About NMEA 0183 networking issues, data loops, and connecting multiple talkers with the Actisense NDC-4 variants -- Actisense educational material

Article HD25104, September 4, 2015

NMEA 0183 networking points to consider with diagrams.


Issues to consider for NMEA 0183 devices and networks

  • Ground loops and Ground Potential Differences are real issues to avoid in all NMEA 0183 installations. Ground loops are the most discussed issue on tech support calls – we’ll discuss how to avoid the damage ground loops can cause. 

  • The differences between the NMEA 0183 Physical Layers of the original version 1 and all other versions (version 2 and above) create compatibility issues and the potential for damage due to lack of isolation on version 1 devices – we will discuss how to minimize these issues and provide isolation. 

  • An NMEA ‘Talker’ with low-drive capability can be difficult or impossible to connect to multiple ‘Listeners’ – we will discuss how to overcome this issue. 

  • The NMEA 0183 Physical Layer stipulates that there can be only one ‘Talker’ - which creates the requirement for Multiplexers to intelligently handle the combination of data from multiple ‘Talkers’. 

  • The power required by the NMEA 0183 Multiplexers and Buffers used to create an NMEA 0183 network can become significant especially on low-power yacht installations – there is no direct solution to this issue (bar using Actisense low-power Multiplexers and Buffers) short of upgrading to NMEA 2000.
  • As soon as NMEA 0183 networks grow beyond a few devices and the requirement to have bi-directional connections occurs, the chances of data-loops increases greatly. Data-loops can be likened to the horrible feedback you get when standing in front of a speaker and talking in to a microphone: data becomes duplicated and consumes all of the available bandwidth, blocking new data.
  • The NMEA 0183 standard 4800 baud rate is very limiting and only allows 480 characters per second to be transferred. 

  • Due to the very limited bandwidth, NMEA 0183 data transfer rates are low – typically once per second is the fastest possible. 

Ground Loops EMI (Electro-magnetic Interference) & PD (Potential Difference)

about EMI and PD in the context ground loops

  • Indicate current flow before data lines of a non-isolated ‘Listener’ are connected. Ground return path is as expected (through the power supply wires): current flows down the +ve wire from the battery to the ‘Listener’ and returns down the –ve wire back to the battery.
  • Indicate current flow once the non-isolated ‘Listener’ data lines are connected. Ground return path is no longer as expected: as current will always choose the shortest/least resistance of the two possible paths, current still flows from the battery to the device down the power supply +ve wire, however it returns through the ‘B/-’ wire in the NMEA 0183 cable, through the output port of the NMEA 0183 ‘Talker’ and back to the battery through the ‘Talker’ power supply –ve wire. This current flow in to the NMEA ‘Talker’ output will damage it – how long that takes depends on a number of things (such as how much current the NMEA ‘Listener’ requires and how big the ground potential difference is) but experience puts this somewhere between 1 minute and 1 month.
  • Galvanic isolation on an device’s input and/or its output will prevent this damage by breaking the ground loop.

Opto-Isolation to prevent ground loops


  • The electrical signal generated by the NMEA 0183 ‘Talker’ is converted in to light by the OPTO’s LED on its input.
  • The light emitted from the LED is detected by the photo-sensor and converted back in to an electrical signal.
  • This breaks the flow of current (from ‘Talker’ to ‘Listener’) and therefore prevents any potential ground loops.
  • As the NMEA 0183 ‘Talker’ power and ‘Listener’ power are electrically isolated from each other, both devices are kept safe.

Notes on NMEA 0183 versions 1 and 2

Types of NMEA 0183

NMEA 0183 Version 1

  • Called a ‘Single-ended’ device as it has one data line. It is an important point to understand that all communications require two connections, so the ‘hidden’ second connection is the ‘Ground’ wire used as the current return path.
  • Signal levels meet the RS232 standard of +/- 15 volts.
  • STOP (1) bit is any signal between -15V & +0.5V, START (0) bit is any signal between +4V & +15V.

NMEA 0183 Version 2 (and above)

  • Called a ‘Differential’ device as the two data lines (A and B) work together as a pair to create the ‘Talker’ or ‘Listener’ connections.
  • Signal levels meet the RS422 standard of (0 to 5 volts). Two data pairs are required for bi-direction communication.
  • STOP (1) bit is when A is less than B (A=0V, B=5V), START (0) bit is when A is higher than B (A=5V, B=0V).
  • Some devices label A as “Data/In +” or “Data/Out +”, and B as “Data/In –” or “Data/Out –”.

Care must be taken when connecting a Version 2 ‘Talker’ to a Version 1 ‘Listener’: never connect the ‘Talker’ NMEA B/- to the ‘Ground’ connection of a single-ended ‘Listener’. Best outcome is ‘Talker’ current consumption increases greatly, worst outcome is terminal damage to the ‘Talker’ occurs.

Isolation and Drive Solutions

Isolation and Drive solutions

  • Any RS422, RS232 or RS485 ‘Talker’ data signal can be buffered to six identical isolated outputs. Any data protocol and baud rate can be handled (as no micro-controller is used).
  • Each output utilizes the Actisense ISO-Drive technology. Connecting more than one ‘Listener’ device to a single ISO-Drive output can create a ground loop between the ‘Listeners’ if they do not have isolated inputs – this will not damage the NBF-3 as it can automatically disconnect to protect itself but it could damage the ‘Listener’ devices.
  • Each output is electrically identical whilst being isolated from ground and all other outputs. If one output is ‘dragged down’ by being connected to a faulty device, that output will disconnect itself until the fault is removed and all other NBF-3 outputs will continue to operate unaffected.
  • Useful for a wide range of non-isolated NMEA 0183 ‘Listener’ devices, or even other non-NMEA 0183 devices with RS422 or RS232 ports on them.

Connecting Multiple Talkers together

  • The Actisense NDC-4 family has many features to help the installer control the sometimes complicated process of combining multiple NMEA 0183 Talkers together.
  • Each variant in the NDC-4 family has its unique features.
  • Marine installers who deploy the NDC-4 do so in two ways:
    • To minimise the number of NDC-4 variants in stock, only the standard NDC-4 is purchased, allowing the installer to configure the NDC-4 to suite the needs of each installation.
    • To minimise the amount of NDC-4 configuration efforts, the specific NDC-4 variant can be ordered for a particular installation allowing it to be installed ‘out-of-the-box’ in most cases.

NDC-4 Standard

NDC-4 Standard

Five NMEA 0183 inputs and two NMEA 0183 outputs :

  • OPTO Inputs 1 to 4 operate at 4800 baud. By default all inputs are set to “Pass All Data”. If filtering is required, use NDC Control Centre.

  • ISO Output 0 operates at 38400 baud and is the recommended connection to an RS232 device like a PC or a Chartplotter. By default no filtering is applied to ISO Output 0.

  • OPTO Input 0 operates at 38400 baud (as it is the pair to ISO Output 0). It is also set to “Pass All Data” by default. OPTO Input 0 is unique in that its data will only go to ISO Output 1 (to prevent data looping on IN/OUT 0).

  • ISO Output 1 operates at 4800 baud. By default all data from all inputs will be sent out of this output (bandwidth permitting). Use NDC Control Centre’s filtering options to manage the data in this output.



  • This is the NDC-4 pre-configure option to use with an AIS device. It has exactly the same configuration as the Standard NDC-4 apart from :

    • OPTO Input 2 is pre-configured to 38400 baud for connection to the AIS device.
    • All data on Input 2 is filtered/blocked from appearing on ISO Output 1. This is to prevent the High-Speed AIS data from flooding this 4800 baud output.
  • Important note: No attempt should ever be made to reduce the baud rate of an AIS stream to 4800 baud as this will result in unpredictable ‘dropping’ of targets in busy AIS areas.



This variant is the NDC-4 pre-configure option to use with two, three or four NMEA 0183 devices of the same type, when only the highest priority source is required to be switched to the output. It has exactly the same configuration as the Standard NDC-4 apart from :

  • Only data one of the four possible inputs (OPTO Input 1 to 4) is sent out of ISO Output 0 and 1.
  • By default, the priority of an input follows its port number, with OPTO Input 1 being the highest priority. This can be configured in NDC Control Centre if necessary.
  • If the primary data source fails (for one of three possible reasons), the NDC-4-ASW will automatically swap over to the second highest priority data source, and so on.
  • If the highest priority source successfully recovers, the NDC-4-ASW will return to this as the chosen data source.



This variant has four NMEA 0183 inputs, a USB input, two NMEA 0183 outputs and a USB output:

  • OPTO Inputs 1 to 4 operate at 4800 baud. By default all inputs are set to “Pass All Data”. If filtering is required, use NDC Control Centre.
  • ISO Output 0 operates at 38400 baud and is the recommended connection to an RS232 device like a PC or a Chartplotter. By default no filtering is applied to ISO Output 0.
  • OPTO Input 0 is disabled when USB power is active.
  • ISO Output 1 operates at 4800 baud. By default all data from all inputs will be sent out of this output (bandwidth permitting). Use NDC Control Centre’s filtering options to manage the data in this output.
  • USB Input (that takes over from OPTO Input 0) and USB Output operate at the same baud rate as ISO Output 0.

The NDC-4-USB variant provides an easy method of sharing the combined NMEA 0183 data with a PC, but only if the PC will remain powered on for the entire time the NDC-4 is required to operate.

When powered by the USB connection, the NDC-4 “Batt” terminals are not used.

If the NDC-4 is also required to operate when the PC is powered off, the NDC-4 USB cable cannot be used : instead connect the PC to the OPTO IN & ISO OUT 0 connections and power the NDC-4 from a battery supply connected to the “Batt” terminals.

Many Talkers, Many Listeners

NMEA network example illustrations with multiple talkers and listeners

Considerations about a typical many-talker, many-listener network

  • An NMEA 0183 network can become complicated when NMEA 0183 Multiplexers and Buffers are used, but if correctly managed and documented it can also be a powerful data network.

  • The network shown is a typical one that combines the various NMEA 0183 Talker stream together using an NDC-4 and then distributes that combined data stream to multiple NMEA 0183 ‘Listeners’ using an NBF-3.

Data Loops

Data Loops in a NMEA 0183 network

  • Connecting both the input and output wire pairs of a device to any NMEA Multiplexer may cause a ‘Data loop’ issue where information is repeatedly re-transmitted as it circles to loop.
  • This will very quickly use up the available NMEA 0183 bandwidth and may cause the NMEA Multiplexer to randomly drop more important data.
  • The filtering options in the Actisense NDC-4 can be used to ‘break’ such a ‘Data loop’ and restore logic to the network.

NDC Control Centre Demonstration

There are 5 windows that make up the NDC Control Centre, each with their own features:

  • The main window displays the current NDC settings for it Port Configs and Inclusion Lists.
  • The Hardware Information window displays the details of any detected NDC including its Serial ID and Firmware version.
  • The Hardware Port Status window displays the loading of all the NDC’s input and output ports – making it a very valuable diagnostics tool.
  • The bar graphs on this panel will be vital in helping to understand what is happening inside the NDC.
  • The Control Centre Port Status window displays all of the NMEA 0183 data received from the NDC. The data shown can be filtered to just show the data that was received by the NDC on a single input.
  • The Hardware Connection Status window displays the success/failure of all communications between the NDC and Control Centre.
  • Diagnose and fix the issue with the device on Input 2.
  • Diagnose and fix the issues with overloaded Output 1, firstly by filtering Input 1 and then Input 2.