Use this guide for the mjbots fdcanusb USB serial interface. Embedder recognizes fdcanusb from the serial device identity and uses its native text protocol. It supports up to 64 payload bytes; its CAN send input does not support remote requests.
Prepare the adapter
- Connect the adapter’s USB serial interface and install any required host driver or device-access permissions. Use the serial port shown in the Monitor.
- Connect CAN H, CAN L, and the required reference ground; check the bus’s termination and voltage requirements.
- Close competing serial applications. Before entering Monitor CAN mode, open the adapter in ordinary UART mode with LF selected.
- Inspect its current settings using
conf get can.bitrate, conf get can.fd_bitrate, and can status, sent one line at a time.
The adapter must be Bus On to receive frames. Its configuration can be changed while Bus Off. For a bus that requires 500 kbit/s arbitration and 2 Mbit/s CAN-FD data, send these lines individually in the adapter’s ordinary serial console:
Replace the rates with the bus’s requirements. Confirm OK responses and mode:BUSON before proceeding. These are adapter console commands, not CAN frames. The mjbots protocol reference describes firmware-specific configuration and persistence options.
Connect the CAN Monitor
Disconnect the ordinary serial console, switch the Monitor tab to CAN, and connect the fdcanusb port. Confirm that its bitrate is shown as device-managed.
The Monitor does not set fdcanusb’s arbitration or data rate and does not send a Bus On command when it connects. Complete the adapter setup above first; changing an SLCAN bitrate control does not configure fdcanusb.
Received frames retain their FD and bitrate-switching indicators. Use CAN Monitor decoding for project database profiles.
Send a deliberate test frame
Review the receiver, ID, payload, and expected effect before sending. These examples demonstrate the Monitor’s accepted input syntax; replace them with frames appropriate for your bus.
The short form accepts hexadecimal id#data:
The short form disables bitrate switching and requests FD for payloads longer than eight bytes. For shorter payloads, the adapter’s default frame format still matters. Use the native form when the frame format must be explicit.
For classic CAN without bitrate switching:
The Monitor currently labels this native send echo as FD even though lowercase f requests classic format. Verify the actual frame format from received bus traffic.
For CAN-FD without bitrate switching, use bF. For CAN-FD with bitrate switching, use BF:
Use only valid payload lengths for the selected frame format and adapter firmware, with a maximum of 64 bytes. The Monitor accepts can send for this native form; it does not expose all vendor console commands through the CAN send input. Send configuration commands in ordinary serial mode as described above.
Troubleshoot
- SLCAN bitrate control appears: confirm the selected serial device identifies itself as fdcanusb; otherwise the wrong protocol may be selected.
- No frames: check
can status, arbitration/data rates, filters, bus activity, wiring, and termination in the adapter console.
- Configuration rejected: enter Bus Off before changing adapter configuration, then return to Bus On.
- Console commands unrecognized: confirm the installed firmware implements the linked mjbots USB CDC protocol before continuing.
- Remote request rejected: use a supported data frame; the fdcanusb CAN send path rejects remote frames.
- Wrong frame format: use explicit native
bf, bF, or BF flags and verify the resulting traffic.
- Payload rejected: check hexadecimal byte pairs, frame-format length limits, and the 64-byte maximum.
Last modified on September 12, 2026
