User's Manual

7 Software interface
Message types
The Tracker transmits two types of messages:
● Unsolicited messages, which include status messages, date/time and GPS information,
and notifications that messages have been received by the Tracker
● Command responses, which include responses to message and power management
commands, as well as notifications that messages have been sent or settings have been
updated
General command structure
All messages to and from the Tracker are NMEA formatted messages. NMEA messages begin
with a $ and end with a single newline ‘\n’ character. In addition, the last three characters
prior to the newline are *xx where xx is a checksum of the characters in the command from
next character after the $ up to, but not including, the *. The checksum is the same as used by
NMEA. Messages with a bad checksum are silently ignored and are not stored anywhere.
A $ will never occur within a command, and may be used to reset the receiving state machine.
A * may occur within a command. The receiving state machine will verify the last three
characters in the command are *xx after the \n is received. The x may be any legal ASCII
character in the range 0..9, A..F, or a..f.
An example command is provided below to illustrate the command structure. The below
command returns the most recent datetime message from the Tracker:
$DT @*70
Command timing
Once the $ is received, the next character must occur within 5 milliseconds of the previous
character. If the inter-character delay exceeds 5 ms, the command will be silently discarded,
and the receiving state machine will consume and ignore any characters received until the next
$.
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