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Use this guide for the mjbots fdcanusb USB interface. Embedder recognizes the adapter when its serial device identifies itself as fdcanusb, and uses the adapter’s own text protocol. It carries up to 64 payload bytes. You can’t send remote requests through it.

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Prepare the adapter

The Monitor doesn’t set fdcanusb’s bus rates or turn the bus on. Configure the adapter in its serial console first.
  1. Connect the adapter’s USB interface and install any host driver or device-access permissions it needs.
  2. Connect CAN H, CAN L, and the required reference ground. Check the bus’s termination and voltage requirements.
  3. Close any other serial program that has the port open.
  4. In a Serial Monitor tab, select the fdcanusb Port, keep Mode set to UART, set NL to LF (\n), and select Connect.
  5. Check the current settings by sending these lines one at a time: conf get can.bitrate, conf get can.fd_bitrate, and can status.
The adapter must be Bus On to receive frames, and you change its configuration while it’s Bus Off. For a bus that uses 500 kbit/s arbitration and a 2 Mbit/s CAN-FD data phase, send these lines one at a time:
Replace the rates with your bus’s requirements. Confirm each OK response and mode:BUSON before you continue. These are adapter console commands, not CAN frames. The mjbots protocol reference describes the firmware’s configuration and persistence options.

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Connect the CAN Monitor

  1. Select Disconnect to close the console connection.
  2. Set Mode to CAN.
  3. Confirm that the tab shows Bitrate: device-managed instead of a Bitrate list.
  4. Select Connect.
If a Bitrate list appears, the selected port didn’t identify itself as fdcanusb and the tab is using SLCAN. That list doesn’t configure fdcanusb.
Received frames keep their FD and BRS markers. Use CAN Monitor decoding to apply a project database profile.

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Send a deliberate test frame

Review the receiver, ID, payload, and expected effect before you send. These examples show the accepted input syntax; replace them with frames for your bus. The short form takes hexadecimal id#data:
The short form turns off bitrate switching. It requests CAN-FD for payloads longer than eight bytes. For eight bytes or fewer, the adapter’s default frame format decides. Use the native form when the frame format must be explicit: The tab’s echo of a native bf send is labeled FD even though lowercase f requests classic format. Check the actual frame format in received bus traffic. Use only payload lengths that are valid for the frame format and the adapter firmware, up to 64 bytes. The send box accepts can send but no other console commands. Send configuration commands in UART mode, as described above.

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Troubleshoot

  • A Bitrate list appears in CAN mode: the selected port didn’t identify itself as fdcanusb, so the tab uses the SLCAN protocol. Check that you selected the right port.
  • No frames: in the adapter console, check can status, the arbitration and data rates, filters, bus activity, wiring, and termination.
  • Configuration rejected: send can off before you change configuration, then can on.
  • Console commands unrecognized: confirm that the installed firmware implements the linked mjbots USB CDC protocol.
  • remote frames are not supported on fdcanusb: send a data frame instead.
  • Wrong frame format: use the native form with bf, bF, or BF, and check the resulting traffic.
  • [CAN send failed: ...]: check for hexadecimal byte pairs, the frame format’s length limits, and the 64-byte maximum.
Last modified on October 12, 2026