CAN Bus Fundamentals: Physical Layer, Frame Structure and Error Detection

ISO 11898-2 physical layer and differential signalling

ISO 11898-2 defines High-Speed CAN — the variant used in almost all building automation and industrial applications. The physical layer uses a differential pair: CAN_H (CAN High) and CAN_L (CAN Low), both referenced to GND. In the recessive state (logic 1), both lines float to approximately 2.5V and the differential voltage is 0V. In the dominant state (logic 0), CAN_H is driven to approximately 3.5V and CAN_L to approximately 1.5V, giving a 2V differential that is immune to common-mode noise up to ±30V.

Use shielded twisted pair with a characteristic impedance of 120Ω — Belden 9841 (single-pair, 24AWG) or equivalent. Connect the shield at one end only (typically at the panel) to avoid ground loops. Both ends of the bus segment must be terminated with a 120Ω resistor between CAN_H and CAN_L. Split termination (two 60Ω resistors with a 4.7nF capacitor to ground at the midpoint) improves EMC performance in high-noise environments.

CAN bus cable (Belden 9841 or equivalent, 120Ω STP):
  CAN_H  → CAN_H terminals on all nodes
  CAN_L  → CAN_L terminals on all nodes
  GND    → GND terminals on all nodes (reference, not signal ground)

Termination resistors:
  Node 1 (first node on bus): 120Ω between CAN_H and CAN_L
  Node N (last node on bus): 120Ω between CAN_H and CAN_L
  All intermediate nodes: NO termination

Measure termination (bus powered off):
  CAN_H to CAN_L resistance should read ~60Ω
  (Two 120Ω resistors in parallel = 60Ω)
  Values outside 54–66Ω indicate missing or extra termination

Bit rate versus cable length

CAN bus bit rate is limited by propagation delay — the time it takes a signal to travel to the farthest node and return before the bit period ends. ISO 11898-2 specifies the standard bit rate and cable length combinations. Building automation systems rarely need 1 Mbit/s; 125 kbit/s or 250 kbit/s is typical for HVAC equipment over cable runs of 100–500m between plant rooms.

CAN frame structure

The standard CAN data frame (ISO 11898-1) consists of seven fields. Understanding the arbitration ID field is critical for CANopen configuration — the COB-ID (Communication Object Identifier) used in CANopen is the 11-bit standard CAN ID. Extended frames use 29-bit IDs and are used in CANopen with 4-byte COB-IDs (extended addressing mode).

Error detection mechanisms

CAN has five independent error detection mechanisms that operate simultaneously. Each detected error causes all nodes to discard the current frame and the transmitter to re-send. A node that repeatedly causes errors transitions from error-active to error-passive to bus-off state, protecting the network from a faulty node.

Non-destructive bitwise arbitration

When two or more nodes attempt to transmit simultaneously, CAN resolves the conflict through bitwise arbitration without destroying either message. Each node monitors the bus while transmitting — if a node transmits a recessive bit (1) but reads a dominant bit (0), it immediately stops transmitting and switches to receiver mode. The node with the lowest numerical arbitration ID always wins.

In CANopen, this means lower COB-IDs have higher priority. Emergency messages (EMCY, COB-ID 0x080 + node ID) and heartbeat messages are assigned lower COB-IDs than PDO data — ensuring alarms are never delayed behind process data traffic. Assign the lowest node IDs to the most critical devices such as fire panel gateways or chiller alarm controllers.

CAN FD — Flexible Data-rate

CAN FD (ISO 11898-1:2015) extends classical CAN with two improvements: a variable data phase bit rate up to 8 Mbit/s (versus 1 Mbit/s for classical CAN) and a payload size up to 64 bytes (versus 8 bytes). The arbitration phase still runs at the classical bit rate for compatibility — only the data phase switches to the higher bit rate after the BRS (Bit Rate Switch) bit.

Diagnostic tools

Use a CAN analyser during commissioning to verify bus traffic, monitor error frames, and confirm node IDs before integrating with a CANopen gateway. Three tools are standard in the industry.