CLA1000 Fault Finding Guide

CLA1000 Bundle HR

Despite our popular CLA1000 Class A transponder being predominately fitted to commercial ships by experienced electronic installers, an increasing number are being fitted to pleasure craft and are often installed by the boat’s owner or an installer who has never fitted a Class A transponder before.

Installation is pretty straight forward, but there are a number of steps and procedures to follow:-

  • Mechanical Installation
  • Power Connections
  • GPS antenna installation
  • VHF Antenna installation
  • Interfacing*
  • Configuration via the supplied configAIS program

*Many CLA1000 units are interfaced to our HSC100, USB to NMEA Adaptor cable, Pilot Plug or PilotLink units.

Things do not always go to plan and with this in mind, we have released a Fault Finding Guide to help users, new to Class A transponders, work out what is wrong. To download a copy of our AIS Class A Transponder Fault Finding Guide, click here.


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.

Support for AIT250 and AIT1000



Usually our posts are about the latest and greatest, but with thoughts turning to the new season, I thought I would give some advice on two of our older but very popular products; the AIT250 and AIT1000 Class B Transponders. Both of these two products are based on the same Transponder PCB, which can still be repaired/serviced although PCB failures are fairly rare and faults can usually be traced to antennas or external wiring.

If you are the proud owner of one of these units, then you can be confident that should you have any problems, we are here to help you sort them out. We have produced a fault finding guide that covers the most common installation problems and also explains how to use the proAIS software to diagnose and fix problems. Click here for a copy of the Class B Transponder Fault finding Guide which is applicable to both of these units but not the latest AIT2000.

If you have lost the manuals for either of these two products click on the relevant link below to download a PDF copy or if you want the latest proAIS software for your PC or the USB drivers for the AIT1000.

 AIT250 Manual

–  AIT1000 Manual

–  proAIS Software

–  Windows USB Drivers for AIT1000


The latest firmware is also available V10.11.14.0, but we would only recommend installing this if you are experiencing interfacing or TX time out problems.

Finally the only known issue with either of these products is intermittent “Phantom” Silent Switch operation on the AIT1000. If you see the occasional flashing of the blue Silent LED or the yellow TX time out keeps coming on, then it is worth either switching the Silent Switch setting in proAIS to “Switch has no function” if you do not have a Silent Switch fitted or alternatively connect the Blue wire in the Power/Data cable to the Black wire. This fault is due to spurious electrical noise “spikes” in some installations triggering the Silent Mode and shorting these two wires together or turning off the Silent Switch function fixes the problem.

NMEA 0183 Display Program

NMEA 0183 has become the most widely adopted standard for data communication in the Marine industry. Based on the old RS232 Serial Interface used on computer terminals data is transmitted as a series of high and low voltages between two pieces of wire. Although fairly simple in principle, NMEA 0183 interfacing causes a great deal of confusion for customers and hopefully this article will give some basic advice for fixing simple connection problems.

1) Electrical Connection

NMEA Outputs can drive 2 or 3 devices so connecting a GPS to a DSC VHF and a Chart Plotter is possible, but you must only ever connect one NMEA output “Talker” to an NMEA input “Listener”. For example, you cannot have the output of the GPS and the output of an Instrument System connected to the same input of a chart plotter.

You must always connect two wires for successful data transfer a +wire and a -wire. As long as you get the Output+ connection to the Input+ and the Output- connection to the Input- everything should be fine. Some manufacturers refer to the + connection as “A” and the negative connection as “B” but this is not that common these days. Some systems will have a common negative connection for the Input and Output in which case you take the Output- and Input- wires from the equipment you wish to connect and wire them both to the Common Negative connection.

Finally, if a piece of equipment just has a single wire Output+ but the equipment you are connecting to has a 2 wire input, take the Input- connection to the negative (0 volt) supply of the boat.

NMEA data can sometimes be measured with an electrical meter set to DC Volts, but the readings can be misleading and it is far better to test NMEA data with a Light Emitting Diode (LED) available for a few pence from electronic shops. Please read our Tech Note on how to use an LED to test NMEA data.

2) NMEA Data Speeds

Originally there was just one “flavour” of NMEA 0183 data, with all of the systems transmitting data at 4800 Baud (bits per second). When AIS systems came along it was clear that to cope with the amount of data output by an AIS receiver or transponder, that NMEA 0183 data would have to get faster. A new “flavour” of NMEA 0183 was introduced – NMEA 0183HS where HS stands for “High Speed” which operates at 38400 baud. As a general rule, all AIS systems work at this faster 38400 baud, whilst all other systems on the boat operate at 4800 baud, with a few exceptions such as NMEA Multiplexers that work at 38400 baud.

You cannot connect a 38400 baud output to a 4800 baud input and vice versa and when connecting to equipment that can have different baud rates, particularly chart plotters it is always best to check that they are set to the right baud rate for the equipment connected to them. Another important note, is that on most systems the Input and Output ports are paired so you cannot have NMEA Output 1 of a chart plotter operating at 4800 baud and NMEA Input 1 at 38400 baud.

3) NMEA Data Sentences

NMEA 0183 data is transmitted in bursts, where each burst is referred to as a “Sentence”. There are lots of different NMEA Sentences, one or more for each type of data transmitted i.e. Depth, Wind, Position, etc. Detailed Specifications of the data structure of NMEA 0183 Sentences are available from the NMEA Organisation, although there are many on-line articles that will give a good basic understanding.

In order to “look at” this data, technicians traditionally used a terminal program to display the data on a computer. In early versions of Windows, Microsoft included a program called Hyper Terminal which was very good for reading NMEA 0183 data but when Windows Vista was released, Hyper Terminal was no longer included, so Digital Yacht created a free NMEA Display Program to help our dealers and customers read NMEA 0183 data. Fully Windows 7 compatible, this NMEA display program not only displays the raw data but also formats and parses the data in to a more understandable format, so that you can actually read the values of depth, wind, position, etc.

A screen shot is shown below and to download the complimentary fully functional program (with no adverts or malware !!) please click here….