# OpenXC JSON Message Format
Each JSON message published by a VI is delimited with a `\0 ` character.
## Table of Contents
1. [Vehicle Messages](#vehicle-messages)
2. [CAN Message](#can-message)
3. [Diagnostic Message](#diagnostic-message)
4. [Commands](#commands)
5. [Extra Values](#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