|
Introduction
The National
Marine Electronics Association NMEA2000® network is designed to
allow high speed serial data communications between electronic devices on board a
vessel easily and securely. These can range from navigation instruments,
engine monitoring systems, tank monitoring systems through to control of
electrical devices such as lights and motors over a simple common data
cabling system. This cabling system is designed to be robust, waterproof
and flexible. One additional benefit is the greatly simplified and
reduced amount of cabling required to connect devices the length and
breadth of the vessel.
Network Topology
The network is based on a single backbone or trunk cable, terminated at
each end, with multiple drop connectors each feeding an attached device.
These devices can be senders or transducers placing data onto the trunk
or display or control heads that utilise the data. The trunk has a
single power feed which is connected to the ships power and provides
power to all the attached devices. When an attached device needs to draw
more than the trunk can supply then it may have it's own higher power
supply feed in addition to that provided by the trunk.
The backbone or trunk cables may be either the larger "mini" size
which uses 12mm diameter cable and 25mm diameter plugs or the smaller
"micro" size which uses 7.5mm diameter cable and 14.5mm plugs.
The drop cables are normally the Micro size but it is
possible that devices having heavier power requirements may use the
larger Mini sized cable as a drop cable.
Network Limitations
Backbone/Trunk Cable length - The distance
between any two points on the network must not exceed 200 metres for a
Mini backbone/trunk cable and 100 metres for a Micro backbone/trunk
cable. In most cases this distance will be measured from terminator to
terminator. However if the distance from the Tee connector over the drop
cable to the last device at each end of the network is greater than the
distance from the Tee connector to it's terminator then the length of
that drop cable must be included in the total length calculation.
Drop Line Lengths - The total drop cable
lengths on the network must not exceed 78 metres and no single drop
cable must exceed 6 metres.
Maximum Number of Devices - A maximum of 50
devices can be attached to a network. It is possible to run multiple
networks in a vessel with gateways between networks if it is required to
share information between more than 50 devices.
Network Power
Power Requirements - The network requires a
power input of 9 - 16 volts DC which is normally provided by connecting
the network to the vessels 12 volt battery. In 24 volt vessels a 24 - 12
volt converter will be required. It is possible to have a number of
isolated power supplies feeding the network but this topology is unusual
and requires special considerations. The total current carrying capacity
of the network depends on the choice of backbone/trunk cable and where
the power is connected into that cable. The Mini cable is rated at 8
Amps and the Micro cable is rated at 4 Amps. Both of these ratings are
at 20 degrees celsius and must be de-rated with temperature. If the
power connection is made to the centre of the backbone/trunk cable then
the current carrying capacity is effectively doubled as the full
capacity is available to each end of the network from the power
connection point. This means that a Mini backbone/trunk cable based
network could support up to 16 Amps over the network. It is important
that the power supply is correctly fused to match the load requirement
and the minimum cable size used in the network.
Power Connection - The network is connected to
the power source by means of a Mini or Micro Powertap connection.
Load Equivalence Numbers - All NMEA2000®
certified devices have a Load Equivalence Number (LEN) attached which
represents the current requirement in multiples of 50mA. I.E. a LEN of 4
means that the device can consume up to 4 x 50 mA = 200mA. The installer
should ensure that the network power can supply sufficient power to
support the attached devices by totalling the LENs of all the attached
devices.
Voltage Drop - A maximum voltage drop of 1.5
Volts between any two devices on the network is permitted. This can be
measured or calculated by using the formula:
Voltage Drop = Total Network LENs x Network Length (in
metres) x Cable resistance (in Ohms per 100 metres) / 100 x 0.1
Network Grounding
The network should be grounded at a SINGLE location.
This is normally done at the power supply connection to the network and
should be robustly connected to the vessels grounding system. There must
be no other ground connections on the network to avoid the problem of
ground loops which can harm the networks performance.
Network Installation
This network has been designed to be robust and easy
to install and configure. However it is recommended that it be installed
by qualified and experienced installers who work to national and NMEA
installation standards.
Click on the graphic below to see details of the
cabling, connector and terminator products we sell:
|
