Invocation

Run ./can2mqtt.py -h to see a list of command line parameters. Default is to read configuration from local directory from file config.json and log to stdout with level INFO.

Configuaration

Connection to CAN bus is configured in canbus section of json config file. Connection to MQTT broker is configured in mqtt section. Translation of CAN messages to MQTT is configured in receivers section. Translation of MQTT messages to CAN is configured in transmitters section.

CAN parameters

Connection to CAN bus is made using python-can package. See documentation on available interfaces here.

MQTT parameters

Cennection to MQTT broker is made using paho-mqtt package. You may set client_id (default is can2mqtt), host (default is 127.0.0.1) and port (default is 1883).

Packing and Unpacking of CAN message

CAN messages are packed and unpacked using bitstruct package. Due to a problem with endianness, packing and unpacking is now done using the struct package. This is a little bit less flexible; it doesn't allow packing single bits into one byte and odd 24 bit or 56 bit integers are not supported, but being able to unpack CANopen data which come in little endian was more importatnt for me. See here for valid syntax of pack_template of transmitter resp. unpack_template of receiver templates. Each value from the template is later referenced by a variable name given in var_names.

Formating of topic and payload string

Multiple topic and payload strings can be formed and sent to the broker from one CAN message via one or more receiver. topic_template and payload_template may be strings or arrays of strings. Each string is formated using Format Specification Mini-Language. Each replacement field has to explicitly reference a variable name from var_names. Additionally to the variable names defined in var_names, topic_template and payload_template may reference the timestamp and CAN id of the received message by the names canidresp. t. The CAN timestamp is also provided under the name dt in datetime format which may be converted to string with e.g. the following pattern: {dt:%Y-%m-%d %H:%M:%S.%f}.

Parsing of incoming MQTT messages

Each transmitter handles MQTT messages received due to its subscriptions. Received topics and payloads are parsed using parse package. Again explicit field names have to be used. var_names gives the order of the parsed named values to be used to pack the CAN message using the pack_template. The CAN id is either a decimal integer, a hex formatted integer string or a single variable name from the set of parsed named values.

Transforming values

Unpacked and parsed values named by variable names in the var_names parameter may be transformed using a python function defined in the can2mqtt_vias.py module. Each such function must expect one scalar parameter and return one scalar parameter. To use a translation the variable name in var_names has to be followed by the key word "via" and then the name of the transformation function.

Logging

For logging python logging is used.

CANopen

The special section canopen allows to configure the bridge to send some CANopen specific messages. If sync_interval is specified a sync message is sent every interval seconds. If sync_count is specified the sync message has one data byte that cyclicly counts from 0 to the given count. If auto_start is true the program listens to CANopen NMT bootup and heartbeat messages and sends a start command to every node that indicates it is not in operational mode.