Safely connecting NMEA 0183 devices or a NMEA 2000 network to a computer -- Actisense educational documentation

Article HD25110, September 29, 2015

Information about how best to connect NMEA 0183 or 2000 to a computer.


About NMEA 0183 interfacing with computers

  • NMEA 0183 is any ASCII data that starts with a ‘$’ or ‘!’ and ends with a line feed and/or carriage return.

  • As an NMEA 0183 signal can be received directly in to a PC’s RS232 COM port or Virtual COM port, there is a great temptation to directly connect the two together.

  • This temptation should always be resisted as connecting a PC directly to an NMEA 0183 device (that doesn’t have isolation on both its input and output) will result in a ground loop or ground potential difference that will probably cause damage to the PC, the NMEA 0183 device, or even both. To correctly protect the PC, a device with full isolation in both directions is a must – and that is far cheaper than the cost of repairing or replacing the PC!

  • As with any other ASCII data, it can be viewed using basic software such as HyperTerminal, however Actisense NMEA Reader provides a far more ‘useable’ option, that we’ll detail later.

RS232 Connections

RS232 connections to PC compared

Contrasting the different ways of connecting RS232:

  • For computers (generally older PCs) with an RS232 port, the left-side diagram (above) shows a connection method deployed by a user unaware of the damage that can occur. Whilst they have avoided the immediate damage scenario of connecting the differential output B/- to the single-ended (PC) input ground (Pin 5), without isolation this connection does risk damaging the computer, the NMEA Talker, or both.

  • The right-side diagram (above) shows the recommended connection method when it is required to share NMEA 0183 with the PC. The OPTO-4 has 2500 volts of isolation to keep the PC input safe. The OPTO-4 can only be used to share NMEA 0183 data back to an NMEA 0183 ‘Listener’ if it has an isolated input (as the OPTO-4 does not have any isolation in that direction, only spike protection).

NMEA 0183 OPTO Isolator (OPTO-4)

About the Actisense OPTO-4

Advantages of using the Actisense OPTO-4 Isolator:

  • OPTO Isolation technology in an over-moulded RS232 connector

  • As it is fully NMEA 0183 and RS232 compliant, it correctly converts the NMEA 0183 RS422 signals in to true RS232 signals for use on a PC. This allows greater cable lengths to be used successfully.

  • As the OPTO-4 can operate up to 115K baud, it can easily handle High-Speed NMEA 0183 data at 38400 baud and way beyond.

  • The IP67 rating applies to the over-moulded housing, including its internal electronics. Naturally the RS232 D-Type connector is not waterproof but once matted to a PC port it will be highly resistant to water spray.
    For a fully waterproof connection, a correctly rated waterproof RS232 port needs to be used.

  • Power is taken from the PC COM ports handshaking lines so typically there is no need for an external power supply.

    For non-PC use, the OPTO-4 can be alternatively powered with +9V to +15V on pin 7 (RTS) OR pin 4 (DTR), and 0V on Pin 5 (Ground).

  • Care must be taken when using an external battery to power the OPTO-4 as to not create a ground loop or ground potential difference. Ensure that the battery ground and device’s ground are at the same potential.

  • Still a very popular product for customers who wish to avoid USB and its drivers.

RS232 serial conversion to USB + virutal COM port

PC-USB-1 adapter explained in brief

About the Actisense USB-1 adapter:

  • The USB-1 can be used directly with the OPTO-4 for PCs without an RS232 COM port.

  • Compatible with Windows and Mac OS X Operating Systems. Drivers available online and install automatically. (On Mac OS X, the terminology is "SerialUSB" instead of "COM port" -- but the principles of operation are the same.)

  • An important note is the USB-1 does not contain any isolation so it can only be used with devices that offer isolation (such as the OPTO-4, and all ISO-drive devices)

The Safest Connection for NMEA 0183 to a computer

About USG-1 unit

About the Actisense USG-1:

  • Provides a fully isolated PC COM port with :
    • an OPTO isolated input offering 2500 volts of isolation.
    • an Actisense ISO-Drive output offering 1500 volts of isolation.
    • all power taken from the USB port for easy of use and installation.
    • The latest Windows and Mac USB drivers are available on the Actisense website. Windows will automatically download the latest USB driver if the PC is connected to the Internet.

  • No firmware updates required as there is no microcontroller.

  • Data in = Data out. Virtual COM port can be opened at any standard baud rate.

  • A great NMEA 0183 ‘Sniffer’ diagnostics tool when used with NMEA Reader.


About NMEA 2000 interfacing with computers

  • For information about NMEA 2000 networking in general, see: Understanding NMEA 2000 networks -- Actisense educational documentation.

  • As there is no standard method of connecting an NMEA 2000 network to a PC – as they do not come with a CAN Bus port – the Actisense NGT-1 PC Interface (see below) has become the leading industry solution. The Actisense NGT-1 is currently supported by over 40 software developers and 3 hardware manufacturers.

  • Whereas each PC software developer added NMEA 0183 support to their software a long time ago, they are required to add NMEA 2000 support via an NGT-1 interface to gain access to the wealth of NMEA 2000 data.

  • NMEA 2000 PGN messages are pure binary, making them unreadable to the human eye – however the freely available Actisense NMEA Reader can decode and display each message, the fields inside and the data values.

  • Currently, NMEA 2000 certification of software is not mandatory. Once the NMEA ratifies the ‘Third Party Gateway’ standard, it will be possible to state that “The software is certified when used with the specific interface”.

  • All NMEA 2000 devices are isolated from each other to prevent ground loops, and the NGT-1 continues that isolation to ensure that the PC remains protected and safe.

NMEA 2000 via NGT1 product

Technical and developer highlighrts of the Actisense NGT-1:

  • Transfers all the NMEA 2000 PGN messages available on the network to the PC application where they can be decoded (and the data used). Also allows the PC application to generate its own NMEA 2000 PGN messages to send back to the network.

  • The NGT-1 is fully configurable via an easy to use software API (Application Programming Interface). The software API is part of the freely available Actisense Windows SDK that contains everything needed to get started with C or C++. There are also C# and vb.Net wrappers available if required. An alternative SDK is available for Mac and Linux developers.

  • Easy installation through USB and DeviceNet plug-n-play connectors.

  • For those that wish to avoid USB drivers, the NGT-1-ISO is also available.

  • The NGT-1 is compatible with all versions of Windows (from XP and above), Mac (from OS 8.6 and above) and Linux (any version but with built-in support from Kernel v4.0 onwards)