NMEA sentence structure

A sentence structure is defined by NMEA to establish the communication between two units. Most other datagram formats are designed using the same, or a similar, structure.

The following provides a summary explanation of the approved parametric sentence structure:

$aaccc,c-c*hh<CR><LF>

This character (Hex: 24) is used to identify the start of a sentence.

aaccc

This is the address field. The first two characters (aa) identify the talker ID, while the last three characters are the sentence formatter mnemonic code identifying the data type and the string format of the successive fields.

The comma (Hex: 2C) is used as a field delimiter. This character starts each field except the address and checksum fields. If it is followed by a null field, it is all that remains to indicate that there are no data in the field.

c-c

This is the data sentence block. This is a series of data fields containing all the data to be transmitted. The data field sentence is fixed and identified by the sentence formatter in the address field. Data fields may be of variable length, and they are preceded by the field delimiter.

This character (Hex: 2A) is the checksum delimiter. This delimiter follows the last field of the sentence and indicates that the following two alphanumerical characters contain the checksum.

This is the checksum.

The carriage return and line feed characters terminate the sentence.

Note

In some proprietary telegrams received from other Kongsberg Maritime equipment, the $ character is replaced by the @ character. The checksum field may then not be in use.