Serial Communication

The information on this page refers to firmware v0.78e and higher.

Communication with the STorM32 board is possible through the serial interfaces USB, UART, and Bluetooth. Three sets of commands are available for serial communication:

• Simple Commands: This set has a very simple command structure, and is used for major tasks of the GUI.
• RC Commands: This set is targeted at a remote control of the STorM32 gimbal.
• Mavlink Commands: This set is "real" Mavlink, with extensions however to fully support the STorM32 features.

The simple serial and the RC commands share many similarities, and could be considered the two sides of a coin (they both are in fact processed by the same code part in the firmware). They tend to be more efficient than the Mavlink commands. Any command of any set can be processed through any of the serial interfaces (USB, UART, and Bluetooth). However, when the Mavlink heartbeat is activated, then the UART accepts only Mavlink commands, and also Bluetooth may not be used anymore (USB still accepts all commands).

For understanding all details of the communication, it is generally best to peek into the source code of the GUI (which is included in any firmware package). It's written in Perl, and Perl is sufficiently primitive to understand the code easily.

Serial Communication - Simple Commands

This protocol for the communication via the serial interfaces follows these rules:

• The board responds to incoming commands by sending back a data stream of one or more characters, which is determined by the received command.
• The board is never sending/transmitting anything by itself, that is, a transmission is always started only as a reaction to an incoming command.
• The board is responding to any incoming command, be it valid or not.

Any data stream returned by the board ends with one of these characters:

• 'o': indicates that everything is ok, i.e. that the received command has been identified
• 'e': indicates that an error has occurred, i.e. that an invalid command has been received
• 't': indicates that a timeout has occurred, i.e. that a command which consists of several characters was not completed within a certain time window
• 'c': indicates that a checksum error has occurred

A checksum is invoked whenever data are transmitted, currently these are the 'p','g', and 'd' commands.

Command 't'

This command simply returns the character 'o'. Can be used by the host to check if the board is still connected.

Command 'v'

This command returns information on the installed firmware version, the name of the board, and the type of the board. The data stream is appended by a crc, and closed with the character 'o'.

Command 'g'

This command returns a data stream containing all parameter values. The data stream is appended by a crc, and closed with the character 'o'.

Command 'p'

This command sets all parameter values. The command character 'p' needs to be followed by a data stream containing all parameter values, plus a crc. It returns the character 'o'.

Command 'd'

Upon receipt of the command 'd' a data stream containing the current live data is transmitted. The live data is appended by a crc, and closed with the character 'o'. The respective C code snippet is as follows:

 //send 32 data values
 uart_prepare_transmit();
 ((u16*)fbuf)[(*len)++]= STATE; //state
 ((u16*)fbuf)[(*len)++]= status; //status
 ((u16*)fbuf)[(*len)++]= status2; //status2
 ((u16*)fbuf)[(*len)++]= i2c_geterrorcntofdevice(IMU_I2CDEVNR)+i2c_geterrorcntofdevice(IMU2_I2CDEVNR);
 ((u16*)fbuf)[(*len)++]= adc_lipovoltage(); //lipo_voltage;
 ((u16*)fbuf)[(*len)++]= (u16)systicks; //timestamp
 ((u16*)fbuf)[(*len)++]= (u16)(1.0E6*fdT); //cycle time
 ((u16*)fbuf)[(*len)++]= (s16)(fImu1.imu.gx);
 ((u16*)fbuf)[(*len)++]= (s16)(fImu1.imu.gy);
 ((u16*)fbuf)[(*len)++]= (s16)(fImu1.imu.gz);
 ((u16*)fbuf)[(*len)++]= (s16)(10000.0f*fImu1.imu.ax);
 ((u16*)fbuf)[(*len)++]= (s16)(10000.0f*fImu1.imu.ay);
 ((u16*)fbuf)[(*len)++]= (s16)(10000.0f*fImu1.imu.az);
 ((u16*)fbuf)[(*len)++]= (s16)(10000.0f*fImu1AHRS.R.x);
 ((u16*)fbuf)[(*len)++]= (s16)(10000.0f*fImu1AHRS.R.y);
 ((u16*)fbuf)[(*len)++]= (s16)(10000.0f*fImu1AHRS.R.z);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*fImu1Angle.Pitch);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*fImu1Angle.Roll);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*fImu1Angle.Yaw);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*cPID[PITCH].Cntrl);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*cPID[ROLL].Cntrl);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*cPID[YAW].Cntrl);
 ((u16*)fbuf)[(*len)++]= InputSrc.Pitch;
 ((u16*)fbuf)[(*len)++]= InputSrc.Roll;
 ((u16*)fbuf)[(*len)++]= InputSrc.Yaw;
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*fImu2Angle.Pitch);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*fImu2Angle.Roll);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*fImu2Angle.Yaw);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*fMag2Angle.Yaw);
 ((u16*)fbuf)[(*len)++]= (s16)(100.0f*fMag2Angle.Pitch);
 ((u16*)fbuf)[(*len)++]= (s16)(10000.0f*fImu1AHRS._imu_acc_confidence);
 ((u16*)fbuf)[(*len)++] = pack_functioninputvalues(&FunctionInputPulse);
 (*len)*=2;
 //add crc
 uint16_t crc= do_crc( fbuf, fbuf_len );
 fbuf[fbuf_len++]= (u8)crc; //low byte
 fbuf[fbuf_len++]= (u8)(crc>>8); //high byte
 //end character
 uart_transmit_ackchar( closewith ); //this sends a 'o'

Command 's'

Upon receipt of the command 's' a data stream containing the current status data is transmitted. The data stream is appended by a crc, and closed with the character 'o'. The command is essentially identical to the 'd' command, except that it transmits only the first five data values.

Checksum

The checksum for protecting some data streams is the x25 16-bit crc used by Mavlink. C code can be found here.

Serial Communication - RC Commands

In addition to the simple serial commands described in the previous section, the o323bgc firmware also understands some messages targeted at remote control of the gimbal. These messages have a stricter data format, and potentially provide a higher level of transmission reliability.

The rules of the communication are exactly as before:

• The STorM32 board emits a message only upon request, but never by itself.
• Any received message is acknowledged by a message send back to the host.

The general structure of a data frame is:

* Startsign: 0xFA for incoming messages, and 0xFB for outgoing messages
* Length: length of the payload, i.e. number of bytes of the data packet excluding Start sign, Length byte, Command byte, and crc word
* Command: the command byte
* Payload: as many bytes as expected by the command
* Checksum: x25 16-bit crc excluding start byte, as used by Mavlink; C code can be found here

The following commands can be send to the STorM32 controller:

CMD_GETVERSION (#1)

0xFA 0x00 0x01 crc-low-byte crc-high-byte

If an error occurred a CMD_ACK message will be emitted. Otherwise a message containing the firmware version, the setup layout version and board capabilities in this format will be emitted:

0xFB 0x06 0x00 data1-low data1-high data2-low data2-high data3-low data3-high crc-low-byte crc-high-byte

Data1 is the firmware version, data2 the setup layout version, and data3 holds the board capabilities value.

CMD_GETVERSIONSTR (#2)

0xFA 0x00 0x02 crc-low-byte crc-high-byte

If an error occurred a CMD_ACK message will be emitted. Otherwise a message containing the version string, name string and board string in this format will be emitted:

0xFB 0x30 0x02 data-stream crc-low-byte crc-high-byte

The data stream contains the 16 bytes version string, the 16 bytes name string and the 16 bytes board string.

CMD_GETPARAMETER (#3)

0xFA 0x02 0x03 data-low-byte data-high-byte crc-low-byte crc-high-byte

The data is of type uint16_t and represents the number of the parameter which is requested. If an error occurred a CMD_ACK message will be emitted. Otherwise a message containing the parameter value in this format will be emitted:

0xFB 0x04 0x03 data1-low-byte data1-high-byte data2-low-byte data2-high-byte crc-low-byte crc-high-byte

Data1 is the parameter number and data2 holds the parameter value.

CMD_SETPARAMETER (#4)

0xFA 0x04 0x04 data1-low-byte data1-high-byte data2-low-byte data2-high-byte crc-low-byte crc-high-byte

Data1 is the parameter number and data2 holds the parameter value. As response to this command a CMD_ACK message will be emitted.

CMD_GETDATA (#5)

0xFA 0x01 0x05 type-byte crc-low-byte crc-high-byte

Type specifies the type of the requested data stream; currently only type 0 is supported. If an error occurred a CMD_ACK message will be emitted. Otherwise a message containing the data stream in this format will be emitted:

0xFB 0x4A 0x05 type-byte 0x00 data-stream crc-low-byte crc-high-byte

The data stream contains the same data as send by the 'd' command.

CMD_GETDATAFIELDS (#6)

0xFA 0x02 0x06 data-low-byte data-high-byte crc-low-byte crc-high-byte

The data is of type uint16_t and represents a bitmask to specify which data should be send. If an error occurred a CMD_ACK message will be emitted. Otherwise a message containing the bitmask word and all the requested data in this format will be emitted:

0xFB LEN 0x06 data1-low-byte data1-high-byte data-stream crc-low-byte crc-high-byte

Data1 is the bitmask word, and data-stream holds the data. The following bits can be raised:

0x0001 = LIVEDATA_STATUS		   
0x0001 = LIVEDATA_TIMES
0x0004 = LIVEDATA_IMU1GYRO
0x0008 = LIVEDATA_IMU1ACC
0x0010 = LIVEDATA_IMU1R
0x0020 = LIVEDATA_IMU1ANGLES
0x0040 = LIVEDATA_PIDCNTRL
0x0080 = LIVEDATA_INPUTS
0x0100 = LIVEDATA_IMU2ANGLES
0x0200 = LIVEDATA_MAGANGLES
0x0400 = LIVEDATA_STORM32LINK
0x0800 = LIVEDATA_IMUACCCONFIDENCE

CMD_SETPITCH (#10)

0xFA 0x02 0x0A data-low-byte data-high-byte crc-low-byte crc-high-byte

The data is of type uint16_t and can assume the values 700...2300. It represents the pitch input value. If the value 0 is send, then the pitch axis will be recentered. Any other values are ignored. As response to this command a CMD_ACK message will be emitted.

CMD_SETROLL (#11)

0xFA 0x02 0x0B data-low-byte data-high-byte crc-low-byte crc-high-byte

The data is of type uint16_t and can assume the values 700...2300. It represents the roll input value. If the value 0 is send, then the roll axis will be recentered. Any other values are ignored. As response to this command a CMD_ACK message will be emitted.

CMD_SETYAW (#12)

0xFA 0x02 0x0C data-low-byte data-high-byte crc-low-byte crc-high-byte

The data is of type uint16_t and can assume the values 700...2300. It represents the yaw input value. If the value 0 is send, then the yaw axis will be recentered. Any other values are ignored. As response to this command a CMD_ACK message will be emitted.

CMD_SETPANMODE (#13)

0xFA 0x01 0x0D data-byte crc-low-byte crc-high-byte

The data is of type uint8_t and can assume these values: 0 = off, 1 = HOLDHOLDPAN, 2 = HOLDHOLDHOLD, 3 = PANPANPAN, 4 = PANHOLDHOLD, 5 = PANHOLDPAN. As response to this command a CMD_ACK message will be emitted.

CMD_SETSTANDBY(#14)

0xFA 0x01 0x0E data-byte crc-low-byte crc-high-byte

The data is of type uint8_t and can assume these values: 0 = off, 1 = on. As response to this command a CMD_ACK message will be emitted.

CMD_DOCAMERA(#15)

0xFA 0x06 0x0F dummy-byte data-byte dummy-byte dummy-byte dummy-byte dummy-byte crc-low-byte crc-high-byte

The data is of type uint8_t and can assume these values: 0 = off, 1 = IRSHUTTER, 2 = IRSHUTTERDELAYED, 3 = IRVIDEOON, 4 = IRVIDEOOFF. As response to this command a CMD_ACK message will be emitted.

CMD_SETSCRIPTCONTROL (#16)

0xFA 0x02 0x10 data1-byte data2-byte crc-low-byte crc-high-byte

The data1 and data2 are of type uint8_t. Data1 is the number of the script and data2 can assume these values: 0 = off, 1 = CASE#DEFAULT, 2 = CASE#1, 3 = CASE#2, 4 = CASE#3. As response to this command a CMD_ACK message will be emitted.

CMD_SETANGLE (#17)

0xFA 0x0E 0x11 float1 float2 float3 flags-byte type-byte crc-low-byte crc-high-byte

The float1, float2, float3 fields represent 4 bytes each. They are of type float, and correspond to the pitch, roll, and yaw angles in degree. The flags byte allows to modify the behavior of the angle setting for each angle. It can be in the limited or unlimited mode. In limited mode the angle setting is subject to the RcMin and RcMax settings, and works only for "absolute". In unlimited mode any angle value can be set without restriction, bypassing the RcMin and RcMax settings, and works for both "relative" and "absolute". The first bit, 0x01, corresponds to pitch, 0x02 to roll, and 0x04 to yaw, and when set the respective axis is in limited mode. The type byte is not used currently and has to be set to zero. As response to this command a CMD_ACK message will be emitted.

CMD_SETPITCHROLLYAW (#18)

0xFA 0x06 0x12 data1-low-byte data1-high-byte data2-low-byte data2-high-byte data3-low-byte data3-high-byte crc-low-byte crc-high-byte

The data1, data2 and data3 are each of type uint16_t and can assume the values 700...2300. They represent the pitch, roll, yaw input values. If a value 0 is send, then the respective axis will be recentered. Any other values are ignored. As response to this command a CMD_ACK message will be emitted.

CMD_SETPWMOUT (#19)

0xFA 0x02 0x13 data-low-byte data-high-byte crc-low-byte crc-high-byte

The data is of type uint16_t and can assume the values 700...2300. It represents the pwm pass through input value. As response to this command a CMD_ACK message will be emitted.

CMD_RESTOREPARAMETER (#20)

0xFA 0x02 0x14 data-low-byte data-high-byte crc-low-byte crc-high-byte

The data is of type uint16_t and holds the parameter number. This command sets the parameter to the value in the EEPROM. As response to this command a CMD_ACK message will be emitted.

CMD_RESTOREALLPARAMETER (#21)

0xFA 0x00 0x15 crc-low-byte crc-high-byte

This command sets all parameters to the values stored in the EEPROM. As response to this command a CMD_ACK message will be emitted.

CMD_ACTIVEPANMODESETTING (#100)

0xFA 0x01 0x64 data-byte crc-low-byte crc-high-byte

The data is of type uint8_t and is a bit field related to the pan mode setting: default setting = 0x00, setting #1 = 0x01, setting #2 = 0x02, setting #3 = 0x03. As response to this command a CMD_ACK message will be emitted.

CMD_ACK (#150)

0xFB 0x01 0x96 data-byte crc-low-byte crc-high-byte

This command is send by the STorM32 controller to acknowledge execution of a received RC command message (if the message itself doesn't lead to a response, such as e.g. the get parameter command). The data is of type uint8_t and can assume these values:

0 = SERIALRCCMD_ACK_OK
1 = SERIALRCCMD_ACK_ERR_FAIL
2 = SERIALRCCMD_ACK_ERR_ACCESS_DENIED
3 = SERIALRCCMD_ACK_ERR_NOT_SUPPORTED
150 = SERIALRCCMD_ACK_ERR_TIMEOUT
151 = SERIALRCCMD_ACK_ERR_CRC 
152 = SERIALRCCMD_ACK_ERR_PAYLOADLEN

Mavlink Communication

In addition to the commands described in the previous sections, the o323bgc firmware also understands "real" Mavlink messages, as defined in the Mavlink standard. Some STorM32 specific Mavlink messages were also added. The implemented Mavlink messages are described in the mavlink_storm32.xml file. For some messages the parameter values were extended to support all STorM32 features, but such as to maintain compatibility.

The STorM32's system ID and component ID can be freely adjusted in the GUI; the defaults are

• System ID: 'G' (= 71 = 0x47)
• Component ID: 'C' (= 67 = 0x43)

The following commands are supported:

MAVLINK_MSG_ID_HEARTBEAT (#0)

See HEARTBEAT. The emission of the heartbeat needs to be activated in the GUI via the Mav Configuration parameter. The heartbeat is emitted at 1 Hz, with the following values:

• type: MAV_TYPE_GIMBAL (= 26)
• autopilot: 'S' (= 83 = 0x53) (may become MAV_AUTOPILOT_STORM32)
• base_mode: either 0 or MAV_MODE_FLAG_SAFETY_ARMED (= 0x80)
• custom_mode: STATE value of the o323firmware
• system_status: either MAV_STATE_BOOT (= 1) or MAV_STATE_ACTIVE (= 4)

MAVLINK_MSG_ID_PARAM_REQUEST_READ (#20)

See PARAM_REQUEST_READ.

MAVLINK_MSG_ID_PARAM_REQUEST_LIST (#21)

See PARAM_REQUEST_LIST.

MAVLINK_MSG_ID_PARAM_VALUE (#22)

See PARAM_VALUE.

MAVLINK_MSG_ID_PARAM_SET (#23)

See PARAM_SET.

MAVLINK_MSG_ID_ATTITUDE (#30)

See ATTITUDE. The emission of the attitude message needs to be activated in the GUI via the Mav Configuration parameter. It is emitted with 2 Hz.

MAVLINK_MSG_ID_COMMAND_LONG (#76)

See COMMAND_LONG. The value ranges of some parameters of the MAV_CMD_DO_SET_PARAMETER, MAV_CMD_DO_DIGICAM_CONTROL and MAV_CMD_DO_MOUNT_CONTROL were extended, to embrace the STorM32's capabilities. These commands are supported (parameter fields not mentioned are ignored):

MAV_CMD_DO_SET_PARAMETER (#180)
* param1 = parameter index
* param2 = parameter value
* param7 = if it is the magic number 83 then the indexed parameter is restored from the EEPROM

MAV_CMD_DO_SET_SERVO (#183)
* param2 = pwm value (only 700...2300 accepted)

MAV_CMD_DO_DIGICAM_CONTROL (#203)
* param5 = shot (0 = off, 1 = IRSHUTTER, 2 = IRSHUTTERDELAYED, 3 = IRVIDEOON, 4 = IRVIDEOOFF)

MAV_CMD_DO_MOUNT_CONFIGURE (#204)
* param1 = mount_mode (1 = MAV_MOUNT_MODE_NEUTRAL recenters camera)

MAV_CMD_DO_MOUNT_CONTROL (#205)
* param1 = pitch, angle in degree or pwm input
* param2 = roll, angle in degree or pwm input
* param3 = yaw, angle in degree or pwm input
* param6 = typeflags (uint16_t)
     0x0001: pitch, 0: unlimited, 1: limited, see CMD_SETANGLE
     0x0002: roll, 0: unlimited, 1: limited, see CMD_SETANGLE
     0x0004: yaw, 0: unlimited, 1: limited, see CMD_SETANGLE
     0x0100: input type, 0: angle input (see CMD_SETANGLE), 1: pwm input (see CMD_SETPITCHROLLYAW)

MAV_CMD_PREFLIGHT_STORAGE (#245)
* param1 = parameter storage
     0: restore parameter values from EEPROM
     1: store parameter values in EEPROM

MAVLINK_MSG_ID_COMMAND_ACK (#77)

See COMMAND_ACK.

MAVLINK_MSG_ID_DIGICAM_CONTROL (#155)

ArduPilot specific message. See DIGICAM_CONTROL. The value range of the shot byte is extended, to embrace the STorM32's camera capabilities.

* target_system: System ID (uint8_t)
* target_component: Component ID (uint8_t)
* shot: 0 = off, 1 = IRSHUTTER, 2 = IRSHUTTERDELAYED, 3 = IRVIDEOON, 4 = IRVIDEOOFF (uint8_t)

All other fields of the message are ignored.

MAVLINK_MSG_ID_MOUNT_CONFIGURE (#156)

Ardupilot specific message. See MOUNT_CONFIGURE.

* target_system: System ID (uint8_t)
* target_component: Component ID (uint8_t)
* mount_mode: MAV_MOUNT_MODE_NEUTRAL recenters camera (uint8_t)

All other fields of the message are ignored.

MAVLINK_MSG_ID_MOUNT_CONTROL (#157)

ArduPilot specific message. See MOUNT_CONTROL. The possible values of the save_position byte is extended to accept more flags, to embrace both the angle and pwm input capabilities of the STorM32.

* target_system: System ID (uint8_t)
* target_component: Component ID (uint8_t)
* input_a: pitch, angle in 0.01° or pwm input (int32_t)
* input_b: roll, angle in 0.01° or pwm input (int32_t)
* input_c: yaw, angle in 0.01° or pwm input (int32_t)
* save_position: (uint8_t) 
   0x01: standard not-save/save position
   0x02: pitch, 0: unlimited, 1: limited, see CMD_SETANGLE
   0x04: roll, 0: unlimited, 1: limited, see CMD_SETANGLE
   0x08: yaw, 0: unlimited, 1: limited, see CMD_SETANGLE
   0x10: input type, 0: angle input (see CMD_SETANGLE), 1: pwm input (see CMD_SETPITCHROLLYAW)

MAVLINK_MSG_ID_COMMAND_TARGET_SPECIFIC (#234)

STorM32 specific message. Container for target specific messages. See Comments to STorM32 specific Mavlink commands below.

* target_system: System ID (uint8_t)
* target_component: Component ID (uint8_t)
* specific_payload: Target specific message as payload, the size can be variable, as needed

MAVLINK_MSG_ID_COMMAND_TARGET_SPECIFIC_ACK (#235)

STorM32 specific message. Container for a target specific responses. May be emitted by the target as response to a COMMAND_TARGET_SPECIFIC message. See Comments to STorM32 specific Mavlink commands below.

* target_system: System ID (uint8_t)
* target_component: Component ID (uint8_t)
* specific_payload: Target specific response as payload, the size can be variable, as needed

MAVLINK_MSG_ID_PARAM_READ2 (#236)

STorM32 specific message. Requests reading the parameter with index param_index. The target should respond with a PARAM_VALUE2 message. See Comments to STorM32 specific Mavlink commands below.

* target_system: System ID (uint8_t)
* target_component: Component ID (uint8_t)
* param_index: Parameter index (uint16_t)

MAVLINK_MSG_ID_PARAM_SET2 (#237)

STorM32 specific message. Sets the parameter with index param_index to the value param_value. The target should respond with a PARAM_VALUE2 message to acknowledge. See Comments to STorM32 specific Mavlink commands below.

* target_system: System ID (uint8_t)
* target_component: Component ID (uint8_t)
* param_index: Parameter index (uint16_t)
* param_value: Parameter value (type depends on parameter)
* param_type: parameter type, see the MAV_PARAM_TYPE enum for supported data types (uint8_t)

MAVLINK_MSG_ID_PARAM_VALUE2 (#238)

STorM32 specific message. Sends the value of the parameter with index param_index. See Comments to STorM32 specific Mavlink commands below.

* target_system: System ID (uint8_t)
* target_component: Component ID (uint8_t)
* param_index: Parameter index (uint16_t)
* param_value: Parameter value (type depends on parameter)
* param_type: Parameter type, see the MAV_PARAM_TYPE enum for supported data types (uint8_t)
* param_count: Total number of parameters (uint16_t)

MAVLINK_MSG_ID_PARAM_REQUEST_LIST2 (#239)

STorM32 specific message. Reserved.

MAVLINK_MSG_ID_PARAM_FIELD2 (#240)

STorM32 specific message. Reserved.

Comments on STorM32 specific Mavlink Messages

PARAM_READ2, PARAM_SET2, PARAM_VALUE2

The set of commands PARAM_READ2, PARAM_SET2, and PARAM_VALUE2 are alternatives to the analogous original commands, using only the parameter index to identify a parameter. This minimizes bandwidth and required processing power.

Comment: These commands will be complemented by two commands PARAM_REQUEST_LIST2 and PARAM_FIELD2, which also are alternatives to the analogous original command, but will provide more detailed info on the parameters such as default value, min and max values, formating info and so on. This will allow to program much more usable GUIs.

COMMAND_TARGET_SPECIFIC, COMMAND_TARGET_SPECIFIC_ACK

A major addition are the commands COMMAND_TARGET_SPECIFIC and COMMAND_TARGET_SPECIFIC_ACK. They are in some way similar to the pair of messages COMMAND_LONG and COMMAND_ACK, but do not impose format restrictions on the payload, and allow also to send information back to the sender. This optimizes not only bandwidth but also establishes a very flexible method to send information, in both directions if needed. These commands could also be viewed as containers for data, those content isn't known by Mavlink. It becomes thus possible to establish a communication channel between two arbitrary devices, using a Mavlink channel.

These commands are intended for peer-to-peer communication. That is the System and Component IDs of the ACK message are the Target System and Target Component IDs of the sending message, and vice versa.

These two commands are however not fully conform with the Mavlink standard, which currently doesn't support variable payloads. Hence, all routines provided by Mavlink which rely on the hardcoded length associated to a message cannot be used. Also, any router, which handles the distribution of the Mavlink messages, should not rely on the hardcoded lengths (at least not for the two discussed commands). These points are however quite easy to realize in practice.

In the o323bgc firmware, these commands allow to embed both the simple serial and the RC commands. That is, for instance, the command CMD_SETANGLE with value 1000 (= 0x03E8) can be transmitted in two ways:

• It can be transmitted as described in the above, with the 0xFA start sign. The sent byte stream is then:

0xFA 0x02 0x11 0xE8 0x03 CRCLOW CRCHIGH

and the received ACK stream

0xFB 0x01 0x96 0x00 0x52 0xE9

• The message can be embedded into the COMMAND_TARGET_SPECIFIC command, i.e. the message id and payload becomes the specific payload. The byte stream is then:

0xFE 0x05 0xXX 0xYY 0xZZ 0xEA 0x47 0x43 0xFA 0x11 0xE8 0x03 CRCLOW CRCHIGH

and the received ACK stream

0xFE 0x04 0xYY 0x47 0x43 0xEB 0xYY 0xZZ 0x96 0x00 CRCLOW CRCHIGH

This of course works for all serial commands, and their responses. Thus, the full command set and functionality is available through Mavlink channel.