How does NMEA 0183 to NMEA 2000 data conversion work?

Understanding how the NGW-1 / NGX-1 converts data 

NGW/NGX converting NMEA 0183 to NMEA 2000 

During the conversion process from NMEA 0183 to 2000, the corresponding data values slated for conversion are integrated into a storage system. These data values are subsequently populated into their respective Parameter Group Numbers (PGNs), as per the conversion list. These populated PGNs are then transmitted at the configured transmission (Tx) rate. This transmitted data remains valid for a duration of up to 4 seconds. In the absence of updates to the data values during this 4-second interval, the last received value continues to be transmitted at the Tx rate if the PGN is set to be transmitted. (Following the Tx rate if periodically. If set to non-periodic it will only transmit once). 

This scenario implies that if a solitary periodic sentence is sent only once, multiple instances of the same PGN will be transmitted. For instance, if a solitary depth (DPT) sentence is sent and the PGN Tx rate for 128267 is established at 1000ms, you should anticipate the PGN to be transmitted 4 times before the 4-second validity period for the data expires and consequently clears the transmission. 

Similarly, if a single heading (HDM) sentence is transmitted and the PGN Tx rate for 127250 is configured to 100ms, the expectation is that the PGN will be transmitted around 39 to 40 times (this figure hinges on the precise timing of the initial transmission) before the 4-second data validity period concludes and stops the transmission. 

NGW/NGX converting NMEA 2000 to NMEA 0183 

When converting NMEA 2000 Parameter Group Numbers (PGNs) into NMEA 0183 sentences, the process involves taking the data values associated with each PGN and placing them into a repository. These data values are then used to populate the relevant information within the NMEA 0183 sentence, following the conversion list. Subsequently, the populated NMEA 0183 sentences are transmitted based on the configured transmission (Tx) rate. 

The transmitted NMEA 0183 sentences remain valid for up to 4 seconds. If there are no updates to the data values during this 4-second period, the last transmitted value continues to be sent repeatedly at the configured Tx rate (for periodic PGNs or only once for non-periodic PGNs). 

In practical terms, this means that if a specific NMEA 2000 PGN is being converted and transmitted as a periodic NMEA 0183 sentence, sending it only once will result in multiple copies of the corresponding NMEA 0183 sentence being transmitted. For example, if a particular depth PGN’s Tx rate is set to 1000ms and the PGN is sent just once, you should expect the NMEA 0183 sentence to be transmitted four times before the 4-second data validity period elapses and clears the transmission. 

Likewise, if a fast-heading NMEA 2000 PGN is being converted and transmitted with a different Tx rate, such as 100ms, sending it once would lead to the NMEA 0183 sentence being transmitted around 39 to 40 times (depending on the exact timing of the initial transmission) before the 4-second data validity period concludes and stops the transmission. 

For more on NMEA 0183 networking, download our free guide.

To learn more about building an NMEA 2000 network, download this guide.