Serial Data Networking of Marine Electronic Devices

NMEA 2000 Bus Description

The NMEA Standards Committee developed the NMEA 2000 interface which is based on the electrical CAN Bus [ISO 11898-2 as physical layer].
The higher-layer protocol is based on J1939 and ISO 11783. The Controller Area Network (CAN) specification defines the Data Link Layer.
In general only the physical layer [mechanical and electrical interfaces] of the OSI model is covered on this site.

The NMEA-2000 interface is a Balanced (differential) 2-wire interface running over either a Shielded Twisted Pair round cable, including power and ground.
Each node uses a one of two different, five pin circular connectors, just as some implementations of CANbus.
The Bit Encoding used is: Non Return to Zero (NRZ) encoding (with bit-stuffing) for data communication on a differential two wire bus.
The use of NRZ encoding ensures compact messages with a minimum number of transitions and high resilience to external disturbance.

NMEA-2000 Bus Electrical Interface Circuit

One data rate is defined at 250kbps (Bits per second) being the top end.
Unlike CANbus the data rate does not appear to change with cable length but may be 250kbps data rate over 200 meters.
The 121 ohm termination resistors are added to the cable, at the two extreme ends of the trunk line.
Devices are connected to the bus via drop cables with Tee into the network cable.
The maximum trunk length or network cable is 200 meters and the maximum drop length off the trunk is 20 meters.
The specification also defines that the maximum number or total drop length should not exceed 78 meters.

The CAN Bus interface uses an asynchronous transmission scheme controlled by start and stop bits at the beginning and end of each character. This interface is used, employing serial binary interchange. Information is passed from transmitters to receivers in a data frame. The data frame is composed of an Arbitration field, Control field, Data field, CRC field, ACK field. The frame begins with a 'Start of frame' [SOF], and ends with an 'End of frame' [EOF] space. The data field may be from 0 to 8 bits. The frame check sequence is derived from a Cyclic Redundancy Code (CRC); the coefficients are generated modulo-2: X15 + X14 + X10 + X8 + X7 + X4 + X3 + 1. CAN implements five error detection mechanisms; 3 at the message level and 2 at the bit level [Also incorporates error flags]. At the message level: Cyclic Redundancy Checks (CRC), Frame Checks, Acknowledgment Error Checks. At the bit level: Bit Monitoring, Bit Stuffing.

The pin out for the 5-pin connector is shown in the table below.
Basically it's the same signal arrangement used with CANbus when found on a circular connector.
However the [CAN_] signals are now called [NET-] net signals.
The power signals are twisted together [red / black], as is the signal pair [blue / white].

NMEA-2000 Pinouts

The Mini-C [Mini-Change] connector is used on thick backbones [heavy cable].
The Micro-C [Micro-Change] connector is used on thin backbones [light cable].
Each cable uses different wire gauges for the different signal lines.
The power lines are 22AWG for the light version and 16AWG for the heavy version.
The signal lines are 24AWG for the light version and 18AWG for the heavy version.
Maximum current is also defined as 3 amps for the light version and 8 amps for the heavy version.
The current difference indicate why one user might prefer a thick backbone over a thin one.

{NMEA-2000 Index}

NMEA-2000 Interface Organizations

NMEA {National Marine Electronics Association} CiA {CAN In Automation - International Users and Manufactures Group} ISO {International Organization for Standardization} ODVA {Open DeviceNet Vendors Association} Listing of Standard Organizations [related to electrical engineering] The electrical layer for NMEA-2000 uses the ISO specification defining CANbus. The physical layer for NMEA-2000 uses an ODVA specification defining DeviceNet for the connector and cable. The connectors screw together, as opposed to bayonet or other styles.

NMEA-2000 Drop Cable

{NMEA-2000 Index}

CAN Bus Interface ICs

Integrated circuits designed to offer CANbus should function as NMEA-2000 ICs.
After all there designed to the same electrical standard.
An integrated circuit manufacturer may not have a separate call-out for a NMEA IC.

CAN Bus uses a Drive Voltage: High; 2.75v to 4.5 volts, Low; 0.5 to 2.25 volts, Differential 1.5v to 3.0 volts

CAN Bus Interface IC Logic Transition Levels

Analog Devices, Inc. {Transformer Isolated CAN Transceiver, ADM3052}

Dallas Semiconductor 'Maxim' {DS80C390 Dual CAN High-Speed Microprocessor, bus controller ICs}

freescale {33389/33388 low speed fault tolerant CAN transceiver}

Infineon {82C900 Stand-alone TwinCan Controller-TLE6250 CAN Transceiver IC Manufacturer}

Intel {CanBus Interface 82527 IC}

Linear Technology {CAN Transceiver IC Manufacturer}

Maxim Integrated Products {DS80C390 Dual CAN High-Speed Microprocessor, bus controller IC}

Melexis {CAN Bus Transceiver IC Manufacturer}

Microchip {MCP2510 Stand-alone CanBus Controller IC}

National Semiconductor {uP with CAN Interface}

NXP {8/16-bit CAN Bus 2.0 Controllers/Transceiver}

Renesas Technology Corp {Micro-Controller [uC] with CAN / LIN Interface}

STMicroelectronics {uP with CAN Interface}

TI {TMS320F241 with CANbus Interface-3.3v Line Transceiver ICs}

The page covering CANbus may contain addition ICs including IP cores [software]
Editor note; NMEA2000 may demand a higher environmental rating, but I doubt it.

IC Manufacturers by Function {non-CANbus related devices}

Topic Navigation: Engineering Home > Interface Buses > Cabled Interface Standards > NMEA2000

{NMEA-2000 Index}

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