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AHRS - Advanced Configuration - User Guide
MU-AHRS-AN-002 Ed. E November 2015 8
3 G
ENERAL
C
ONVENTIONS
3.1 Command Syntax
All frames are compatible with NMEA 0183 standard and are formatted as follows:
$CMD[,xxx,…,…y*hh<CR><LF> for configuration commands
$PHTXT,CMD[,xxx,…,..y]*hh<CR><LF> for configuration text retrieve commands
$ is a header and “CMD” depends on the command
Brackets [ ] indicates optional parameters, depending on the command. Most commands
can be used either to send configuration parameters “xxx,…,..y” to the system, or to ask
for the current value for the parameter. In such case, the “,xxx,…,..y” should be replaced
by “,,”.
“hh” is the checksum of the sentence, and allows for a control during the transmission. It is
calculated by exclusive-OR’ing (XOR) the 8 bits (no start bits or stop bits) of each
character in the sentence, excluding “$” and “*”. The hexadecimal value of the most
significant and least significant 4 bits of the result are converted to two ASCII (0-9, A-F) for
transmission. The most significant character is transmitted first.
The checksum field is required in all transmitted sentences. All frames should be ended by
the two characters <CR><LF> (0D 0A hexadecimal).
A Web based NMEA calculator is given in Appendix A to help computing checksums.
3.2 How to Send and Receive Commands
The AHRS will listen for COMMAND protocol on repeater port.
On repeater port, serial configuration is fixed and set to 19200 bauds, odd parity bit, 2
stop bits. Repeater port will also output OCTANS STANDARD protocol. Answers to
commands sent to this port will thus be mixed in OCTANS STANDARD output frames
(refer to AHRS Interface Library (Ref.: MU-AHRS-AN-003) for details).
If a command is not correctly formatted, the AHRS will ignore it and no answer will be
sent. If a parameter is out of allowed range, it will be ignored by the AHRS and set to
default value (0 or none). Thus, to confirm that a command was correctly handled by the
AHRS, the control application should systematically send the command to change a
parameter, and then send the read back command to check stored value. After all
changes are made, the application can send a SAVE command to store parameters in
non volatile memory inside the AHRS.