When we purchased our Bavaria 40 Ocean “Millie” last year she was fitted with a good range of electronics, but with some limitations. In particular she had a Raymarine Classic C70 chart plotter/MFD on the helm console, which worked fine but was impossible to see in sunlight (at least with my eyesight). She also had Raymarine ST60 instrumentation and autopilot, an ICOM MX-5000 AIS receiver and a Raymarine 49e VHF. I decided to upgrade the MFD to a Raymarine E90, which was much brighter, larger and easier to see. This needed an adaptor to link the Seatalk ng bus (a proprietary version of NMEA 2000) to the old Seatalk 1 network of the instruments. It also had inputs for NMEA0183 so was able to take the data from the AIS and VHF.
However all was not well with the AIS and during a short trip across the Channel last summer AIS targets started to disappear as we entered the shipping lanes! We had excellent visibility so it wasn’t really a problem, but it was rather disconcerting. I wasn’t able to establish whether the problem was with the ICOM unit or the cabling. The NMEA0183 cable did run through the engine compartment and so could have been getting some interference although the problem also seemed to occur under sail.
Having worked in data communications for 15 years, spending a lot of that time project managing large network installations, I am very familiar with the problems with data cabling. The electircal layer of the NMEA0183 standard is based on RS-422 , which uses a pair of wires to transmit a differential signal. The output from each device can be fed into a number of other inputs, but outputs cannot be linked together, except through a proper multiplexer device. RS-422 is a robust standard, and the use of differential signalling should help to reject noise (the theory is that electrical noise is picked up equally on both conductors, so that it cancels out). Basic NMEA0183 also runs at a very slow speed (4800 bps) so should be very tolerant and even the higher rate used by AIS (38400) is considered slow by modern standards.
On many boats the problems start with the quality of the cable used, the fact that it is often unshielded, very thin and doesn’t use twisted pairs. Very often these are connected together using unreliable screw terminals or worse, so connections may be subject to corrosion and loss of electrical conductivity. Add to that the complexity of cabling multiple devices (remember you can’t link two outputs) and problems are bound to arise.
If you are having problems with an NMEA0183 installation, start by tracing each connection and drawing a network diagram. Make sure you understand which are inputs and which are outputs. Remember one output can feed a number of inputs but you must not feed more than one output into a single input. If you are using an AIS device remember the output runs at 38400 and you may need to change the settings on the device it is feeding. Consider replacing any long runs of cable, especially if they run past the engine. Ethernet cable (often described as Cat5 or Cat6) is ideal if you can find the stranded (as opposed to solid copper conductors) and shielded, such as Wentronic 93953-GB which is available on Amazon and eBay (2018). Tinned copper conductors would be even better if you can find it.
NMEA2000 (N2K) addresses many of these issues by specifying standards for cabling and connectors which are far more robust, and using a multidrop cabling architecture which is much simpler – each device just plugs in to a T-piece on the backbone – and includes a power feed on the cable, which is fully screened so should be more immune to electrical noise. Garmin and most other equipment manufacturers (apart from Raymarine) use standard Devicenet cabling and connectors, designed for industrial use, so well able to withstand conditions on the average boat.
The downside of N2K is that the cost of cabling can be high. A ‘T-piece’ and drop cable (required for each device) can cost £30-40. Also in an area such as the nav table, with several devices to connect, linking a number of Ts together can be bulky and untidy. It is possible to buy multidrop connectors but these are still quite expensive. As these are essentially just connector blocks. I decided it would not be too difficult to make my own – more details of that in my next blog.
And the AIS problem?
Did I solve the AIS problem? Not really. It could have been an issue with the cabling, or it might have been a problem with the receiver. In the end I decided I would prefer to have a Class B Transponder (that sends and receives) and decided to buy the Digital Yacht AIT1500 with an N2k output. And so I have a different problem – building an N2k network and linking it to the Seatalk NG used by the chartplotter.