14.9.4. NMEA 2000 Instances
NMEA 2000 defines three different instances:
1. Data instance
2. Device instance
3. System instance
For all battery monitors and other devices that the GX device makes available on the CAN-bus, each of the above types of
instances is available, and can be individually configured.
Per Virtual-device, there is one Device instance and one System instance. And depending on the type of the Virtual-device, there
are one or multiple Data instances.
For example, for a BMV-712 there are two Data instances, one DC Instance for the main battery and another one for the Starter
battery voltage.
How to configure the instances depends on the equipment and software that is used to read them from the CAN-bus. Examples
of equipment and software meant here are MFDs such as from Garmin, Raymarine or Navico; as well as more software oriented
solutions from for example Actisense and Maretron.
Most of those solutions identify parameters and products by requiring unique Device instances, or using the PGN 60928 NAME
Unique Identity Numbers and do not rely on the Data instances to be globally unique.
There are, however, a few exceptions:
• For Raymarine and Furuno MFDs the Data instance needs to be changed in order to properly show data. Please refer to the
MFD integration page for more details.
The NMEA 2000 specification specifies the following: “Data instances shall be unique in the same PGNs transmitted by a device.
Data instances shall not be globally unique on the network. Field programmability shall be implemented through the use of PGN
126208, Write Fields Group Function.”.
In other words, Data instances need to be unique only within a single device. There is no requirement for them to be globally
unique – the only exception is “Engine Instance” that at least for now, to cope with legacy devices, needs to be globally unique
(e.g. Port = 0, Starboard = 1). For example, some of our BMV battery monitors can measure two voltages, one for the main
battery and one for the starter battery, and that's where data instancing is used. Similar for multiple-output battery chargers.
Note that there is no need for the installer to change those data instances, as those products are pre-configured to transmit the
relevant PGNs with unique Data instances (Battery instance & DC Detailed instance, in this case).
Whilst it is possible to change the Data instances, changing them on a Victron devices will render that device
impossible to read correctly by other Victron devices.
A note about the Device instances: it is not necessary to assign a unique Device instance to each device on the CAN-bus.
Its no problem for a battery monitor and a solar charger to both be configured with (their default) Device instance 0. Also when
having multiple battery monitors or solar chargers, it is not always necessary to assign each of them a unique Device instance. If
at all necessary, they only need to be unique between the devices that use the same Function.
And note that changing the Device instance on a Victron device can change its operation, see below.
System instances
As per NMEA 2000 specification, this instance is a 4-bit field with a valid range from 0 to 15 that indicates the occurrence of
devices in additional network segments, redundant or parallel networks, or sub networks.
The System Instance Field can be utilised to facilitate multiple NMEA 2000 networks on these larger marine platforms. NMEA
2000 devices behind a bridge, router, gateway, or as part of some network segment could all indicate this by use and application
of the System Instance Field.
The ECU instance and Function instance
In some documentation and software tools, yet other terminology is used:
• ECU Instance
• Function Instance
• Device Instance Lower
• Device Instance Upper
Color Control GX Manual
Page 94 Marine MFD integration by NMEA 2000
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