The Controller Area Network bus is a broadcast real-time bus which is widely used in industrial or automotive applications. It has the following characteristics:

• its physical layer used a differential signal pair, which make the bus more robust against electromagnetic noises ;
• the protocol includes by default a CRC on each message to ensure integrity.
• the bus is a broadcast medium to allow efficient communication between multiple nodes (e.g. implementing producer/consumer scheme);
• it works upto 1Mbps bitrate,
• each message on the bus has a strict priority;
• each message is at most 8-bytes long to ensure real-time reactiveness on the bus

The QuimBox uses the CAN protocol internally to allow communication between the embedded Linux and the controller boards. It also offers an external CAN connection to the bus to extend the system with CAN-compliant modules.

On top of the physical CAN bus, devices and applications have to agree on a high-level protocol to exchange data. A popular choice is CANopen which defines a standard way to access CAN nodes on a bus. It typically defines how to format the messages and what interfaces to expose on the bus.

An interesting concept is the Object Dictionary (OD). The OD is a standardized way to describe a device through a list of entries. Each entry has a specific semantic and can be read or written by another node. Once you know the layout of a particular node OD, you know how to communicate with it.

The QuimBox controllers implement a subset of the CANopen protocol. Namely the message layout is completely compatible and the OD concept is reused.

An SDO can frame is structured as follow:

The can id is composed of the RX/TX identifier and the board identifier. (can id = RX/TX-id + Node-id)

TX-id = 600
RX-id = 580

Note: RX/TX from host point of view. On board the convention is opposite.

The command byte is either 0x23 for 32bits writing request, 0x2f for 8bits writing request or 0x40 for reading request (always 32bits response).

One can test the connection with a linux host using can-utils tools with cansend and candump commands.

ex. A request to enter a QuimDC in operational mode. (32bits write request) (Node-id = 7)

cansend 607#23.3333.00.01.00.00.00

See The QuimDC Object Dictionary for more information.