# Demo\_app.py utilizing can as transport layer **URL:** \ **Category:** Applications & Usage\ **Created:** [January 5, 2024, 5:10pm UTC](https://forum.opencyphal.org/t/demo-app-py-utilizing-can-as-transport-layer/2065 "2024-01-05T17:10:13Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Scoeerg](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/scoeerg/32/1713_2.png) [@Scoeerg](https://forum.opencyphal.org/u/Scoeerg)\ **Post date:** [January 5, 2024, 5:10pm UTC](https://forum.opencyphal.org/t/demo-app-py-utilizing-can-as-transport-layer/2065/1 "2024-01-05T17:10:13Z")
Hi all, I am fairly new to the game of CAN and Cyphal. I did the [pycyphal.com demo](https://pycyphal.readthedocs.io/en/stable/pages/demo.html), which works like a charm for UDP as transport layer. Now, I would like to utilize CAN as transport layer, instead of UDP. To do this I changed the code accordingly: ```auto export UAVCAN __NODE__ ID=42 # Set the local node-ID 42 (anonymous by default) export UAVCAN __CAN__ IFACE="socketcan:can0" # Use CAN can0 as transport layer export UAVCAN __CAN__ BITRATE="500000 5000000" # 500kbits export UAVCAN __CAN__ MTU=16 # Maximum Transmission Unit (largest package size for a single frame) export UAVCAN __SUB__ TEMPERATURE_SETPOINT__ID=2345 # Subject "temperature_setpoint" on ID 2345 export UAVCAN __SUB__ TEMPERATURE_MEASUREMENT__ID=2346 # Subject "temperature_measurement" on ID 2346 export UAVCAN __PUB__ HEATER_VOLTAGE__ID=2347 # Subject "heater_voltage" on ID 2347 export UAVCAN __SRV__ LEAST_SQUARES__ID=123 # Service "least_squares" on ID 123 export UAVCAN __DIAGNOSTIC__ SEVERITY=2 ``` based on scarcely scattered information on this forum. `can0` of course exists, hence ```auto sudo ip link set can0 up type can bitrate 500000 ifconfig ``` returns ```auto can0: flags=193 mtu 16 unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 10 (UNSPEC) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 ``` and then when running `python3 demo_app.py` simply returns `Running. Press Ctrl+C to stop.`, as expected. But when aborting, it prints ```auto CDiagnosticPublisher(Publisher(dtype=uavcan.diagnostic.Record.1.1, transport_session=BroadcastCANOutputSession(OutputSessionSpecifier (data_specifier=MessageDataSpecifier(subject_id=8184), remote_node_id=None), PayloadMetadata(extent_bytes=300)))) TIMEOUT uavcan.diagnostic.Record.1.1(timestamp=uavcan.time.SynchronizedTimestamp.1.0(microsecond=0), severity=uavcan.diagnostic.Severity.1.0(value=2), text="pycyphal.transport.can.media.socketcan._socketcan: SocketCANMedia('can0', mtu=16) FIXME: acceptance filter configuration is not yet implemented; please submit patches! Requested configuration: ext:xxx1x0xxxxxxxxx0101010xxxxxxx, ext:xxxxx0xxxxxxxxxxxxxxxx0") Traceback (most recent call last): File "$mypath/demo_app.py", line 159, in asyncio.run(main()) File "/usr/lib/python3.10/asyncio/runners.py", line 44, in run return loop.run_until_complete(main) File "/usr/lib/python3.10/asyncio/base_events.py", line 636, in run_until_complete self.run_forever() File "/usr/lib/python3.10/asyncio/base_events.py", line 603, in run_forever self._run_once() File "/usr/lib/python3.10/asyncio/base_events.py", line 1871, in _run_once event_list = self._selector.select(timeout) File "/usr/lib/python3.10/selectors.py", line 469, in select fd_event_list = self._selector.poll(timeout, max_ev) KeyboardInterrupt ``` tells me this has not yet been implemented ( **FIXME: acceptance filter configuration is not yet implemented; please submit patches!** ), and `yakut` cannot echo the heartbeat as intended, doing: ```auto export UAVCAN __CAN__ IFACE="socketcan:can0" yakut sub --with-metadata uavcan.node.heartbeat uavcan.diagnostic.record ``` returns nothing. `ifconfig` also tells me there’s nothing happening on the BUS (aka RX/TX-packets are 0). Any advice? — Nice to Know— When switching between UDP and CAN as transport layer, one must delete the `demo_app.db`, otherwise things go boomboom. — Nice to Know — ---
**Author:** ![pavel.kirienko](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/pavel.kirienko/32/9_2.png) [@pavel.kirienko](https://forum.opencyphal.org/u/pavel.kirienko)\ **Post date:** [January 5, 2024, 6:13pm UTC](https://forum.opencyphal.org/t/demo-app-py-utilizing-can-as-transport-layer/2065/2 "2024-01-05T18:13:07Z")
The MTU is set incorrectly. According to ifconfig, your `can0` is a Classic CAN, because its kernel MTU is 16 bytes (8 bytes of payload plus 8 bytes of overhead); whereas you’re setting the transport MTU to 16 bytes, which requires CAN FD. Set the MTU to 8 bytes to fix the problem: ```auto export UAVCAN __CAN__ MTU=8 ``` I regularly see much confusion caused by the fact that the MTU reported by the Linux tools includes the 8 bytes of overhead. > [@Scoeerg](#): > > tells me this has not yet been implemented This message is irrelevant. ---
**Author:** ![Scoeerg](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/scoeerg/32/1713_2.png) [@Scoeerg](https://forum.opencyphal.org/u/Scoeerg)\ **Post date:** [January 6, 2024, 3:14pm UTC](https://forum.opencyphal.org/t/demo-app-py-utilizing-can-as-transport-layer/2065/3 "2024-01-06T15:14:40Z")
Hi @pavel.kirienko this worked, thank you. New config is now ```auto export UAVCAN __NODE__ ID=42 # Set the local node-ID 42 (anonymous by default) export UAVCAN __CAN__ IFACE="socketcan:can0" # Use CAN can0 as transport layer export UAVCAN __CAN__ BITRATE="500000 5000000" # 500kbits, 500kbits arbitration export UAVCAN __CAN__ MTU=8 # Maximum Transmission Unit (largest package size for a single frame) export UAVCAN __SUB__ TEMPERATURE_SETPOINT__ID=2345 # Subject "temperature_setpoint" on ID 2345 export UAVCAN __SUB__ TEMPERATURE_MEASUREMENT__ID=2346 # Subject "temperature_measurement" on ID 2346 export UAVCAN __PUB__ HEATER_VOLTAGE__ID=2347 # Subject "heater_voltage" on ID 2347 export UAVCAN __SRV__ LEAST_SQUARES__ID=123 # Service "least_squares" on ID 123 export UAVCAN __DIAGNOSTIC__ SEVERITY=2 ``` and ifconfig now replied with ```auto can0: flags=193 mtu 16 unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 10 (UNSPEC) RX packets 165121 bytes 1320968 (1.3 MB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 3 bytes 24 (24.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 ``` i.e. there’s traffic on the CAN bus now. Every once in a while `demo_app.py` prints ```auto DiagnosticPublisher(Publisher(dtype=uavcan.diagnostic.Record.1.1, transport_session=BroadcastCANOutputSession(OutputSessionSpecifier(data_specifier= MessageDataSpecifier(subject_id=8184), remote_node_id=None), PayloadMetadata(extent_bytes=300)))) TIMEOUT uavcan.diagnostic.Record.1.1(timestamp=uavcan.time.SynchronizedTimestamp.1.0(microsecond=0), severity=uavcan.diagnostic.Severity.1.0(value=2), text="pycyphal.transport.can._can: CANTransport(SocketCANMedia('can0', mtu=8), local_node_id=42): 1 frames of 1 total with CAN ID 0x107d552a could not be sent before the deadline") ``` which I will ignore (for now). **But** sourcing the data types ```auto export YAKUT_PATH="$mypath/uavcan" export YAKUT_PATH="$mypath/sirius_cyber_corp" # Setting up the Cyphal/UAVCAN parameters, in this case we are using Cyphal via CAN export UAVCAN __CAN__ IFACE="socketcan:can0" yakut sub --with-metadata uavcan.node.heartbeat uavcan.diagnostic.record ``` **still won’t print the heartbeat as expected.** — `demo_app.py` problem — Also, restarting `demo_app.py` responds with ```auto DiagnosticPublisher(Publisher(dtype=uavcan.diagnostic.Record.1.1, transport_session=BroadcastCANOutputSession(OutputSessionSpecifier (data_specifier=MessageDataSpecifier(subject_id=8184), remote_node_id=None), PayloadMetadata(extent_bytes=300)))) ERROR OSError [Errno 105] No buffer space available ``` **Fixed** by restarting `can0`, i.e. `sudo ip link set can0 down`, `sudo ip link set can0 up type can bitrate 500000` — `demo_app.py` problem — ---
**Author:** ![pavel.kirienko](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/pavel.kirienko/32/9_2.png) [@pavel.kirienko](https://forum.opencyphal.org/u/pavel.kirienko)\ **Post date:** [January 6, 2024, 5:17pm UTC](https://forum.opencyphal.org/t/demo-app-py-utilizing-can-as-transport-layer/2065/4 "2024-01-06T17:17:46Z")
If your `can0` is a physical bus, ensure there is at least one other node available for the interface to function. Alternatively, use vcan. ---
**Author:** ![Scoeerg](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/scoeerg/32/1713_2.png) [@Scoeerg](https://forum.opencyphal.org/u/Scoeerg)\ **Post date:** [January 6, 2024, 7:27pm UTC](https://forum.opencyphal.org/t/demo-app-py-utilizing-can-as-transport-layer/2065/5 "2024-01-06T19:27:01Z")
Yes `can0` is a physical bus, something like [CAN USB adapter](https://www.google.com/search?q=can+usb+adapter), which registers as a [Geschwister Schneider GmbH / Candlelight CAN USB adapter as used for Unix Kernel](https://linux-automation.com/de/products/candlelight.html) ([or here](https://python-can.readthedocs.io/en/stable/interfaces/gs_usb.html)). Meaning for the UDP tutorial, since its’s looped(?) to `localhost/127.0.0.1` it works even without a second node? I am note sure I tried `yakut` in UDP with or without the second node up. Please elaborate. ---
**Author:** ![pavel.kirienko](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/pavel.kirienko/32/9_2.png) [@pavel.kirienko](https://forum.opencyphal.org/u/pavel.kirienko)\ **Post date:** [January 7, 2024, 1:10pm UTC](https://forum.opencyphal.org/t/demo-app-py-utilizing-can-as-transport-layer/2065/6 "2024-01-07T13:10:46Z")
The requirement to have at least one node follows from the design of the CAN bus itself and it has no relation to Cyphal. A CAN controller will keep retransmitting the frame until at least one node has confirmed its reception. Since you have no nodes on the physical bus, the frames cannot be confirmed, so transmission cannot make progress. As a result, the frames you’re attempting to transmit just get stuck in the TX queue until it’s full; this is where you get the warnings about timed out transmissions. Virtual CAN ifaces do not require TX frames to be confirmed so they can work without physical nodes (and they can’t have physical nodes indeed since they are virtual). UDP/IP does not require frame transmission confirmation since it is built on completely different principles. ---
**Author:** ![Scoeerg](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/scoeerg/32/1713_2.png) [@Scoeerg](https://forum.opencyphal.org/u/Scoeerg)\ **Post date:** [January 8, 2024, 9:27am UTC](https://forum.opencyphal.org/t/demo-app-py-utilizing-can-as-transport-layer/2065/7 "2024-01-08T09:27:55Z")
Thank you for the elaborate answer. I wrongly assumed `yakut` is just another node on the bus.