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Intermediate
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Boat Electrics
6 Lessons
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This module gives an overview of onboard networks. The focus is on NMEA2000 networks. We learn how they are set up and how to troubleshoot the physical layer if problems arise.
The videos for this module are still a work in progress. But you can access all the content in the form of lecture notes.
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In this short introduction, we learn how data is transmitted digitally between various devices.
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Although NMEA0183 is considered by many to be outdated, there are still many devices on older boats that use this standard. We cover the main principles of connecting NMEA0183 devices but also the limitations of the standard.
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The current industry standard for onboard networks is NMEA2000. We explain the principle of the underlying CAN-bus technology and the requirements for the physical layer.
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We learn how to plan and set up a standards-compliant NMEA2000 network, including the required voltage drop limitations. We also explore simple troubleshooting techniques.
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This lesson is a short excursion into the future of onboard networks based on ethernet technology and the NMEA OneNet standard.
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This course has been created in a joint effort by Nigel Calder and Dr. Jan Athenstädt. Nigel is responsible for the lecture notes and Jan will take the role of presenter and guide you through the lessons.
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I was told that if the voltage drop would exceed 1.5 V, you’re allowed to plug in a second power cable from the same power source.
Is that correct ?
Ron,
We are talking two different things here. One is voltage drop between the battery and the network, and the other is voltage drop along the length of the network. Voltage drop between the battery and the network is unlikely because the network loads are so low. However, you can have a second power supply, but this is for redundancy, not voltage drop. Voltage drop in the network itself is a function of the number of loads on the power conductors in the network cable. A redundant power supply will not reduce this. There are a couple of ways to address this. One is to reduce the loads! the other is to change the power injection point (PIP), the point at which the battery is connected to the system. If it is moved, for example, from one end of the network to the center of the network, it is feeding power out in both directions which will, in effect, cut the voltage drop in half (assuming the loads are equally distributed on both sides of the PIP.
Nigel
Hi Jan and Nigel
It is my understanding that in a N2K network there are no receipts meaning that a sender never gets a confirmation that a message is received. For example, a chart plotter sending an instruction to an autopilot computer has no way of knowing if the instruction was received and acted upon. So if the message is lost along the way by an error in the network or in the receiver this will not be detected by the sender.
Is this correct?
How is this in the OneNet standard?
-Tor
Tor,
Electronics are not our specialty – we focus on the connections to the ‘black box’ rather than what is going on inside it – but I think you are correct. However, one of the features of J1939, from which NMEA 2000 is derived, and also of NMEA 2000, is the extremely robust messaging protocols such that messages don’t get dropped or lost. I don’t think this is a concern.
I know even less about the inner workings of OneNet but knowing the NMEA I am guessing there are also robust protocols to ensure critical information does not get lost.
If you do some more investigation and have anything to add, please share it with the BoatHowTo community.
Nigel