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94c09b2 Apr 21, 2016
@peplin @mgiannikouris @emarsman
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OpenXC JSON Message Format

Each JSON message published by a VI is delimited with a \0 character.

Table of Contents

  1. Vehicle Messages
  2. CAN Message
  3. Diagnostic Message
  4. Commands
  5. Extra Values

Vehicle Messages

Simple Vehicle Message

There may not be a 1:1 relationship between input and output signals - i.e. engine timing CAN signals may be summarized in an "engine performance" metric on the abstract side of the interface.

The expected format of a single valued message is:

{"name": "steering_wheel_angle", "value": 45}

Evented Simple Vehicle Message

The expected format of an event message is:

{"name": "button_event", "value": "up", "event": "pressed"}

This format is good for something like a button event, where there are two discrete pieces of information in the measurement.

CAN Message

The format for a plain CAN message:

{"bus": 1, "id": 1234, "data": "0x12345678"}

bus - the numerical identifier of the CAN bus where this message originated, most likely 1 or 2 (for a vehicle interface with 2 CAN controllers).

id - the CAN message ID

data - up to 8 bytes of data from the CAN message's payload, represented as a hexidecimal number in a string. Many JSON parser cannot handle 64-bit integers, which is why we are not using a numerical data type. Each byte in the string must be represented with 2 characters, e.g. 0x1 is 0x01 - the complete string must have an even number of characters. The 0x prefix is optional.

format - (optional) explicitly set the frame format for the CAN message, one of standard or extended. If the id is greater than 0x7ff, the extended frame format will be selected automatically.

Diagnostic Message

Requests

A diagnostic request is added or cancelled with a JSON object like this example:

{ "command": "diagnostic_request",
  "action": "add",
  "diagnostic_request": {
      "bus": 1,
      "message_id": 1234,
      "mode": 1,
      "pid": 5,
      "payload": "0x1234",
      "multiple_responses": false,
      "frequency": 1,
      "name": "my_pid"
    }
  }
}
  • The command must be diagnostic_request.
  • The action must be included, and must be one of:
    • add - create a new one-off or recurring diagnostic request.
    • cancel - cancel an existing request.
  • The details of the request must be included in the request field, using the sub-fields defined below.

A diagnostic request's bus, id, mode and pid (or lack of a pid) combine to create a unique key to identify a request. These four fields will be referred to as the key of the diagnostic request. For example, to create a simple one-time diagnostic request:

{ "command": "diagnostic_request",
  "action": "add",
  "diagnostic_request": {
      "bus": 1,
      "message_id": 1234,
      "mode": 1,
      "pid": 5
    }
  }
}

Requests are completed after any responses are received (unless multiple_responses is set), or the request has timed out after a certain number of seconds. After a request is completed, you can re-create the same key to make another request.

Requests with a frequency are added as recurring requests, e.g. to add the previous example as a recurring request at 1Hz:

{ "command": "diagnostic_request",
  "action": "add",
  "diagnostic_request": {
      "bus": 1,
      "message_id": 1234,
      "mode": 1,
      "pid": 5,
      "frequency": 1
    }
  }
}

To cancel a recurring request, send a cancel action with the same key, e.g.:

{ "command": "diagnostic_request",
  "action": "cancel",
  "diagnostic_request": {
      "bus": 1,
      "message_id": 1234,
      "mode": 1,
      "pid": 5
    }
  }
}

Simultaneous recurring requests for the same key at different rates (e.g. 1Hz and 2Hz) is not supported. However, non-recurring ("one-off") requests may exist in parallel with a recurring request for the same key.

bus - the numerical identifier of the CAN bus where this request should be sent, most likely 1 or 2 (for a vehicle interface with 2 CAN controllers).

message_id - the CAN message ID for the request.

mode - the OBD-II mode of the request - 0x1 through 0xff (1 through 9 are the standardized modes and 0x22 is a common proprietary mode).

pid - (optional) the PID for the request, if applicable.

payload - (optional) up to 7 bytes of data for the request's payload represented as a hexadecimal number in a string. Many JSON parser cannot handle 64-bit integers, which is why we are not using a numerical data type. Each byte in the string must be represented with 2 characters, e.g. 0x1 is 0x01 - the complete string must have an even number of characters. The 0x prefix is optional.

name - (optional, defaults to nothing) A human readable, string name for this request. If provided, the response will have a name field (much like a simple vehicle message) with this value in place of bus, id, mode and pid.

multiple_responses - (optional, false by default) if true, request will stay active for a full 100ms, even after receiving a diagnostic response message. This is useful for requests to the functional broadcast message ID (0x7df) when you need to get responses from multiple modules. It's possible to set this to true for non-broadcast requests, but in practice you won't see any additional responses after the first and it will just take up memory in the VI for longer.

frequency - (optional) Make this request a recurring request, at a this frequency in Hz. To send a single non-recurring request, leave this field out.

decoded_type - (optional, defaults to "obd2" if the request is a recognized OBD-II mode 1 request, otherwise "none") If specified, the valid values are "none" and "obd2". If obd2, the payload will be decoded according to the OBD-II specification and returned in the value field. Set this to none to manually override the OBD-II decoding feature for a known PID.

Responses

Requests to add or cancel a diagnostic request are first acknowledged by the VI, before any responses to the request are returned. The response uses the standard command response format:

{ "command_response": "diagnostic_request", "status": true}

status - true if the request was successfully created or cancelled.

When a node on the network response to the request and the result is published by the VI, the result looks like:

{"bus": 1,
  "message_id": 1234,
  "mode": 1,
  "pid": 5,
  "success": true,
  "payload": "0x1234",
  "value": 4660}

and to an unsuccessful request, with the negative_response_code and no pid echo:

{"bus": 1,
  "message_id": 1234,
  "mode": 1,
  "success": false,
  "negative_response_code": 17}

bus - the numerical identifier of the CAN bus where this response was received.

message_id - the CAN message ID for this response.

mode - the OBD-II mode of the original diagnostic request.

pid - (optional) the PID for the request, if applicable.

success - true if the response received was a positive response. If this field is false, the remote node returned an error and the negative_response_code field should be populated.

negative_response_code - (optional) If requested node returned an error, success will be false and this field will contain the negative response code (NRC).

Finally, the payload and value fields are mutually exclusive:

payload - (optional) up to 7 bytes of data returned in the response, represented as a hexadecimal number in a string. Many JSON parser cannot handle 64-bit integers, which is why we are not using a numerical data type.

value - (optional) if the response had a payload, this may be the payload interpreted as an integer.

The response to a simple PID request would look like this:

{"success": true, "bus": 1, "message_id": 1234, "mode": 1, "pid": 5, "payload": "0x2"}

Commands

In addition to the diagnostic_request command described earlier, there are other possible values for the command field.

All commands immediately return a command_response, e.g.:

{ "command_response": "version", "message": "v6.0-dev (default)", "status": true}

command_response - an echo of the command this is a ACKing.

status - true if the command was understood and performed succesfully.

message - (optional) a string message from the VI, e.g. to return a version descriptor or error message.

Version Query

The version command triggers the VI to inject a firmware version identifier response into the outgoing data stream.

Request

{ "command": "version"}

Response

{ "command_response": "version", "message": "v6.0-dev (default)", "status": true}

Device ID Query

The device_id command triggers the VI to inject a unique device ID (e.g. the MAC address of an included Bluetooth module) into into the outgoing data stream.

If no device ID is available, the response message will be "Unknown".

Request

{ "command": "device_id"}

Response

{ "command_response": "device_id", "message": "0012345678", "status": true}

Passthrough CAN Mode

The passthrough command controls whether low-level CAN messages are passed through from the CAN bus through the VI to the output stream. If the CAN acceptance filter is in bypass mode and passthrough is enabled, the output stream will include all received CAN messages. If the bypass filter is enabled, only those CAN messages that have been pre-defined in the firmware are forwarded.

Request

{ "command": "passthrough",
  "bus": 1,
  "enabled": true
}

Response

If the bus in the request was valid and the passthrough mode was changed, the status field in the response will be true. If false, the passthrough mode was not changed.

{ "command_response": "passthrough", "status": true}

Acceptance Filter Bypass

The af_bypass command controls whether the CAN message acceptance filter is bypassed for each CAN controller. By default, hardware acceptance filter (AF) is enabled in the VI - only previously defined CAN message IDs will be received. Send this command with bypass: true to force the filters to bypassed.

If passthrough mode is also enabled, when the AF is bypassed, the output will include all CAN messages received.

Request

{ "command": "af_bypass",
  "bus": 1,
  "bypass": true
}

Response

If the bus in the request was valid and the AF mode was changed, the status field in the response will be true. If false, the passthrough mode was not changed.

{ "command_response": "af_bypass", "status": true}

Payload Format Control

The payload_format command determines the format for output data from the VI and the expected format of commands sent to the VI.

Valid formats are json and protobuf.

Request

{ "command": "payload_format",
  "format": "json"
}

Response

If the format was changed successfully, the status in the response will be true. The response will be in the original message format, and all subsequent messages will be in the new format.

{ "command_response": "payload_format", "status": true}

Automatic Pre-Defined OBD-II PID Requests

The predefined_obd2 command enables and disables the querying for and translating of a set of pre-defined OBD-II PIDs from the attached vehicle. When enabled, the VI will query the vehicle to see if these PIDs are claimed to be supported and for those that are, it will set up recurring requests. The responses will be output as simple vehicle messages, with the names defined in the "Signals Defined from Diagnostic Messages" section below.

Request

{ "command": "predefined_obd2",
  "enabled": true
}

Response

If the predefined requests were enabled or disabled successfully, the status in the response will be true.

{ "command_response": "predefined_obd2", "status": true}

C5 Cellular Configuration

The ModemConfigurationCommand message allows users to change certain aspects of modem operation on-the-fly (at runtime). The modem configuration settings are stored in flash memory and are untouched by the bootloader during a software update (assuming the correct cellular_c5 linker file is used during compilation of vi-firmware). Thus, new modem settings persistent across power cycles.

The ModemConfigurationCommand message provides three sub-messages for particular groups of modem settings. These are NetworkOperatorSettings, NetworkDataSettings, and ServerConnectSettings. These configuration messages are described in great detail within the c5_cellular_config documentation.

Currently, only the ServerConnectSettings sub-message is supported in the vi-firmware's command interpreter. All other settings are currently compile-time only.

The ServerConnectSettings part of ModemConfigurationCommand allows the user to set the host server name and port that the device will use when opening a TCP socket to upload data. This destination must be running an HTTP server similar to OpenXCWebServer, which defines a set of supported HTTP transactions where the body is comprised of data in the familiar OpenXC Message Format.

Request

{ "command": "modem_configuration",
  "server": {
    "host": "www.myhost.com",
    "port": 10000
  }
}

Response

{ "command_response": "modem_configuration", "status": true}

C5 SD Card Status

In order to check the status of the SD card, the following command is available:

{ "command": "sd_mount_status"}

Command response if the SD card is mounted correctly:

{ "command_response": "sd_mount_status", "status": true}

If the SD card is full, not enabled, or connected as a MSD, the device will respond with:

{ "command_response": "sd_mount_status", "status": false}

For more info see c5_msd.

C5 RTC Configuration

To set the current time of the RTC, the following

{ "command": "rtc_configuration", "unix_time": "1448551563"}

The response is

{ "command_response": "rtc_configuration", "status": true}

For more info see c5_rtc.

Extra Values

Any of the following JSON objects may optionally include an extras field. The value may be any valid JSON object or array. The client libraries will do their best to parse this information into a generic format and pass it to your application. For example:

{"name": "steering_wheel_angle",
    "value": 45,
    "extras": {
        "calibrated": false
    }
}