-head to NMEA 2000 developers/users.

NMEA 2000 might have slightly one-time fee, but over a long run, the J1939 annual web subscription may cost even more.

[Mikeduan]

On NMEA 2000, CAN shield is not connected to the board level. It is always the 4 wire (Power, Ground, CAN-H and CAN-L) connected with the circuit board in NMEA 2000 device.
On J1939-11, CAN shield is connected to the board with a Resistor-Capacitor filter. At one point of the J1939 backbone, the CAN shield is connected to battery negative.
On J1939-15, CAN shield is not used at all.

[Administrator]

Under NMEA 2000:
Maximum physical nodes: up to 50 connections
Maximum Functional nodes: up to 254 network addresses
Maximum backbone length: up to 200 meters (at 250Kbits/second bit rate)
Maximum stub drop length: up to 6 meter
Physical media: 5 wires (CAN-H, CAN-L, Power, Ground, Shield)
Shield: not connect with circuit board

SAE J1939-11 (Jacketed Shielded Twisted Pair (STP) 0.5mm2/20AWG and 0.8mm2/18AWG wires can be used):
Maximum number of ECUs: is fixed to 30
Maximum Bus Length (not including cable stubs): 40 meters
Maximum Cable Stub length: 1 meter, the cable stub length for the diagnostic connector is 0.66 meter maximum for the vehicle and 0.33 meter maximum for the off-board.
Physical media: 3 wires (CAN-H, CAN-L, Shield), require 3-pin connector concept when used.
Shield: connected to network drain wire.

SAE J1939-15 (Jacked un-shielded twisted pair (UTP) 0.5mm2/20AWG and 0.8mm2/18AWG wires can be used; allows the vehicle integrator to design a reduced network to meet design and cost goals with comparable performance to the J1939-11 network):
Maximum number of ECUs: is fixed at 10. (Due to the extended stub lengths from 1 meter to 3 meters.)
Maximum Bus Length: 40 meters
Cable Stub Length (included in Bus Length): 3 meters
An ECU that contains an internal 120 terminal resistor shall be designated as a Type II J1939-15 ECU, and shall have a unique marking on the outside housing to easily determine the internal terminal resistor feature. (J1939-11 doesn't allow Terminal resistor locate within an ECU)
The type of connector is not specified for implementing the J1939-15 network and a "standard" connector is not required. An ECU may be connected to the network with either a hard splice or connector. If a connector is used, it shall meet the connector electrical performance requirements in J1939-11.
Physical media: 2 wires (CAN-H, CAN-L) No shield wire.

[Mikeduan]

According to the committee, Some of the following capabilities will be added in the near future:

•  Alarms and Faults – a set of tools for a suite of alarms and faults.  This subcommittee has received input from the IEC INS group
 
•  Power Generation – PGNs are being created for status and control of power generation devices, such as generators, alternators, inverters, hybrids, and shore power on ships
 
•  Electrical Power Generation and Distribution – PGNs are being developed for the delivery of power on vessels; identifying loads, load sharing, and virtual breakers.
 
•  24-Volt Systems – There is a subcommittee studying the complexity of adding 24 volt systems to NMEA 2000, defining the rules and requirements.
 
•  Common Configuration – There has been a great deal of demand that manufacturer “A” devices on the network can configure manufacturer “B” devices on the network
 
•  Intelligent Gateways – Gateway that are “smart” and can communicate and recognize devices on both sides of the gateway and be able to transmit on the NMEA 2000 network status of the device on the
other side of the gateway.  Some companies today are in development of NMEA 0183 to NMEA 2000 Gateways.
 
•  NMEA 2000 Bridges – the ability to “bridge” to other network protocols   
 
•  Galileo – collaborating with Working Group

•  E-Loran – have had discussion with Working Group
 
   

[Mikeduan]

NMEA 2000 Product certification is granted for two classes and at two levels within those classes.

1.Class 1 devices: generally have a single NMEA 2000 interface that is used for all communication with other devices.  

2. Class 2 devices: have dual-redundant NMEA 2000 interfaces intended to be connected to redundant NMEA 2000 backbones, where parameter groups are intended to be transmitted in parallel on both interfaces.  

Note 1: Additional requirements are placed on Class 2 devices to ensure correct identification of identical parameter groups when received by another Class 2 device connected to the same two NMEA 2000 backbones. 

Note 2: Bridges may be certified for either class; however, a bridge with two NMEA 2000 backbone connections would usually be a Class 1 device because parameter groups are transmitted on each backbone independent of the other.
 
 

[Mikeduan]

Two levels, Level A and Level B, have been established in order to accommodate a wide range of devices with varying processing resources.

In particular, Level B was established to accommodate simple devices with minimal processing power and memory.  One main impediment to low-power, minimal-memory implementations was implementing the ISO Transport Protocol, which depends on a large amount of memory for buffering.  Level B was further simplified by omitting requirements to implement complex parameter groups with multiple functions that were only apparent after examining the data contained within the frame.  With these reductions in complexity, Level B devices are now possible using simple mailbox-style interrupt mechanisms used in less-powerful systems.   

The table below summarizes the minimum parameter groups required for both Level A and Level B implementations.


In addition to certification of standard self-contained products, NMEA 2000 contains provisions for third-party products.  Third-party products are those products that do not entirely meet the requirements for NMEA 2000 Certification until connected to or installed on another product, usually manufactured by another manufacturer.  Software packages, such as chart-plotting programs or instrumentation programs, are clear examples of third-party products.  Software packages are not functional until installed on a computer or embedded processing device, and they cannot be tested for certification separately.