Network management (NMT)

The NMT protocols are used to issue state machine change commands (e.g. to start and stop the devices), detect remote device bootups and error conditions.

The Module control protocol is used by the NMT master to change the state of the devices. The CAN-frame COB-ID of this protocol is always 0, meaning that it has a function code 0 and node ID 0, which means that every node in the network will process this message. The actual node ID, to which the command is meant to, is given in the data part of the message (at the second byte). This can also be 0, meaning that all the devices on the bus should go to the indicated state.

The Heartbeat protocol is used to monitor the nodes in the network and verify that they are alive. A heartbeat producer (usually a slave device) periodically sends a message with the binary function code of 1110 and its node ID (COB-ID = 0x700 + node ID). The data part of the frame contains a byte indicating the node status. The heartbeat consumer reads these messages.

CANopen devices are required to make the transition from the state Initializing to Pre-operational automatically during bootup. When this transition is made, a single heartbeat message is sent to the bus. This is the bootup protocol.

Examples

Access the NMT functionality using the canopen.Node.nmt attribute. Changing state can be done using the state attribute:

node.nmt.state = 'OPERATIONAL'
# Same as sending NMT start
node.nmt.send_command(0x1)

You can also change state of all nodes simulaneously as a broadcast message:

network.nmt.state = 'OPERATIONAL'

If the node transmits heartbeat messages, the state attribute gets automatically updated with current state:

# Send NMT start to all nodes
network.send_message(0x0, [0x1, 0])
node.nmt.wait_for_heartbeat()
assert node.nmt.state == 'OPERATIONAL'

API

class canopen.nmt.NmtMaster(node_id)

add_hearbeat_callback(callback)

Add function to be called on heartbeat reception.

Parameters:

callback (Callable[[int], None]) – Function that should accept an NMT state as only argument.

send_command(code)

Send an NMT command code to the node.

Parameters:

code (int) – NMT command code.

start_node_guarding(period)

Starts the node guarding mechanism.

Parameters:

period (float) – Period (in seconds) at which the node guarding should be advertised to the slave node.

stop_node_guarding()

Stops the node guarding mechanism.

timestamp: Optional[float]

Timestamp of last heartbeat message

wait_for_bootup(timeout=10)

Wait until a boot-up message is received.

Return type:

None

wait_for_heartbeat(timeout=10)

Wait until a heartbeat message is received.

exception canopen.nmt.NmtError

Some NMT operation failed.