Corrupted NMEA Data

Corrupted Data

Today we had an interesting telephone call from a customer with one of our AIT3000 units that was not providing wireless data to his iPad. The unit had been working perfectly when installed earlier in the year, but on conducting some last minute system checks, prior to leaving for the Baltic, the customer found that the AIS data to his iPad appeared to be corrupted (see image above).

On seeing this screen shot, we immediately confirmed that the NMEA data was indeed corrupted as the majority of the characters were not your normal alpha-numeric ASCII dataset. Our first thought was that the unit had an internal fault but on further investigation we found that instead of trimming off and isolating the unused wires in the Power/Data cable, the wires had been tied back and taped up. Unfortunately the wires had now started to touch together with NMEA output wires touching NMEA Input wires resulting in the data corruption we had seen.

We asked the customer to isolate and trim back all of the wires and “voilà” the wireless data went back to normal and the iPad App could display the AIS and GPS data again. The customer was very relieved that his system was working again and we were reminded again how simple installation errors can cause the most unusual of fault symptoms.

Installation Tip – always trim back and isolate any unused wires.

RJ45 Ethernet Connections


RJ45 Connector

With more and more marine electronics using Ethernet networking onboard boats, being able to assemble RJ45 Connectors is a useful skill to have. This week we were aboard a 50ft yacht on the Hamble that was having problems accessing our WL510 long range Wi-Fi adaptor. The WL510 was connected to our iNavConnect wireless router and by looking at the iNavConnect web interface, on a browser, we were able to confirm that it was not getting an IP address from the WL510 on its WAN socket.

First step was to connect a PC directly to the WL510, using a spare network cable and on doing this the PC immediately got an IP address (in the range) from the WL510.

On further investigation, we found that the installer had run a long network cable between the iNavConnect and the WL510 and it was this cable that was causing the problem. Careful inspection of the RJ45 connectors showed that the connections between the wires and the RJ45 IDC connector (insulation displacement connection) had not been made correctly. Talking to the installer, it was clear that despite using the right crimping tool and new RJ45 connectors from popular UK reseller Maplins, the correct assembly procedure had not been followed.

The image above shows the type of popular RJ45 connector used and it features a small wire guide through which you insert the eight wires of the Cat5 or CAT6 network cable. It is important to wire the connector as per the international EIA/TIA-568B specification as shown in the image below.


The first step is to poke the cable through the plastic connector cover (if you are using one). It is probably the most common mistake that people make and it is so frustrating if you finish making the cable and find that you forgot this step, as you cannot fit a cover to an assembled connector unless it is already on the cable.

Carefully remove the outer insulation of the network cable and the foil shield if you are using a shielded cable (recommended for extra mechanical strength rather than electrical properties). Be very careful not to cut/damage the individual wires. Once the outer insulation (and shield) are removed, untwist the pairs of wire, spreading them out and arranging them in the order shown above. Now insert each wire in to the wire guide in the correct order, noting the bevel on the guide which needs to point upwards towards the connectors.

Wires in guide

A good tip is to mark the top edge of the bevel on the wire guide with a black marker pen, so that you can see it when you insert it in to the main connector. Push the wires in to the wire guide as far as you can, so that the wire guide is as close to the outer cable insulation as possible.

Now trim the wires as close to the wire guide as you can using side cutters.

Wires Cutting

Now insert the wire guide in to the main connector, ensuring that the bevel you have marked with the black pen, is facing upwards towards the gold connectors. Push the wire guide in to the connector as far as possible and it should end up right underneath the connectors with the black bevel visible just behind the gold connectors.

Cable Inserted

Once you are sure that the cable and wire guide are pushed in as far as you can go, use a proper RJ45 crimp tool to compress the IDC connectors, forcing them down so that they cut through the insulation on each wire and make good electrical connections.

RJ45 Crimping Tool

Finally if you are going to be assembling a lot of these RJ45 connectors, it is definitely worth investing in a network cable tester, which will allow you to check long cable runs after you have installed them through the boat. The unit shown below is a typical example and has a detachable “Loop Back” module that you plug in to one end of the cable and then plug the other end in to the main tester and LEDs illuminate to tell you if the cable is OK. This same tester will also check USB and coax cables.

RJ45 Cable tester


Raymarine a, c and e Series Plotters with AIS over NMEA2000

Raymarine MFD Family

This week we have been helping one of our dealers; Marine Electronic Installations (MEI) in Portsmouth, to find a solution to an interesting NMEA2000 problem with the latest range of Raymarine a, c and e Series Multi-Function Displays.

Most of these MFDs feature an internal GPS, a SeaTalkNG (NMEA2000) interface and an NMEA0183 interface. Under normal circumstances, if you connect one of our AIT2000 or AIT3000 transponders to a Raymarine MFD via SeaTalkNG (NMEA2000) the MFD will use its own internal GPS as the position source and receive the AIS target information from the transponder.

What MEI discovered, was that when a Raymarine SeaTalk 1 to SeaTalkNG converter (E22158) is fitted to the network, that the MFD stops using its internal GPS and tries to use a GPS source on the SeaTalkNG network. As our transponders are outputting the Rapid Update GPS PGNs on the NMEA2000 network, the Raymarine sees this as a GPS source and tries to use it but then reports an “AIS Position lost” alarm because the GNSS PGN that provides GPS status information is not being received.

This seems to be a unique problem to Raymarine and not one, so far, reported on other systems. For instance the latest Garmin plotters always default to using their own GPS and ignore the GPS data from the AIS transponder. Also it only occurs when this SeaTalk 1 to SeaTalkNG converter is in the system, although this is quite a popular accessory and used when boats have older ST50/ST50+/ST60/ST60+ instruments or autopilots with a SeatTalk 1 interface.

We have found two ways to fix this problem;

1)  Connect the AIS Transponder to the Raymarine MFD via NMEA0183, which is fine for units that have an NMEA0183 input (all units except a6X and a7X )

2)  Send a special configuration command to the AIT2000/AIT3000 transponder via the proAIS2 software that turns off the NMEA2000 GPS data

For more information on this configuration command please email

No GPS on Transponder due to loose FME connector

FME Connector Apart

Our AIT2000 and AIT3000 Class B transponders are supplied with a GPS antenna that has a 10m cable terminated in an FME connector (right hand connector in the above image).

These connectors are very slim, not much larger than the coax cable and are much easier to route through the boat. We also supply an FME to TNC adaptor for connecting the cable to the transponder (left hand connector in the above image).

Today we were reminded of the importance of ensuring this adaptor is firmly screwed on to the cable, when a US customer reported that their AIT2000 had stopped getting a GPS fix. After using the proAIS2 software to confirm that the GPS signals were very low, we asked the customer to check the GPS antenna connections to the AIT2000 and sure enough found that the FME connector had become loose and was no longer making a good connection.

A quick tighten of the FME connector in to the adaptor and the AIT2000 started to get a GPS fix again and the customer could continue their cruise.

The nut on the FME connector is 8mm (AF) and the TNC adaptor has two flat indents that are 9.5mm (AF). You can tighten the two connectors quite tightly but avoid using too much force which could damage the connector and cause a different set of problems. The image below shows the FME fully tightened in to the adaptor and there should be about a 1.5mm gap between the FME nut and the collar of the adaptor when properly tightened.

FME Connector Joined

Our GV30 combo GPS and VHF antenna also uses FME connectors and is supplied with a TNC and BNC adaptor, which should also be tightened in the same way.

Crimping Small Wires

Crimping Small Wires

Time and again we see faults in marine electronic installations caused by bad or failed connections. Normally the harsh marine environment is the cause, with corrosion eating away at the electrical contacts but poor assembly of wiring connections is also a major culprit.

Digital Yacht’s products, like many other modern marine electronic systems, feature multi-core cables with relatively small power and signal wires. Lower power consumption, digital interfaces and the drive for smaller and smaller product sizes, has led to most electronic wiring in modern boats, using 24 AWG sized wires or smaller.

These smaller wires do create a challenge though when it comes to connecting them to the boats DC electrical system, which generally uses much larger gauges of wire and has large crimp and screw terminal type connectors.

Traditionally, those distinctive Red, Blue and Yellow insulated terminal crimps have been the most popular way of connecting low voltage DC wires in Boats, Cars and Caravans. They are cheap, commonly available and assembled correctly can make a very reliable connection. However, as we were reminded again this week, they can also create problems when poorly assembled.

Take the image above, where the two crimp connections appear at first glance to be OK. You can see the wire protruding slightly through the insulated ring and the crimp has been compressed to push down on the wire. However, appearances can be deceptive and the upper crimp which was attached to the positive power wire of one of our AIS units, was not making a connection with the red wire. When we cut the crimp off and did a resistance test, we found it to be completely open circuit (no electrical connection).

There are five simple steps to successfully crimping small wires:

1)  Strip the wire, being extra careful to not damage any of the internal strands of wire

2)  Give the strands a quick 180º degree twist to make them less likely to splay out

3)  Fold the exposed strands of wire back on the insulated wire (as shown in the image)

4)  Insert the wire in to the crimp so that it just protrudes out of the insulated ring

5)  Use a proper ratchet type crimp tool (see image below) and make sure the crimp sits in the “Red” labelled recess of the jaws

NOTE:  Always use the smallest “Red” range of crimps and never the Blue or Yellow ranges


There are two reasons for bending the stripped strands of wire back on to the insulated wire, firstly it provides more material for the crimp to bite down on and secondly if the wire is subsequently pulled or tugged, the strain is not taken just on the stripped wire but also on the insulation as well, creating a much stronger mechanical join.

Having the right tool for the job is also important and those cheaper crimp tools that you often see included in the box with a mixed set of crimps are just not up to the job. For approximately £10-£15 pounds you can buy a proper ratchet type tool that will consistently make good crimp connections for the life of the tool.

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.

Testing Wireless NMEA Data

iAIS TCP-IP Screen


With more and more wireless NMEA systems being installed on-board boats, it is very useful for dealers, installers and enthusiastic end users to have simple tools to “view” this wireless NMEA data.

Traditionally, wired NMEA0183 data was viewed using an NMEA to USB cable connected to a PC and then a program such as Hyperterminal (included with Windows up to WinXP) would be used to display the data. In fact Digital Yacht released a free, dedicated NMEA Display program to use on Windows Vista/7/8 and this proved to be a popular tool for testing wired NMEA0183 systems.

With wireless NMEA systems it is much easier to test using a smart phone or tablet and we would recommend the following free apps;

For Apple iOS Devices – our own free iAIS app (see image above) has a very simple raw data view window that can be used to display the wireless NMEA data in TCP or UDP mode. Alternatively iNMEA Logger is another free app, written by the company that developed the popular iRegatta App that can log 30 seconds of received data and create a text file of the results, useful if you do not understand NMEA0183 and want to send it to someone who does.

For Android Devices – there are no specific wireless NMEA Apps, but there are a lot of terminal programs that display TCP and UDP data and after trawling through a fair number of apps, we came across TCP/UDP Terminal App which we think is the best Android App found so far.

Once you have installed your app for displaying wireless NMEA data, then you need to know what you are looking at. To buy the NMEA0183 Specification costs quite a lot of money but there is quite a lot of data on-line, you just need to hunt it out.

Unfortunately a lot of the information on-line is quite old and some of the newer sentences are not fully explained. The NMEA do in fact publish a complete list of all Sentence Identifiers with a short description of what they are (not the complete sentence description) and this list also includes the proprietary Manufacturer’s ID – these sentences start $Pxxx, where xxx = the manufacturer identifier.

It should be noted that most wireless NMEA data is “human readable” (ASCII Text) but the AIS sentences VDM and VDO have what is called a “binary encapsulated” section (bit like a zip file) to reduce the sentence length – see example below.


This means that you will not be able to make sense of the AIS target information in the VDM and VDO messages, but our free iAIS app does display this information on the main plotter screen, so worth having a copy of this app.