3 common mistakes when installing NMEA devices
Installing new and exciting pieces of equipment onto a vessel should be a stress free and easy process, however it doesn’t always go to plan. We know there are always going to be challenges, changes in scenario and requests, or sometimes things just simply will not work as they were expected to. Unfortunately, some of the issues we see are down to bad practice and installation and a lot of this can be avoided.
This article is focused on some of the simple mistakes we have seen, the easy to avoid issues and how to ensure that everything is operating as it should be. The following points are going to be covered:
NMEA 2000 Network Termination
NMEA 2000 is a CAN (Controller Area Network) system, designed to be plug and play with simple and easy ways of expanding and manipulating the network layout to suit any installation requirements. Whilst this is done successfully most of the time, there are still some issues that can potentially cause problems.
The most common mistake we see is terminator usage, be that only one present, or having more than two present. There should only be 2 NMEA 2000 Terminators on a network, one located at either extremity of the backbone (in line terminators are an exception to this). The terminators act as a resistive load to correctly terminate the network and help to reduce reflections of signals.
A correctly terminated network will have a resistance of 60 Ohms on the backbone, which can easily be tested using a multimeter. A quick indication that something is wrong is if the resistance is 120 or 40 Ohms. 120 means that there is only 1 terminator present, and 40 means there is 3. This issue is very frequently seen when using devices which have internal terminators. These terminators operate exactly the same as your common external one, and should be treated as such.
NMEA 2000 Network Design and Installation
Given how forgiving CAN is in its nature, it is entirely plausible that an NMEA 2000 network can be installed incorrectly, for example with the wrong topology and it will still operate correctly under the current conditions. However, this is not good practice and can result in issues over time, especially if the network is expanded.
Certain practices such as daisy chaining of devices, or running a 4 way drop off of a 4 way drop are not ideal by any means, however chances are that if the network is not overly busy, the devices will all seem to operate and transfer data as they should. However, these can create ‘star network’ style set-ups, rather than the typical bus network that is expected in NMEA 2000 installations.
As the number of devices on the network is increased, you may experience things like data loss and corrupt data, along with devices starting to disappear from the network.
There is also the second problem caused by having an ‘incorrect’ network topology, which is the troubles it can cause an installer. If there is an issue on the network which needs a certified technicians support, they need to be able to easily understand your network and perform diagnostic tests on it. With a poor installation choice with incorrect layout, the technician is already fighting an uphill battle before they have really begun.
To avoid this, always be sure to follow the BUS network method, where each device is connected individually to the backbone via a drop cable or 4 way drop cable. This keeps the network easier to trace, and follows all NMEA 2000 standards, plus it makes it easier to document the network with diagrams etc…
*Read our article on how to manage more elaborate NMEA networks.*
NMEA 0183 Wiring
NMEA 0183 wiring poses a few problems and a level of confusion, especially amongst those who are not experienced in installations and standards. There are 2 ‘main’ versions of NMEA 0183;
NMEA 0183 V1.5 is RS-232, which utilise single ended talker / listener connections.
NMEA 0183 V2.0+ is RS-422, which utilises differential talker / listener connections.
Principally, connecting RS-232 to RS-232 and like-wise for RS-422 is easy. The mistakes usually happen when trying to connect a combination of the two. There is a fundamental flaw with the way RS-422 and RS-232 work when connecting together, where connecting a V2.0+ Differential Talker to a V1.5 Single Ended Listener can have drastic consequences if not done safely and correctly. It is possible to damage or completely overload the Differential device.
The reasoning behind this is that the older Single Ended devices use ground as return, but Differential devices actually fluctuate by driving 0v-5v through the ‘-/B’ connection. If they were connected together, the Talker would draw too much current as it’s trying to output to ground which causes over heating and component failure, along with device damage.
Fortunately, Actisense have a solution which makes this risky connection completely safe and isolated. Read about our ISO-Drive technology and more on NMEA 0183 here:
One other topic to cover briefly here is the wiring colours in NMEA 0183. For a long time there was no wire colour standards in the NMEA 0183 standards, so manufacturers essentially had free reign on deciding which wire colours to use. Whilst there is now a standard, it is a little complicated and doesn’t work as it should, because the wires would have to change colour half way down.
Unlike NMEA 2000 which had a wire colour standard from the get-go which makes wiring much easier, NMEA 0183 doesn’t have the same level of continuity. For the safest method, please always refer to the user manuals of the products you are connecting, and do not just go on a colour match method for wiring. In NMEA 0183, one companies Rx – may be red, and in another company that is their Tx + colour code. Whilst this wouldn’t damage the devices, it can still be confusing, especially when trying to diagnose a fault.