Appendix B — CAN Frame Format Reference

OpenLCB uses CAN 2.0B extended frames with 29-bit identifiers. This appendix documents the bit-field layout of the CAN identifier and the different frame types used by the protocol.

B.1 29-Bit Identifier Layout

The 29-bit CAN extended identifier is divided into fields that differ based on whether the frame is a control frame or an OpenLCB message frame:

Bit:  28  27  26 25 24  23 22 21 20 19 18 17 16 15 14 13 12  11 10 9 8 7 6 5 4 3 2 1 0
      [R] [M] [--- Frame Type/Seq ---] [------ Variable Field ------] [-- Source Alias --]

R   = Reserved (bit 28): always 1 on transmit, ignored on receive
M   = OpenLCB Message flag (bit 27): 1 = OpenLCB message, 0 = CAN control frame
Bits 26-24 = Frame Type (messages) or Sequence Number (control frames)
Bits 23-12 = Variable Field (MTI for messages, sub-type for control frames)
Bits 11-0  = Source Alias (12-bit CAN alias of the sending node)

Identifier Masks

Constant	Value	Purpose
`RESERVED_TOP_BIT`	`0x10000000`	Bit 28 -- always set to 1
`CAN_OPENLCB_MSG`	`0x08000000`	Bit 27 -- 1 = OpenLCB message frame
`MASK_CAN_FRAME_SEQUENCE_NUMBER`	`0x07000000`	Bits 26-24 -- frame type or sequence number
`MASK_CAN_VARIABLE_FIELD`	`0x00FFF000`	Bits 23-12 -- MTI or control frame sub-type
`MASK_CAN_SOURCE_ALIAS`	`0x00000FFF`	Bits 11-0 -- source node alias

B.2 Frame Type Classification

flowchart TD A["CAN Frame Received"] --> B{"Bit 27 set?"} B -->|"Yes (1)"| C["OpenLCB Message Frame"] B -->|"No (0)"| D["CAN Control Frame"] C --> E{"Frame Type
bits 26-24"} E -->|"001"| F["Standard Message"] E -->|"010"| G["Datagram Only"] E -->|"011"| H["Datagram First"] E -->|"100"| I["Datagram Middle"] E -->|"101"| J["Datagram Final"] E -->|"110"| K["Reserved"] E -->|"111"| L["Stream"] D --> M{"Sequence bits
26-24 + Variable"} M --> N["CID / RID / AMD / AME / AMR"]

B.3 OpenLCB Message Frame Types

Constant	Value	Bits 26-24	Description
`OPENLCB_MESSAGE_STANDARD_FRAME_TYPE`	`0x01000000`	001	Global or addressed OpenLCB message (MTI in variable field)
`CAN_FRAME_TYPE_DATAGRAM_ONLY`	`0x02000000`	010	Complete datagram in a single CAN frame
`CAN_FRAME_TYPE_DATAGRAM_FIRST`	`0x03000000`	011	First frame of a multi-frame datagram
`CAN_FRAME_TYPE_DATAGRAM_MIDDLE`	`0x04000000`	100	Middle frame of a multi-frame datagram
`CAN_FRAME_TYPE_DATAGRAM_FINAL`	`0x05000000`	101	Final frame of a multi-frame datagram
`CAN_FRAME_TYPE_RESERVED`	`0x06000000`	110	Reserved for future use
`CAN_FRAME_TYPE_STREAM`	`0x07000000`	111	Stream data frame

B.4 CAN Control Frames

Control frames have bit 27 = 0. They handle alias allocation and mapping management:

Check ID (CID) Frames

Constant	Value	Bits 26-24	Variable Field Contains
`CAN_CONTROL_FRAME_CID7`	`0x07000000`	111	Node ID bits 47-36 (first 12 bits)
`CAN_CONTROL_FRAME_CID6`	`0x06000000`	110	Node ID bits 35-24 (2nd 12 bits)
`CAN_CONTROL_FRAME_CID5`	`0x05000000`	101	Node ID bits 23-12 (3rd 12 bits)
`CAN_CONTROL_FRAME_CID4`	`0x04000000`	100	Node ID bits 11-0 (last 12 bits)

Alias Management Frames

Constant	Value	Description	Data Field
`CAN_CONTROL_FRAME_RID`	`0x00700000`	Reserve ID -- claims the alias	Empty (0 bytes)
`CAN_CONTROL_FRAME_AMD`	`0x00701000`	Alias Map Definition -- maps alias to Node ID	6-byte Node ID
`CAN_CONTROL_FRAME_AME`	`0x00702000`	Alias Mapping Enquiry -- query alias ownership	6-byte Node ID (or empty for global query)
`CAN_CONTROL_FRAME_AMR`	`0x00703000`	Alias Map Reset -- node releasing alias	6-byte Node ID

B.5 Multi-Frame Message Indicator Bits

For standard OpenLCB messages that span multiple CAN frames (not datagrams), the first data byte contains framing bits:

Constant	Value	Bits 5-4	Meaning
`MULTIFRAME_ONLY`	`0x00`	00	Single frame (complete message)
`MULTIFRAME_FIRST`	`0x10`	01	First frame of multi-frame sequence
`MULTIFRAME_FINAL`	`0x20`	10	Final frame of multi-frame sequence
`MULTIFRAME_MIDDLE`	`0x30`	11	Middle frame of multi-frame sequence

Mask

Use MASK_MULTIFRAME_BITS (0x30) to extract the framing indicator from the first data byte.

B.6 Datagram Frame Sequencing

Datagrams up to 8 bytes use a single "Datagram Only" frame. Larger datagrams (up to 72 bytes) are split across multiple frames:

sequenceDiagram participant S as Sender participant B as CAN Bus participant R as Receiver Note over S,R: Datagram ≤ 8 bytes S->>B: Datagram Only (type 0x02) B->>R: Complete datagram Note over S,R: Datagram > 8 bytes S->>B: Datagram First (type 0x03) [8 bytes] S->>B: Datagram Middle (type 0x04) [8 bytes] S->>B: Datagram Middle (type 0x04) [8 bytes] S->>B: Datagram Final (type 0x05) [2-8 bytes] B->>R: Assembled datagram R->>B: Datagram OK Reply B->>S: Acknowledgment

For datagram frames, the variable field (bits 23-12) carries the destination alias instead of the MTI. The frame type bits (26-24) identify the frame as a datagram.

B.7 CAN Identifier Field Masks (OpenLCB Messages)

When the frame is a standard OpenLCB message (frame type = 001), the variable field contains the MTI. These masks extract specific MTI sub-fields from the CAN identifier:

Constant	Value	Purpose
`MASK_CAN_STREAM_OR_DATAGRAM`	`0x01000000`	Stream/datagram indicator in CAN ID
`MASK_CAN_PRIORITY`	`0x00C00000`	Priority field in CAN ID
`MASK_CAN_SIMPLE_PROTOCOL`	`0x00010000`	Simple protocol indicator
`MASK_CAN_DEST_ADDRESS_PRESENT`	`0x00008000`	Destination address present
`MASK_CAN_EVENT_PRESENT`	`0x00004000`	Event ID present in payload
`MASK_CAN_PRIORITY_MODIFIER`	`0x00003000`	Priority modifier bits

Source Reference

All CAN frame format constants are defined in openlcb_defines.h. CAN frame type definitions are in the can_frame_format and can_control_frames Doxygen groups.

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