The ZMQ framework (actually ØMQ) is used to interconnect applications or parts of applications to other applications via a range of interfaces. It's really nice since there's lots of language support and it's easy to use.

The application uses 4 ports for communicating:

• 2000: Main command/response socket (server/client)
• 2001: Logging data and events (publish)
• 2002: Param events and values(publish)
• 2003: Connection events (publish)
• 2004: Control data (pull)

All communication is done using JSON. To test the implementation we have a test client that could be useful to have a look at. Each message sent contains a version field that should always be included.

The client is run using the command line:

$ bin/cfzmq -h
usage: cfzmq [-h] [-u URL] [-d]

optional arguments:
  -h, --help         show this help message and exit
  -u URL, --url URL  URL where ZMQ will accept connections
  -d, --debug        Enable debug output

tcp://127.0.0.1

$bin/cfqmq --url "tcp://*"

The command messages are implemented as server/client, where each request to the server is answered with a response. Each message to the server contains version, command and fields related to the command, Each response from the server will contain version and status, where status 0 means everything was ok. This makes all the calls on this port synchronous, where the server will not reply until the action is completed or it fails.

{
  "version": 1,
  "cmd": "command",
  "arg1": "some argument",
  "arg2": "some other argument"
}

{
  "version": 1,
  "status": 0
}

{
  "version": 1,
  "status": 1,
  "msg": "Something went wrong..."
}

The scan command will trigger a scanning of all of the available interfaces on the server (USB and Crazyradio) and return all the Crazyflies found. If no interfaces are available (no Crazyradio or Crazyflie) the command will return an empty list. Therefore there's no error conditions for this command, status will always be 0.

{
  "version": 1,
  "cmd": "scan"
}

{
  "version": 1,
  "status": 0,
  "interfaces":
    [
      {
        "uri": "radio://0/100/250K",
        "info": "This is a Crazyflie"
      },
      {
        "uri": "debug://0/0",
        "info": "Normal connection"
      }
    ]
}

The connect command will connect to the supplied URI, download the logging TOC and parameter TOC/values and return everything. There's a timeout on the server-side that will be hit if the server can't connect to a Crazyflie on the supplied URI (of if there's some other error).

The log TOC will be found in the log dictionary, where the first level is group, the second level is name and the third is the attributes (see below). So the type of altHold.target will be found in log→altHold→target→type.

The param TOC will be found in the param dictionary, where the first level is group, the second level is name and the third is the attributes (see blow). So the RO/RW attribute for altHold.aslAlpha will be found in param→altHold→aslAlpha→access.

{
  "version": 1,
  "cmd": "connect",
  "uri": "radio://0/10/250K"
}

{
  "version": 1,
  "status": 0,
  "log": {
    "acc": {
      "mag2": {"type": "float"}, 
      "x": {"type": "float"},
      "y": {"type": "float"}, 
      "z": {"type": "float"}, 
      "zw": {"type": "float"}
    }, 
    "altHold": {
      "err": {"type": "float"}, 
      "target": {"type": "float"}, 
      "vSpeed": {"type": "float"}, 
      "vSpeedASL": {"type": "float"}, 
      "vSpeedAcc": {"type": "float"}, 
      "zSpeed": {"type": "float"}
    }, 
    "baro": {
      "asl": {"type": "float"}, 
      "aslLong": {"type": "float"}, 
      "aslRaw": {"type": "float"}, 
      "pressure": {"type": "float"}, 
      "temp": {"type": "float"}
    }, 
    "gyro": {
      "x": {"type": "float"}, 
      "y": {"type": "float"}, 
      "z": {"type": "float"}
    }, 
    "mag": {
      "x": {"type": "float"}, 
      "y": {"type": "float"}, 
      "z": {"type": "float"}
    }, 
    "mag_raw": {
      "x": {"type": "int16_t"}, 
      "y": {"type": "int16_t"}, 
      "z": {"type": "int16_t"}
    }, 
    "motor": {
      "m1": {"type": "int32_t"}, 
      "m2": {"type": "int32_t"}, 
      "m3": {"type": "int32_t"}, 
      "m4": {"type": "int32_t"}
    }
}, 
"param": {
  "altHold": {
    "altHoldChangeSens": {
      "access": "RW", 
      "type": "float", 
      "value": "200.0"
    }, 
    "altHoldErrMax": {
      "access": "RW", 
      "type": "float", 
      "value": "1.0"
    }, 
    "aslAlpha": {
      "access": "RW", 
      "type": "float", 
      "value": "0.920000016689"
    }
  }
}

{
  "version": 1,
  "status": 1,
  "msg": "Too many packages lost"
}

Logging data from the Crazyflie is done by setting up log configurations that will push log data at a specified interval (more info here). There are four command associated with log configurations: create, start, stop and delete. Create and delete handles if the log configuration is stored in the Crazyflie memory or not. Start and stop handles if the log data is actually being sent or not from the Crazyflie to the host. Before a log config can be started is has to be created, before it can be stopped it has to be started and before it can be deleted is has to be created. Note that log block are automatically started once they have been created.

Note: When a host connects to a Crazyflie the log configurations are all deleted. So if you connect, set up log configurations, disconnect and then connect again the configurations will be deleted.

Below is an example for creating a logging configuration and starting it. The configuration contains the two variables pm.vbat and stabilizer.roll that will be sent at 1 Hz. Data will be published to the log data socket.

Each action for log configurations (create, start, stop, delete) will be broadcasted on the log data socket. Log data will also be boardcasted on the same socket.

{
  "version": 1,
  "cmd": "log",
  "action": "create",
  "name": "Test log block",
  "period": 1000,
  "variables": [
      "pm.vbat",
      "stabilizer.roll"
  ]
}

{
  "version": 1,
  "status": 0
}

{
  "version": 1,
  "cmd": "log",
  "action": "start",
  "name": "Test log block"
}

{
  "version": 1,
  "status": 0
}

During run-time it's possible to set parameters that are mapped directly to variables in the firmware (more info here). Each parameter update is also published on the param socket.

Below is an example command to set the flightctrl.xmode parameter.

{
    "version": 1,
    "cmd": "param",
    "name": "flightctrl.xmode",
    "value": True
}

{
    "version": 1,
    "status": 0,
    "name": "flightctrl.xmode", 
    "value": "1" 
}

{
    "version": 1,
    "status": 1,
    "msg": "Could not find flightctrl.xmode in TOC" 
}

This socket is used for sending log configuration events as well as log data. The events that are sent is for creating, starting, stopping and deleting a configuration. For every started configuration the log data will be sent over this socket. To control this see the log configuration above.

Each message contains an event field (see below) and a name field referring to the log configuration name.

{
  "version": 1,
  "name": "Test log block",
  "event": "started"
}

{
  "version": 1,
  "name": "Test log block",
  "event": "data",
  "timestamp": 1004,
  "variables":
    {
      "pm.vbat": 3.5,
      "stabilizer.roll": -80.0
    }
}

This socket is used to broadcast parameter updates done on the command socket.

{
    "version": 1,
    "name": "flightctrl.xmode", 
    "value": "1" 
}

This socket is used to broadcast changes in the connection state as events. Connecting the Crazyflie is a synchronous call to the command socket, but for instance a lost connection will be asynchronous and boardcasted on this socket.

Each event has a name and uri, there might also be an optional message. Note that disconnected is always sent no matter the reason. So a requested disconnect will send a disconnected event, and a lost connection will send a lost event as well as a disconnected event.

{
    "version": 1,
    "event": "failed", 
    "uri": "radio://0/10/250K",
    "msg": "Too many packets lost!" 
}

Control commands can be sent at any time after the Crazyflie has been connected and has the following scaling/format:

{
  "version": 1,
  "roll": 0.0,
  "pitch": 0.0,
  "yaw": 0.0,
  "thrust": 0.0
}