Binary Output Format¶
For applications that need to maximize the amount of data transferred from the vehicle, the firmware includes an experimental binary output format. Instead of JSON, it uses Google Protocol Buffers to more efficiently pack the data. Translated-style OpenXC messages are on average 30% smaller when using protobufs instead of JSON, and raw messages are around 60% smaller. This space savings comes at the cost of decreased flexibility and increased complexity in receiving and parsing the data.
The firmware does not currently support receiving binary-encoded messages - CAN write requests must still be sent in JSON.
This output format is supported by the official OpenXC Android library and OpenXC Python library, both of which will auto-detect the output format being used by an attached VI. The protobuf objects are defined in an experimental branch in the openxc-message-format repository if you wish to add support to another language or environment.
The output stream uses the common delimiting technique of writing the length of each protobuf message before the message itself in the stream.
Compiling with Binary Output¶
To use the binary output format, compile with the BINARY_OUTPUT flag (see all compile-time flags).
The default output format encodes data from the vehicle as JSON, using the OpenXC message format. There are JSON formats for both translated and raw messages. The format is human-readable and very simple to parse, but it’s not very compact.
For one, it representes everything in human-readable text, so for example, the number 999 (which could be stored in only 10 bits) takes 3 bytes - more than twice the space.
This isn’t an issue for most users, as the output interfaces have plenty of bandwidth (USB is around 125-160 KB/s and the popular RN-42 Bluetooth tops out at 23KB/s) and the data rate of most release VI firwmares is on the order of only 3-6KB/s using JSON.
However, some applications need to pull a lot more data out of the car, perhaps by reading raw CAN messages or reading many signals at high frequencies. These can quickly overwhelm the output pipe using JSON.