6.2 Controller Area Network (CAN) and Garmin HSDB Architectures

6.2 Controller Area Network (CAN) and Garmin HSDB Architectures

Quick Answer: The ISO 11898 Controller Area Network (CAN) is a multi-master, linear broadcast databus where all nodes share a single differential pair (CAN_H and CAN_L). In the recessive state (Logic '1'), both lines rest at $2.5\text{ V}$ ($0\text{ V}$ differential). In the dominant state (Logic '0'), CAN_H is driven to $3.5\text{ V}$ and CAN_L to $1.5\text{ V}$ ($2.0\text{ V}$ differential). Exactly two $120\ \Omega$ termination resistors are placed at the extreme physical ends of the bus trunk, producing a measured unpowered bus resistance of precisely $60\ \Omega$. Bus collisions are resolved non-destructively using CSMA/CD with bitwise arbitration, whereby dominant '0's override recessive '1's, ensuring the lowest numerical identifier transmits without latency. For high-bandwidth payloads such as digital weather radar and terrain rendering, modern avionics employ the Garmin High-Speed Data Bus (HSDB), an IEEE 802.3u 100BASE-TX full-duplex switched Ethernet architecture operating at $100\text{ Mbps}$ over dedicated 4-wire shielded twisted pair cabling.

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ISO 11898 CAN Bus Architecture and Multi-Master Topology

Originally developed by Robert Bosch GmbH in the 1980s for robust vehicular systems and standardized internationally under ISO 11898, Controller Area Network (CAN) has emerged as a cornerstone digital databus in modern general aviation, light business jets, and unmanned aircraft systems (UAS). For example, in the Garmin G3X Touch system the displays, air data and AHRS units, engine interface, and autopilot servos share a CAN network, and many other light-aircraft and experimental avionics systems use CAN for sensors and servos.

Linear Trunk-and-Drop Topology

Unlike ARINC 429's point-to-point simplex architecture, CAN operates on a linear trunk-and-drop topology:

• Shared Multi-Master Bus: All connected Line Replaceable Units (LRUs) connect in parallel across a single twisted pair comprising CAN_H (CAN High) and CAN_L (CAN Low). There is no master computer governing bus access; any node can initiate a transmission whenever the bus is idle.
• Trunk Line: The continuous main transmission line that routes through the aircraft cabin, nose bay, and wing roots.
• Drop Lines (Stubs): Short stub connections that tee off the main trunk to interface individual LRUs. To prevent transmission line reflections and capacitive loading, avionics installation guidelines strictly limit drop stub lengths (typically to less than 0.3 meters to 1.0 meter).
• Star and Ring Topologies Prohibited: True star or ring topologies are forbidden in standard ISO 11898 high-speed CAN networks because branch reflections destroy high-frequency edge transitions.

[120-ohm]                                                              [120-ohm]
[Term R ]                                                              [Term R ]
   |                                                                      |
===+====================+======================+==========================+===
   |                    |                      |                          |
   | Drop <= 0.3m       | Drop <= 0.3m         | Drop <= 0.3m             |
+--+---+             +--+---+               +--+---+                   +--+---+
| Node |             | Node |               | Node |                   | Node |
| AHRS |             | EIS  |               | Servo|                   | PFD  |
+------+             +------+               +------+                   +------+

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Physical Layer Signaling: Recessive vs. Dominant

CAN transceivers communicate by altering the differential potential between CAN_H and CAN_L across a common-mode DC baseline.

1. Recessive State (Logic '1')

• Transceiver Output Stage: Both output driver transistors are turned OFF (high-impedance state).
• Line Voltage: Internal resistive biasing networks within each connected transceiver maintain both CAN_H and CAN_L at approximately $+2.5\text{ V}$ nominal relative to airframe ground (typical range: $2.0\text{ V}$ to $3.0\text{ V}$).
• Differential Voltage ($V_{\text{diff}}$): $V_{\text{diff}} = V_{\text{CAN\_H}} - V_{\text{CAN\_L}} = 2.5\text{ V} - 2.5\text{ V} = 0.0\text{ V}$ ISO 11898-2 requires a transmitter's recessive output to fall between $-0.5\text{ V}$ and $+0.05\text{ V}$ differential, and a receiver treats any differential voltage below 0.5 V as a Recessive Bit (Logic '1').

2. Dominant State (Logic '0')

• Transceiver Output Stage: The line driver transistors turn ON, actively driving current through the terminating resistors.
• Line Voltage: CAN_H is driven up to $+3.5\text{ V}$ (allowable range: $2.75\text{ V}$ to $4.50\text{ V}$), while CAN_L is pulled down to $+1.5\text{ V}$ (allowable range: $0.50\text{ V}$ to $2.25\text{ V}$).
• Differential Voltage ($V_{\text{diff}}$): $V_{\text{diff}} = 3.5\text{ V} - 1.5\text{ V} = +2.0\text{ V}$ A transmitter drives $+1.5\text{ V}$ to $+3.0\text{ V}$ differential for dominant, and a receiver treats any differential voltage above 0.9 V as a Dominant Bit (Logic '0').

Voltage (V)
  4.0V |
  3.5V |        +-------------+                 CAN_H (Dominant 3.5V)
       |        |             |
  2.5V |--------+             +--------+-----   CAN_H & CAN_L (Recessive 2.5V)
       |        |             |
  1.5V |        +-------------+                 CAN_L (Dominant 1.5V)
  1.0V |
       +--------------------------------------
       | Recessive (1)| Dominant (0) | Recessive (1)|
       | V_diff = 0V  | V_diff = 2V  | V_diff = 0V  |

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Bus Termination and Resistance Troubleshooting

High-speed CAN signals propagate down the transmission line at approximately $200\text{ meters per microsecond}$ ($0.66c$). When the steep voltage transition of a dominant bit encounters an open circuit at the end of the wire harness, electromagnetic energy reflects back into the cable in-phase. These signal reflections generate constructive and destructive ringing, corrupting bit decoding.

Termination Requirements

To absorb reflections, high-speed ISO 11898 CAN networks mandate two $120\ \Omega$ termination resistors ($1/4\text{W}, 1\%$ metal film), installed across CAN_H and CAN_L strictly at the two extreme physical ends of the main trunk line. Intermediate nodes along the trunk must remain unterminated.

De-Energized Resistance Testing

Avionics technicians verify CAN physical layer integrity using a Digital Multimeter (DMM) in resistance mode. Crucial Rule: Power to the aircraft electrical buses must be completely OFF/de-energized prior to measuring resistance, as energized transceiver bias voltages will corrupt DMM resistance readings.

Because the two $120\ \Omega$ termination resistors are connected in parallel across the continuous CAN_H and CAN_L lines, total equivalent resistance ($R_{\text{total}}$) is calculated using the parallel resistance formula:

$R_{\text{total}} = \frac{R_1 \times R_2}{R_1 + R_2} = \frac{120\ \Omega \times 120\ \Omega}{120\ \Omega + 120\ \Omega} = \frac{14{,}400}{240} = 60\ \Omega$

In-Circuit Fault Diagnosis Matrix

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CSMA/CD with Bitwise Arbitration

In a multi-master network where multiple LRUs can transmit simultaneously, data collisions would normally corrupt packets. CAN resolves collisions without losing data using Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and Bitwise Arbitration, which is why CAN is often described as CSMA with collision resolution (CSMA/CR).

Non-Destructive Bitwise Arbitration

The fundamental principle of CAN arbitration is that a Dominant bit ('0') electrically overrides a Recessive bit ('1'):

• If one transceiver attempts to output a recessive state (transistors OFF, floating at $2.5\text{ V}$), but another transceiver simultaneously drives a dominant state (transistors ON, driving to $3.5\text{ V}$ and $1.5\text{ V}$), the physical bus voltage is pulled to the dominant state ($2.0\text{ V}$ differential).
• The bus acts as an electrical wired-AND logic gate.

Arbitration Sequence in Action

1. Carrier Sense: Nodes listen to the bus. A node may start a frame only when the bus is idle, after the previous frame's end-of-frame and intermission bits.
2. Synchronized Transmission: If two nodes (e.g., Autopilot Pitch Servo and Engine Monitor) begin transmitting simultaneously, each outputs the Start of Frame (SOF) dominant bit followed by the bits of its Message Identifier (11-bit standard CAN 2.0A or 29-bit extended CAN 2.0B).
3. Continuous Bus Readback: Crucially, while a transceiver is transmitting each bit, its internal differential receiver continuously samples the actual voltage present on the physical bus.
4. Collision Detection and Backoff: As long as both nodes transmit identical bits, neither detects a conflict. However, the instant Node A transmits a recessive bit ('1') while Node B transmits a dominant bit ('0'):
   • Node A expects to see a recessive state on the bus.
   • Instead, Node A's receiver senses the dominant state enforced by Node B.
   • Node A recognizes that another higher-priority message is on the bus.
   • Node A instantly halts transmission and switches to receive-only mode, without corrupting Node B's transmission.
   • Node B never notices the conflict and continues transmitting its message uninterrupted.

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Aviation CAN Protocols: CANaerospace and CANopen

While ISO 11898 standardizes the physical and data link layers, the aeronautics industry requires upper-layer software protocols to define data structures, transmission intervals, and network health monitoring.

• CANaerospace: An open protocol developed by Stock Flight Systems in the late 1990s and adopted for NASA's AGATE general-aviation research program, CANaerospace defines an open communication protocol specifically for flight-critical avionics. It maps standard aeronautical parameters into defined 11-bit and 29-bit CAN identifiers. It incorporates Heartbeat Monitoring (periodic health pings from every LRU), High-Frequency Periodic Channels (e.g., AHRS pitch/roll at 50 Hz to 100 Hz), and Emergency Event Data channels that immediately preempt ordinary bus traffic.
• CANopen: A European-standardized higher-layer profile (EN 50325-4) adapted for aircraft flight control surfaces, electro-mechanical actuators, and environmental control systems. CANopen uses an Object Dictionary to standardize device configuration parameters and uses Service Data Objects (SDO) for configuration and Process Data Objects (PDO) for real-time sensor streaming.

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Garmin High-Speed Data Bus (HSDB)

While CAN bus is exceptionally robust for sensor telemetry and servo control at data rates up to $1\text{ Mbps}$, modern flight decks require bandwidths two orders of magnitude higher to transfer megabyte-scale graphic databases, digital radar sweeps, and synthetic vision terrain.

To meet this demand, Garmin developed the Garmin High-Speed Data Bus (HSDB), which forms the high-speed data backbone of the G1000, G3000, G5000, and GTN 650/750 avionics suites.

HSDB Architecture and Physical Layer

• Ethernet Foundation: Garmin HSDB is an implementation of IEEE 802.3u 100BASE-TX Fast Ethernet.
• Bandwidth: Operates at 100 Mbps, full duplex (simultaneous bidirectional transmission without collisions).
• Physical Media: 4-wire shielded twisted pair cable (two distinct twisted pairs within an overall outer braided shield):
  • Pair 1: Transmit data lines (TX+ and TX-).
  • Pair 2: Receive data lines (RX+ and RX-).
  • Characteristic impedance is $100\ \Omega$ differential.
• Point-to-Point Switched Architecture: Unlike CAN's multi-drop linear bus, HSDB is strictly a point-to-point switched network. Each LRU connects point to point over dedicated cable runs to Ethernet ports or switching built into the Garmin units, rather than sharing one multi-drop wire pair.

Avionics Systems Linked by Garmin HSDB

1. Airborne Digital Weather Radar: Streaming raw digital sweep and Doppler turbulence targets from GWX 70 or GWX 75 radar units directly to cockpit displays.
2. Synthetic Vision System (SVS): Transferring 3D terrain mesh data, obstacle warning elevations, and airport runway diagrams between the integrated flight computer and the PFD/MFD.
3. Navigation / Chart Databases: High-speed cross-loading of Jeppesen electronic terminal charts and flight plans between GTN 750 navigators.
4. Cross-Cockpit Flight Display Synchronization: Mirroring flight plans, autopilot mode annunciations, and active radio tuning between Captain and First Officer displays.