How to make sense of NMEA 2000 messaging formats

NMEA 2000 Messaging

NMEA 2000 data is sent as messages, which we refer to as a PGN. A Parameter Group Number (PGN) is a unique numerical identifier used to categorize and define specific sets of data within the NMEA 2000 protocol. Think of it as a code that groups together related data, allowing devices on a network to recognize and interpret information efficiently. The PGNs define all data transmitted from a device.

There are 3 types of PGN;

• Mandatory: These PGN types are defined by the NMEA 2000 Standard as ones which all devices must support.

• Common / Data: These PGN types contain the normal data from instruments such as Depth, Position, and Temperature.

• Proprietary: These PGN types are defined by the NMEA to allow transfer of data which is beyond the scope of the approved PGN specification.
  An example of this would be manufacturers sending specific data to targeted devices for firmware updates or configurations. This is outside of the scope of the NMEA 2000 standard message spec and also has no value to any other manufacturer or connected device, thus it can be kept proprietary. These messages are the lowest priority on the network, and will not take priority over data or command PGNs.

There are three conditions which the NMEA defines as acceptable use of the Proprietary PGN:

The serial data frame used by CAN has a 29-bit identification field and from zero to eight data bytes.

In addition, the frame contains the start of frame and end of frame bits, reserved bits, frame control bits, a 15-bit CRC error detection field, and acknowledgement bits. 29-bit Header 8-byte Data field CRC field End of Frame Start of frame Control field ACK field.

So why are they important?

Understanding these messages and their formats is useful for installers and boaters when trying to make sense of data. Of course, you can’t look at raw binary data and understand it, so we need the PGN contents to be decoded. There are several NMEA 2000 PC Applications available which do this for you, such as Actisense NMEA Reader.

Using NMEA Reader, you can see the contents of each NMEA 2000 PGN (excluding other manufacturers’ proprietary messages), in a human-readable, decoded format, allowing you to make sense of the data you are receiving. This allows for basic investigation and fault-finding exercises, as now we can see what data a GPS is outputting for example, or if it is even outputting at all.