RPi Communication Protocol

This page describes the communication protocol used by the Raspberry Pi 4 servers. This is the protocol used by RPiDataClient.

All communication is done via JSON encoded strings. All JSON strings have a common header, indicated by the msg_type keyword. The msg_type keyword can have the values

• command, indicating a command

• response, indicating a response to a command

• telemetry, indicating telemetry sent by the Raspberry Pi 4’s

This keyword is needed to be able to distinguish between responses and telemetry in a fail safe manner.

Commands

Apart from the msg_type keyword with the command value, all commands have the following keywords:

• cmd_name: the name of the command

• cmd_id: a unique, increasing long value determined by the CSC. This is the UNIX timestamp in milliseconds when the command was sent

• parameters: a set of name/value pairs where each name represents the name of a parameter and each value represents the value of the parameter

Note: only the configure command sends a non-emtpy parameter set.

Examples

The start command looks as follows:

{
  "msg_type": "command",
  "cmd_name": "start",
  "cmd_id": 1,
  "parameters": {}
}

The configure command for, for instance, two sensors looks as follows:

{
  "msg_type": "command",
  "cmd_name": "configure",
  "cmd_id": 23,
  "parameters": {
      "devices": [
          {
              "name": "Test01",
              "num_channels": 4,
              "dev_type": "FTDI",
              "dev_id": "ABC",
              "sens_type": "Temperature"
          },
          {
              "name": "Test02",
              "num_channels": 2,
              "dev_type": "Serial",
              "dev_id": "port_1",
              "sens_type": "Wind"
          }
      ]
  }
}

Responses

Apart from the msg_type keyword with the response value, all responses have the following keywords:

• cmd_id: the unique, increasing id of the command that the response is sent for

• cmd_status: can be

  • ack: in case the received command has been accepted It is assumed that ack only gets sent if and when the command has been executed successfully since for this CSC no long running commands have been implemented.

  • command_failed: the received command syntatically was correct but the execution failed

  • unknown_command: in case the received command in unknown

  • bad_parameter: in case an unknow parameter was received, or a parameter is missing, or there are too many parameters

Examples

An ack command response looks as follows:

{
  "msg_type": "response",
  "cmd_id": 1,
  "cmd_status": "ack"
}

Telemetry

Apart from the msg_type keyword with the telemetry value, all telemetry messages have the following keywords:

• telemetry: a sensor specific string representing the telemetry.

Examples

A telemetry message looks as follows:

{
  "msg_type": "telemetry",
  "telemetry": "['Test01', 1624900703.949579, 0, 24.0131, 18.5856, 19.5273, 21.4308]"
}

which, in this case, means:

• The temperature sensor name (see the configuration example above)

• The UNIX timestamp of the measurement

• The temperatures were measured OK

• The four measured temperatures