Are you Positive about that Negative ?

A customer recently contacted us asking why he was measuring a negative voltage on his NMEA0183 Output and suggesting that his AIS had a fault. He confirmed that he had his multi-meter’s Red lead on the Output+ and the Black lead on the Output-, so how on earth could it be measuring a negative voltage unless the AIS was faulty.

Well normally when you measure the voltage of say a battery, the two terminals are clearly labelled + and – so as long as you get your multi-meter’s leads round the right way the measured voltage is always positive. However, with most NMEA0183 signals, you are dealing with a “differential” voltage where the positive and negative connections both have pulsing signals on them and it is the difference between the voltages on the two connections that you are measuring.

Looking at the diagram above, you can see the pulsing signals are going between 0v and +3.3v on both connections, but they are inverted so when one signal goes to 3.3v the other goes to 0v and vice versa. This has the interesting effect of making the measured voltages alternate between +3.3v, when the + connection is at 3.3v and the – at 0v, and then -3.3v when the + is 0v and the – is 3.3v.

Most AIS units, particularly receivers only output data when they receive an AIS target report, so if there are not many AIS vessels in the area, it is not unusual to measure -3.3v on the output. Couple this with the fact that AIS data is at a higher baud rate (38400) which means the pulses are much shorter and may not be detected by the multi-meter, it is always good practice to use an LED to test NMEA0183 outputs as detailed in our Tech Note.

New iKommunicate Developer’s Guide (SDK)

iK GitHub Site

This week we launched a new iKommunicate Developer’s Guide on GitHub that aims to make it easy for developers to start writing HTML5 Web Apps or mobile Apps that read data from NMEA networks via the new Signal K open data format.

iKommunicate is designed to be a powerful and intelligent gateway that efficiently converts NMEA data in to Signal K data, but that is just half of the jigsaw. The other key elements are the apps and software that work with iKommunicate to make it a fun and useful addition to your boat.

Out of the box, iKommunicate will work with all Navigation Software packages and Apps that support NMEA network data over TCP or UDP and it will also include a couple of Signal K web apps to get you started. However, it is hoped, that as more and more developers realise the potential of Signal K, that a plethora of innovate and useful apps will start to appear, transforming the way we use out boats.

In putting together the Developer’s Guide, we realised just how easy it is to start developing apps and there is an awful lot of information, libraries and source code on the internet that you can draw upon as you become more experienced. Even if you have never looked at HTML5, http REST APIs or WebSockets, our wiki and example apps, will give you a good starting point and have you displaying Signal K data in no time.

Testing NMEA0183

NMEA Display Program

Back in the 1980s the National Marine Electronics Association (NMEA) defined an interfacing standard that would revolutionise the way marine electronics operated together. For the first time a common standard was defined that allowed equipment from different manufacturers to talk to each other and do things that had previously been impossible.

After a few iterations (NMEA0180 and NMEA0182) the NMEA0183 standard was published and gradually manufacturers developed products that had NMEA0183 Inputs and Outputs that could be connected together. There were some teething problems but over time NMEA0183 established itself as a very compatible and reliable interface standard and even today most marine electronic systems have at least one NMEA0183 interface, although the newer NMEA2000 standard is gradually taking over.

Online information on NMEA0183 is fairly limited and often quite old, but Actisense publish a useful booklet on NMEA0183 and this website has also collected a lot of useful information on NMEA0183.

Testing NMEA0183 systems can be done in a number of ways. The simplest method, just to see if there is data being transmitted or not, is to place an LED across an NMEA0183 output. One way round the LED should flash and the other way round it will not flash. If the LED fails to flash in either direction then no NMEA0183 data is present. For more information on this type of LED test, please download our Tech Note by clicking here.

If after establishing that there is NMEA0183 present, you wish to go a step further and look at the raw data to see what messages are being transmitted then you will need an old laptop and an NMEA to USB adaptor cable like our Part# ZDIGUSBNMEA. This adaptor cable can easily be connected to any NMEA0183 Output (two wires) and then with suitable software running on the PC, the NMEA0183 data can be displayed and interpreted.


A few years ago, when Microsoft stopped including a utility called HyperTerminal in Windows Vista/7/8,  Digital Yacht developed a useful NMEA Display program that is free to download from here.  Our NMEA Display program is perfect for not only viewing the raw NMEA0183 data but also for interpreting and displaying the different fields of data i.e. Wind Speed, Depth or Heading. For more information on using this program and NMEA0183 in general, please refer to an earlier post we wrote, by clicking here.

With these simple tools and techniques, anyone can test and fault find NMEA0183 systems and with so many systems out there, it is a useful “string to your bow” that might just get you out of trouble.

Digital Yacht Tech Training – A great resource

We’ve decided to publish our dealer training presentations on line so that the whole boating community can benefit.  Paul Sumpner, Digital Yacht’s CTO, was at the CA Clase distributor training event this week and presented a great overview on some of our core technologies including:

  • AIS – Hints and tips on transponder operation, programming and antenna selection
  • Wireless NMEA – differences between TCP/IP and UDP connections and app integration
  • GPS antennas and interfacing
  • Wireless internet and routers including common problems with accessing marina hotspots

Most importantly he covered lots of installation issues with hints and tips and important product selection advice.  Of course, reading a presentation on line is not as good as hearing the real thing so if you do have questions, feel free to contact us.

You can download the presentation from HERE  or click the picture below

tech training front cover


“Quick Fit” RF Connectors

Quick Fit

When you are feeding cables through a boat, it is often necessary to remove bulky connectors to fit the cable through tight spaces, or you may need to extend the cable and fitting proper mating connectors is ideal. Soldering connectors can be fiddly and crimping requires a specialist tool that many people do not have, so using a solderless, quick fit RF connector is very tempting.

However, I learnt a lesson today that I hope others can learn from. I had not come across this particular type of connector before (see image above) and the idea is that you place the coax cable firmly up inside the connector where a sharp center pin presses in to the inner conductor of the cable. Then you tighten down the small screw shown in the image above and it cuts through the outer insulation and makes contact with the outer, shield conductor of the cable. At first sight, this seems a very clever idea but as I was to discover this type of connector can create more problems than it solves.

The boat that I visited had one of our AIT2000 Class B Transponders on which was giving a high VSWR reading in the proAIS2 diagnostics software. The VSWR reading is one of the built-in self tests of the AIT2000 where it measures the power it sends to the antenna and also measures the “reflected” power that comes back down the aerial. In an ideal world all of the power sent to the antenna should be transmitted in to the air, but this is never the case and a small amount is always reflected back to the AIT2000. If there is an antenna fault or a short or open circuit in the antenna cable then the VSWR reading will be very high (>5:1) and trigger an alarm.

On this particular boat, there were a number of these quick fit connectors used to extend the AIS antenna, but they appeared to be well made so I started by quickly connecting a spare VHF antenna directly to the AIT2000. Voila, on the next AIS transmission (every 3 minutes when stationary) the VSWR  reading came right down to a very healthy 1.2:1. The installed AIS antenna was mounted on the mast so replacing it would be a pain, so just to be sure I thought I would check the extra piece of antenna cable that had been added to the cable. A quick DC continuity and isolation test showed a short between the inner and outer of the extension cable.

On closer examination of the quick fit connectors, I found that removing one of them, the short disappeared. My only conclusion was that either the small screw that is intended to just make contact with the outer shield had been over tightened shorting the inner and outer conductors or that blunt wire cutters had been used and some poorly cut strands of the shield had shorted to the inner.  Any way the fault had been found and I quickly rejoined the “Quick Fit” connector to the cable. Just to be sure I repeated the continuity and isolation test and this time it showed the short had gone BUT there was no continuity on the inner connection.

In total I tried remaking the “Quick Fit” connectors three or four times and on every occasion there was no continuity on the center conductor and in the end I gave up and replaced them all with crimp connectors which I have never had a problem with. You might have to spend £10-£15 on a crimp tool (see below) but if you are regularly doing RF connections or you simply want the best possible connection then I would definitely recommend the crimp connectors.

Crimp Tool

Integrating a PC into your boat’s navigation system

Utilising a PC on board brings important benefits not just for regular PC tasks such as email and web access but also for entertainment and navigation.  This document explains how to integrate a PC into a boat’s navigation system and this approach offers substantial cost savings over buying a multi station traditional navigation suite.  PCs also provide a much more powerful platform and with an internet connection, data such as weather can be integrated into the system.  Navigation software for a PC is sophisticated yet easy to use.

Installed PC or notebook…

Whilst a laptop or notebook provides a solution to run e-navigation software, it’s not ideal.  Power consumption is high and often you will need an inverter or adaptor to connect to the boat’s DC supply which introduces more losses and electrical noise.  Notebooks don’t like the constant vibration and momentum from the boat and of course they’re not designed for salty air.  They also don’t have interfaces for the boat’s NMEA system so integration can be tricky.  We’ve found that most users trial a system with a notebook but move on quickly to a dedicated, installed system like our Aqua range of PCs.

Aqua PCs from Digital Yacht

Aqua 2 PC with Logos-web

The Aqua range of PCs from Digital Yacht are designed to be permanently installed and can connect direct to the boats DC electrical system.  They are capable of operation from 8-18V so can deal with a fluctuating battery supply.  They also consume minimal power (typically around 1A at 12V DC) and are completely solid state with no moving parts.  There’s no cooling fan to suck in moist air and the solid state hard drives offer exceptional reliability and speed.  They also feature a dual NMEA input to integrate straight into your boats navigation system.  Despite their impressive performance they are as affordable as a good quality laptop and can support multiple monitors.

Using the AIS transponder as the navigation feed…

If you’re contemplating a PC system then you’ll probably also look at having an AIS Transponder.  Our AIT2000 features a USB output to connect direct to an Aqua PC and provides a feed of AIS and GPS information.  It also incorporates a NMEA multiplexer so will take your boat’s instrument NMEA data and combine with the AIS data and output via USB.  One  connection to your PC with AIS, GPS and NMEA boat data!

Navigation Software for your PC…

One of the beauties of a PC based system is that you can constantly upgrade, change and evolve with the addition of new software.  Of course, we’d like you to use our SmarterTrack navigation package which offers great value for money, amazing charting capabilities and one of the best AIS presentations available today.  SmarterTrack uses Navionic’s charts and you’re now able to copy a Navionic’s chart (on SD or CF card – like the ones used on plotters such as Lowrance, Raymarine and Simrad) direct to you’re PCs hard drive – that means no duplicated charting costs!   SmarterTrack supports weather overlays, instrument displays and can be supplied with a NMEA 2000 interface

Add wifi internet access with the WL510…

Internet access afloat really adds to boating.  Not only can you keep abreast of news and email, but it’s a great source of entertainment, TV and media when in port.  It can also be used to dowload the latest weather information.  Free GRIB files are available to integrate with PC navigation programs to provide animated weather forecasts with wind, swell, pressure and temperature information.  Whilst on board, you can access the internet through 3G/4G systems, WiFi and satellite.  Digital Yacht make a range of high power wifi systems which offer easy and cheap data access with ranges of up to 5NM.  WiFi is cheap, global and fast and for many coastal sailors, it’s ideal.  Satellite provides a trans ocean solution but at a high price and substantial running cost.  The WL510 will integrate with your on board PC system to provide internet access

Fitting it all together

So here’s how it all interconnects.  In the cockpit or flybridge you can use any standard 5, 7, 10, 12 or 15″ multi function display/plotter, linking via NMEA direct to the Aqua PC system.  5″ plotters from Garmin, Raymarine and Lowrance offer incredible value for money, sunlight viewing capability and waterproof performance.  Even a 5″ screen is ideal for use by the helsman and if you choose a Navionic’s based plotter, you can share the charting with SmarterTrack on the Aqua PC.  Make sure your plotter choice accepts an AIS input too

In this system, we’ve added the WL510 for internet access, connected to our WiFi hub called iNavHub.  This allows internet data as well as navigation NMEA data to be shared with mobile devices such as iPads/Phones.

We’ve also utilised our SPL2000 VHF-AIS antenna splitter to share the VHF antenna with the AIS transponder and provide an aerial feed to an on board stereo.

Its a powerful system that offers incredible value for money

2015 Update:  Since writing this we’ve introduced our new Aqua Adapt range – more powerful, lower power and better value.  Details Here 


Problem with NMEA (COM Port) Connections on Ubuntu

Nautical Tux

In response to a comment on my recent “Managing the Device Manager”, where Michael reported problems getting his AIS100USB working on LINUX, I decided to download the latest OpenCPN and make sure that the new NMEA Connections code that they have recently added to V3.20 did in fact still work like the previous versions.

It has been a while since I played with the LINUX version of OpenCPN and with a normal Ubuntu V12.04 32 bit setup, I was surprised when I could not get the NMEA data from my AIS receiver or transponder to be received. To investigate further, I opened a Terminal Window and typed…

dmesg  | grep tty

Which returned the following information about all of the serial ports – “tty” is a legacy term in LINUX from the days of Teletypewriters which then evolved in to computer terminals that used serial ports to connect to the mainframe computer.


The Dmesg command extracts information from various log files and returns the current and historical information about USB devices that have been connected to the PC. In the screen above, you can see that a USB to Serial converter was connected and given the name ttyUSB0 (this was my AIS receiver) but that this was disconnected. Then you can see that another USB ACM device was  connected and given the name ttyACM0 (this was my AIS transponder) which is still connected.

So I now knew which serial port had been setup by LINUX for my AIS transponder, so the next step was to use a LINUX program called Screen to send the NMEA data coming in to ttyACM0 to my terminal screen. First I installed the Screen program by typing the following command in to the terminal…

sudo apt-get install screen

Once installed, the following command should have displayed the NMEA data coming in on port ttyACM0…

screen /dev/ttyACM0 38400

However, when I ran this command, I very briefly saw a warning displayed saying that I did not have the necessary permissions to access this device. This was my first clue as to the problem with no NMEA in OpenCPN. I re-entered the command this time running it as the “Root” user using the sudo command…

sudo screen /dev/ttyACM0 38400

This time the Screen command worked and I saw the following display…


So it would appear that for some reason I was being denied access to the COM port devices. My first thought was that this was a problem with OpenCPN and I looked at ways to run OpenCPN as root. Fortunately advice was on hand from a Digital Yacht customer called Fulup who had previously helped me in getting OpenCPN to work with our AIT2000.

Once again Fulup’s good LINUX advice was invaluable and he highlighted that the problem lay with changes in the way the latest versions of Ubuntu set default user permissions. The COM ports use to be available to all Users but now you need dialout permissions to access them. Fulup advised that I should add my Username to the dialout Group and then I should be able to access the COM ports, without opening up a potential security risk of running OpenCPN as root.

To do this, it was necessary to open a Terminal Window again and to run the following command exactly as written, note that $LOGNAME is a shell variable and will automatically get the name of the currently logged on user;

sudo usermod -a -G dialout $LOGNAME 

So now your username has been added to the dialout group and you just need to logout and log back in again to make the changes take effect. Once logged back in, run OpenCPN to make sure you can now read data from the COM ports.

I was very pleased to find that this fixed the problem (thanks Fulup) and I now have NMEA data coming in from my AIS Transponder or my AIS Receiver. Hope this helps anyone else having similar problems on the latest 12.04 or higher versions of Ubuntu.