Advanced usage
This article provides a brief overview of some of the more advanced features of Pyuavcan.
It does not aim to provide an exhaustive user guide; for that you should refer to the
Python’s help system (the function help()) and to the source code,
particularly to the sub-package uavcan.app.
The examples below assume that the local node instance is bound to the variable node,
unless stated otherwise.
Bus monitoring
IO hooks allow the application to monitor all incoming and outgoing CAN frames. The Bus Monitor feature of the GUI Tool is based on this capability of Pyuavcan.
def frame_hook(direction, frame):
if direction == 'rx':
print('<', frame)
elif direction == 'tx':
print('>', frame)
else:
raise Exception('Invalid direction specification: %r' % direction)
handle = node.can_driver.add_io_hook(frame_hook)
# The hook can be removed like that:
handle.remove()
File server
File server can be used to perform firmware update on remote nodes, as well as for general purpose file exchange.
import uavcan
# File server need to be provided with file lookup pathes.
# These can be directories or files.
file_server = uavcan.app.file_server.FileServer(node, ['/home/pavel'])
file_server.lookup_paths.append('/etc/fstab')
print(file_server.lookup_paths)
print(file_server.path_hit_counters)
# The file server will be running in the background, requiring no additional attention
# When it is no longer needed, it should be finalized by calling close():
file_server.close()
You can test the file server using the following command in the Interactive Console of the GUI Tool:
request(uavcan.protocol.file.Read.Request(path=uavcan.protocol.file.Path(path='/etc/fstab')), 123)
The file server uses the following logic for relative file path resolution (simplified). It can be seen that locations specified earlier in the lookup path list take precedence.
# This is simplified
def _try_resolve_relative_path(lookup_paths, rel_path):
for p in lookup_paths:
if p.endswith(rel_path) and os.path.isfile(p):
return p
if os.path.isfile(os.path.join(p, rel_path)):
return os.path.join(p, rel_path)
Dynamic node ID allocation
The logic pertaining to the dynamic node ID allocation feature is located in the module
uavcan.app.dynamic_node_id.
The code below demonstrates how to run an instance of a centralized node ID allocator.
node_monitor = uavcan.app.node_monitor.NodeMonitor(node)
# It is NOT necessary to specify the database storage.
# If it is not specified, the allocation table will be kept in memory, thus it will not be persistent.
allocator = uavcan.app.dynamic_node_id.CentralizedServer(node, node_monitor,
database_storage='/home/pavel/allocation_table.db')
# The allocator and the node monitor will be running in the background, requiring no additional attention
# When they are no longer needed, they should be finalized by calling close():
allocator.close()
node_monitor.close()