Most users do not need to know the details of the device driver, but for reference it is documented here.
The CAN translator initializes its USB 2.0 controller as a USB device with three endpoints. The Android tablet or computer you connect to the translator acts as the USB host, and must initiate all transfers.
This is the standard USB control transfer endpoint. The CAN transalator has a few control commands:
Version control command: 0x80
The host can retrieve the version of the CAN translator using the 0x80 control request. The data returned is a string containing the software version of the firmware and the configured vehicle platform in the format:
Version: 1.0 (c346)
where 1.0 is the software version and c346 is the configured vehicle.
Reset control command: 0x81
The CAN transceivers can be re-initialized by sending the 0x81 control request. This command was introduced to work around a bug that caused the CAN translator to periodically stop responding. The bug still exists, but there are now workarounds in the code to automatically re-initialize the transceivers if they stop receiving messages.
Endpoint 1 is configured as a bulk transfer endpoint with the IN direction (device to host). OpenXC JSON messages read from the vehicle are sent to the host via IN transactions. When the host is ready to receive, it should issue a request to read data from this endpoint. A larger sized request will allow more messages to be batched together into one USB request and give high overall throughput (with the downside of introducing delay depending on the size of the request).
OpenXC JSON messages created by the host to send to the vehicle (i.e. to write to the CAN bus) are sent via OUT transactions. The CAN translator is prepared to accept writes from the host as soon as it initializes USB, so they can be sent at any time. The messages must be separated by a NULL character.
There is no special demarcation on these messages to indicate they are writes - the fact that they are written in the OUT direction is sufficient. Write messages must be no more than 4 USB packets in size, i.e. 4 * 64 = 256 bytes.
In the same way the CAN translator is pre-configured with a list of CAN signals to read and parse from the CAN bus, it is configured with a whitelist of messages and signals for which to accept writes from the host. If a message is sent with an unlisted ID it is silently ignored.