=====Communication Protocol=====
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An adoption of NMEA0183 (National Marine electronics Association) at RS-232 voltage levels is used for the data link between level 1 and level 2. The standard was chosen on the following basis: It has a low power consumption, the bandwidth
requirements are low, it is human readable, and the signal can be tapped for easy debugging. Level 1 will once per second transmit a request for rudder and commands to the Level 2, in the form of a NMEA message. 

{{:l2-l1-comm.png?700|}}

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There are three kind of NMEA-like messages involved for different communications:
- Level 2 - Level 1 communication;
- Level 1 - Operator bilateral communication.

Level 2 sends messages to Level 1 containing the desired rudder angle and thruster intensity. These come as a result of high-level control (PIDs) implemented in DUNE. The format of this message is:
\\
;#;
**MSG = BBB01 , RUD , THR , PWR_SETT , FALLB_MODE**
;#;
Where:\\
- BBB01 is the msg ID; \\
- RUD is the rudder angle [-450,450]; \\
- THR is the thruster intensity [-100,100]; \\
- PWR_SETT are the power settings: 6-bits string where 1 disables and 0 enables; \\
- FALLB_MODE selects the fallback mode: 0 for //rudder 0//, 1 for //circle//, 2 for //autopilot//.

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The operator can comminicate to Level 1 through Neptus (Java Plugin), sending messages that respect this format:\\
;#;
**MSG = RCC01 , RUD , THR , Kp , Ki , PWR_SETT , FALLB_MODE , MANUAL_CTRL**
;#;
Where:\\
- RCC01 is the msg ID; \\
- RUD is the rudder angle [-450,450]; \\
- THR is the thruster intensity [-100,100]; \\
- Kp and Ki set the gains for the Level 1 //Fallback Autopilot//; \\
- PWR_SETT are the power settings: 6-bits string where 1 disables and 0 enables; \\
- FALLB_MODE selects the fallback mode: 0 for //rudder 0//, 1 for //circle//, 2 for //autopilot//; \\
- MANUAL_CTRL is a boolean: 1 for taking control (manual), 0 for giving control (autopilot).

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Level 1 can send messages to the Operator showing the status of the system. These massage have the following format:\\
;#;
**MSG = CR601 , LEAK , PWR_SETT , LOAD, PANEL1_2 , BATT , COG , SOG , N_SAT , LAT , LONG , Kp , Ki , e , int_e , RUD , THRUST , L1_STATE , FALLB_MODE**
;#;
Where:\\
- CR601 is the msg ID;\\
- LEAK indicates a leak is happening: 1 for TRUE, 0 for FALSE;\\
- PWR_SETT indicates the power settings: 6-bits string where 1 means disabled and 0 enabled; \\
- LOAD is the load power (W);\\
- PANEL1_2 is the panel 1 and panel 2 power (W);\\
- BATT is the voltage battery (V); \\
- COG is the course over ground (°); \\
- SOG is the speed over ground (kn); \\
- N_SAT is the satellites number used by the GPS; \\
- LAT is the latitude; \\
- LONG is the longitude; \\
- Kp and Ki are the gains used by the PI fallback autopilot controller; \\
- e is the error; \\
- int_e is the integrated error; \\
- RUD is the rudder computed by the PI fallback autopilot controller; \\
- THR is the thruster applied to the servo; \\
- L1_STATE shows the operating state of L1: 1 = normal, 2=fallback, 3=manual; \\
- FALLB_MODE selects the fallback mode: 0 for //rudder 0//, 1 for //circle//, 2 for //autopilot//.