CAN Monitor

Switch a serial tab to CAN mode when a USB adapter carries CAN frames instead of UART text. The Monitor normalizes frames before it displays or decodes them.

Choose the adapter protocol

Embedder supports two CAN-over-serial codecs:

Codec	Typical hardware	Frame size	Bus configuration
Lawicel SLCAN	CANable, CANtact, CANUSB, and compatible adapters	Classic CAN, up to 8 data bytes	Select the CAN bitrate in the Monitor
`fdcanusb`	mjbots fdcanusb	Classic CAN and CAN-FD, up to 64 data bytes	Configure bitrate on the device

Embedder selects fdcanusb when the port’s product metadata contains fdcanusb. Other serial CAN adapters use SLCAN.

Connect to classic CAN

Choose one of the SLCAN rates:

10, 20, 50, 100, 125, 250, 500, or 800 kbit/s
1 Mbit/s

The default is 500 kbit/s. On connect, Embedder closes the SLCAN channel, sets the bitrate, and opens the channel.

Open the selected CANable port in CAN mode at 500 kbit/s. Read frames for five
seconds without transmitting.

Status output can report RX or TX FIFO full, error warning, data overrun, error passive, arbitration lost, and bus error.

Connect to CAN-FD

The fdcanusb codec does not send a bitrate handshake. Set the arbitration and data rates with the adapter’s own configuration before you connect.

Use the fdcanusb port in CAN mode. Read for ten seconds and preserve the FD and
BRS markers. Do not change the device configuration or transmit.

A normalized frame looks like:

00000123  [12]  01 02 03 04 05 06 07 08 09 0A 0B 0C  FD BRS

The eight-digit ID denotes an extended frame. FD marks a CAN-FD frame, and BRS marks bit-rate switching.

Send a test frame

A transmitted CAN frame can command actuators or change ECU state. Name the bus, arbitration ID, payload, and expected receiver before you ask Embedder to send. Reading and decoding are observational by default.

Use id#data for either adapter:

123#DEADBEEF

IDs with more than three hexadecimal digits become 29-bit extended IDs. SLCAN accepts remote requests such as 123#R8. fdcanusb also accepts its native form:

can send 123 DEADBEEF bF

An fdcanusb frame longer than eight bytes is sent as CAN-FD. Remote frames are not supported by the fdcanusb codec.

Decode application signals

CAN payload bytes do not identify their application meaning. Before Embedder interprets them, point it at a database or let it search the project. The database workflow recognizes:

DBC (.dbc)
KCD (.kcd)
SYM (.sym)
AUTOSAR XML (.arxml)

These files are the source of truth. The Monitor itself reads normalized JSON profiles from .embedder/can/; it does not load a DBC, KCD, SYM, or ARXML file directly.

Find the database

Ask Embedder to search project files and configuration for CAN database references. If several files exist, identify the bus or ECU you are monitoring.

Convert the needed definitions

Embedder writes only the messages and signals required for the investigation to a version 1 JSON profile. It preserves the database IDs, frame format, lengths, bit positions, byte order, signedness, scale, offset, units, and enumerated values that the Monitor can decode.

Select the profile

Put profiles in .embedder/can/. The CAN toolbar lists each .json file and lets you pin one.

Compare decoded and raw data

Enable Decoded, then keep the raw bytes visible in your report. Frames without a matching definition remain raw.

If no database exists, Embedder asks before it infers a profile from source code. It follows serialization code, masks, shifts, constants, and enums only after you approve that fallback.

Write a Monitor profile

Use this shape for an ordinary bit-field message:

.embedder/can/codebase-derived.json

{
  "version": 1,
  "source": "db/vehicle.dbc",
  "messages": [
    {
      "id": "0x123",
      "extended": false,
      "name": "Status",
      "length": 8,
      "signals": [
        {
          "name": "rpm",
          "startBit": 0,
          "length": 16,
          "byteOrder": "little_endian",
          "signed": false,
          "factor": 1,
          "offset": 0,
          "unit": "rpm"
        },
        {
          "name": "state",
          "startBit": 16,
          "length": 8,
          "byteOrder": "little_endian",
          "choices": {
            "0": "Stopped",
            "1": "Running"
          }
        }
      ]
    }
  ]
}

You can add mask when a family of arbitration IDs shares one layout. The parser also accepts byte-indexed integer fields and packed BCD values. For project-specific ASCII records, set payload to ascii_record and define prefixed fields. Keep hexadecimal or decimal IDs consistent within one profile. Validate 11-bit versus 29-bit format and the exact payload length against captured traffic.

Select and refresh profiles

The Monitor watches .embedder/can/*.json. It refreshes when a profile is created, changed, or deleted. Auto selection uses this order:

A profile you pinned in the CAN toolbar
.embedder/can/codebase-derived.json
The first profile by filename

Keep database versions as separate JSON files when you need to compare them. Do not overwrite the project database.

Read decoded output

A report should pair meaning with evidence:

Status (0x123)  rpm=1500 rpm  state=Running
raw:  DC 05 01 00 00 00 00 00  dlc=8

Do not decode a higher-layer protocol unless the project identifies it. J1939, CANopen, ISO-TP, and UDS each add rules that raw CAN framing cannot supply.

Fix CAN problems

The adapter rejects the connection

For SLCAN, select one of the supported bitrates. For fdcanusb, configure the bus on the device and leave the Monitor bitrate alone.

Every frame is raw

Confirm that CAN mode is enabled, the profile is valid version 1 JSON, the selected path is active, and the message ID, extended flag, and length match.

A signal value is wrong

Recheck start bit, length, byte order, signedness, factor, and offset against the database. Keep the raw payload beside the decoded value.

No database was found

Attach or name a DBC, KCD, SYM, or ARXML file. Approve source inference only when no authoritative database exists.

An unresolved frame disappeared in decoded mode

Switch to Raw and verify the input line. A frame without a matching message should remain visible as raw output; an invalid line may not parse as a CAN frame.

Serial transports

Configure the serial port that carries CAN traffic.

Logic analyzer

Capture the physical bus when the adapter or transceiver is suspect.

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API

Choose the adapter protocol

Connect to classic CAN

Connect to CAN-FD

Send a test frame

Decode application signals

Write a Monitor profile

Select and refresh profiles

Read decoded output

Fix CAN problems

Serial transports

Logic analyzer

​Choose the adapter protocol

​Connect to classic CAN

​Connect to CAN-FD

​Send a test frame

​Decode application signals

​Write a Monitor profile

​Select and refresh profiles

​Read decoded output

​Fix CAN problems

Serial transports

Logic analyzer

Choose the adapter protocol

Connect to classic CAN

Connect to CAN-FD

Send a test frame

Decode application signals

Write a Monitor profile

Select and refresh profiles

Read decoded output

Fix CAN problems