CAN Bus Input

Overview

A CAN Bus Input decodes a single CANbus signal directly out of a raw incoming CAN frame, using a byte/bit layout you define by hand. Use it when you know the frame layout (e.g. from a DBC) for an arbitrary or custom device and there is no CAN Preset for it. This is the raw frame signal approach: you give the message ID, say how the bits are encoded and where they sit, then scale the raw value to engineering units. To read a named object from a supported device instead, use a CAN Object Input.

Prerequisites & hardware

  • A defined CAN bus for the channel the frame arrives on — see Working with the CAN Bus.
  • The frame's message ID and bit layout for the signal you want (length, offset, signedness, endianness).

Add it in the app

  1. Add a new input and choose CAN Bus Input as the type.
  2. Give it a clear alias (e.g. Engine RPM).
  3. Enter the Message ID and its ID type, then the decoding type, length and offset that locate the signal in the frame.
  4. Set the raw → out scaling and the fault / timeout behaviour below.
Setting Meaning Unit Range / values Notes
Message ID CAN frame identifier to match hex fixed32 The arbitration ID of the frame carrying the signal.
Message ID type Identifier width — Extended (29-bit), Standard (11-bit) Must match how the sending device addresses the frame.
Decoding type How the raw bits are interpreted — UnsignedBigEndian, UnsignedLittleEndian, SignedBigEndian, SignedLittleEndian, BitField, BitFieldBigEndian, SignedBitField, SignedBitFieldBigEndian, FloatLittleEndian, FloatBigEndian Selects signedness, endianness, bitfield or float decoding. Match the sender's encoding.
Length Signal length bytes / bits uint32 In bytes for the integer/float types; in bits for the bitfield types (BitField, BitFieldBigEndian, SignedBitField, SignedBitFieldBigEndian).
Offset Position of the signal in the frame bytes / bits uint32 Same units as Length: bytes for integer/float types, bits for bitfield types.
Enable interpolation Linearly interpolates between the raw/out points — on / off Off = step/clamped mapping.
Raw min / Raw max Raw signal range to scale from — sint32 The decoded low/high points.
Out min / Out max Engineering range to scale to signal unit sint32 The value reported at Raw min / Raw max.
Fault input An input whose state flags this signal as faulted — input alias When that input is in its fault band the signal is treated as faulted.
Fault min / Fault max Values that indicate a fault — sint32 Decoded values inside this band mark the signal as faulted.
Saturate Clamps the output to the out range — on / off On = limit to Out min … Out max instead of extrapolating.
Default value Value used before any frame arrives / when default selected signal unit sint32 Reported until the first valid frame.
Fault value Value reported on fault signal unit sint32 Substituted while the signal is faulted.
Timeout Maximum time between frames ms uint32 A gap longer than this triggers the timeout behaviour.
Timeout mode What to report on timeout — UsePrevious, UseDefault, UseFault See the Timeouts section.
Scaling: the decoded value is mapped from Raw min…Raw max to Out min…Out max using two-point interpolation.

Common settings

CAN Bus Input also uses the shared setting — alias. See Common IO Settings.

Example — engine RPM from a 500 kbit/s bus

  1. Type CAN Bus Input, alias Engine RPM.
  2. Message ID 0x0CF00400, ID type Extended (29-bit).
  3. Decoding type UnsignedLittleEndian, Length 2 (bytes), Offset 3 (starts at byte 3) — a 16-bit value. (A bitfield type would use bits here.)
  4. Raw min/max 0/65535 → Out min/max 0/8191 (0.125 rpm/bit gives the right scale); Saturate on.
  5. Timeout 100 ms, Timeout mode UseFault, Fault value 0 so a lost ECU is detected rather than the last rpm being held.

Troubleshooting

  • Value is garbage / wildly wrong: check the decoding type (signedness and endianness) and that Length and Offset match the frame layout — remember they are in bytes for integer/float types and bits for bitfield types.
  • Reads zero / never updates: confirm the Message ID and ID type, and that the CAN bus baud rate matches the sender.
  • Scaled wrong: verify Raw min/max and Out min/max.
  • Holds a stale value when the device drops off: set a Timeout and a Timeout mode of UseDefault or UseFault.